Stomach parasite burden and health status of stranded common dolphins, Delphinus delphis

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Alwis, Sofia Albrecht, Sinéad Murphy, Jim O'Donovan, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4906242/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 03 Apr, 2025 Read the published version in Marine Biology → Version 1 posted 5 You are reading this latest preprint version Abstract Short-beaked common dolphins are important top predators in marine ecosystems, and inevitably they encounter a range of pressures in their environment. Parasite infections pose one such possible pressure and examining parasite burdens of common dolphins, and any possible impacts of infection, enables us to better understand these pressures. Here we examined the stomach parasite burden of 69 stranded common dolphins collected along the Irish coastline between 2017–2019. After isolating the parasites from the stomachs, all were identified as nematodes belonging to the genus Anisakis . Additional to adult nematodes, L3 and L4 larval stages were observed. A total of 319,344 anisakid specimens were estimated from all stomach compartments of the animals. Parasite prevalence was 94%, mean abundance was 4,630 worms, and mean intensity was 4,910 worms. A generalised linear model with negative binomial error structure revealed that dolphin body length, nutritional status and year of stranding were significantly associated with parasite burden. The results presented highlight the high parasite burdens that may be carried by common dolphins and their potential interaction with health parameters such as nutritional status. To gain a comprehensive overview it is important to include parasite infection investigations along with other parameters when evaluating the health status of marine mammals. Anisakis cetacean North-East Atlantic strandings marine mammal health Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Parasites are an integral part of natural ecosystems, influencing ecosystem functioning and enhancing biodiversity, while occurring in virtually all free-living organisms (Marcogliese, 2005 ). However, parasites can be a natural stressor on their hosts as they display an obligate physical association to obtain critical resources (Stewart and Schnitzer, 2017 ). Infection with parasites can have substantial effects on the health of host organisms, by causing diseases, impairing growth and reproduction, and even leading to mortality (Dailey and Walker, 1978 ; Geraci et al., 1978 ; Bressem et al., 1998 ; Fauquier et al., 2009 ). Their impacts extend to population levels influencing population dynamics and community structure (Geraci et al., 1978 ; Terracciano et al., 2020 ) demonstrating the important ecological role they occupy. The transmission of parasites from intermediate hosts to definitive hosts, such as marine mammals, typically occurs trophically through the food web (Luque and Poulin, 2004 ). Marine mammals occupy important ecological roles as top predators in marine ecosystems and thereby can affect ecosystem function and structure (Nelms et al., 2021 ). These animals are considered sentinel species or indicators of their environs, and can provide early warnings of potential changes to their ecosystem (Aguirre and Tabor, 2004 ; Bossart, 2011 ). Furthermore, marine mammals can be used as indicators of human health (O’Brien et al., 1993). Up to certain thresholds in parasite burden, the consequences of parasitic infections may manifest as localised reactions within the host, rather than systemic reactions. For instance, the embedding of the proboscis of the acanthocephalan Bolbosoma into the intestinal wall in the fin whale, Balaenoptera physalus , leads to localised reactions in the host (Santoro et al., 2021 ). These local reactions are unlikely to have a substantial impact on the overall health and survival of the host, enabling some levels of tolerance. However, more severe reactions, such as granulomas, ulcers and haemorrhages in response to parasites, such as the nematodes Anisakis and Pseudoterranova , can have greater effects on host health (Jaber et al., 2006 ; Di Azevedo et al., 2017 ; Pons-Bordas et al., 2020 ). In extreme cases, severe infections can cause debilitation and death (Geraci and Aubin, 1987 ; Stroud and Roffe, 1979 ; Terracciano et al., 2020 ). For instance, parasitosis, such as cerebral nasitremosis caused by trematodes in the genus Nasitrema may lead to inflammation and death in brain tissue and has resulted in strandings of striped dolphins ( Stenella coeruleoalba ) (Dailey and Walker, 1978 ; O’Shea et al., 1991 ). Some studies have reported the influence of parasites on marine mammal population dynamics with parasites affecting the survival and reproduction of the host. It is suggested that transplacental lungworm infections in stillborn bottlenose dolphins ( Tursiops truncatus ) leading to increased neonatal mortality could affect recruitment (Fauquier et al., 2009 ), although the effect of such infections on neonatal survival has yet to be fully discovered. Geraci et al. ( 1978 ) reported that mastitis in Atlantic white-sided dolphins ( Lagenorhynchus acutus ), caused by the nematode Crassicauda grampicola , affected the quality and quantity of milk production, compromising the reproductive success of the pod studied. Short-beaked common dolphins, Delphinus delphis , have a global distribution encompassing temperate and tropical waters. (Perrin, 2018 ). They are one of the most abundant cetacean species in the North-East Atlantic, with one population reported to date in the region, ranging from waters off Macaronesia and North-West Africa to Norway (Murphy et al., 2019 ). A recent abundance estimate reported 439,212 dolphins (coefficient of variation (CV) = 0.18; 95% confidence interval (CI) 309,153–623,987) for the European continental shelf and adjacent waters, excluding the Irish EEZ, for the summer of 2022 (Gilles et al., 2023 ). Within Irish waters, common dolphins are present year-round, and are the most frequently reported species stranded on coastlines (McGovern et al., 2018 ; Levesque and Berrow, 2022 , 2024 ). The species faces numerous anthropogenic stressors across the North-East Atlantic, including by-catch in fisheries, persistent pollutants, underwater noise, marine debris and wildlife tourism (Atkinson and Dierauf, 2018 ; Lusher et al., 2018 ; Murphy et al., 2019 ). Among natural stressors are competition for resources, environmental changes, including shifts in ocean temperatures resulting in changes in prey availability and/or quality, and alterations in habitat conditions, parasites and disease, and predator attacks, all of which can impose stressful conditions with sublethal or lethal consequences (Murphy et al., 2005 ; Marcogliese and Pietrock, 2011 ; Murphy et al., 2013 ; Atkinson and Dierauf, 2018 ; Murphy et al., 2019 ; Methion and López, 2021 ). The severity of parasitic infections in marine mammals is dependent on factors involved in the host-parasite interaction such as the host species, health status of the host, type(s) of parasite, parasite burden, and co-occurrence of other anthropogenic or natural stressors (Bull et al., 2006 ; Raga et al., 2009 ; Cable et al., 2017 ). Among the parasites affecting common dolphins, various microparasites have been documented. For instance, the bacteria Helicobacter spp., associated with gastritis and gastric ulcers, can impact their digestive system (Harper et al., 2000 ). Another bacterium, Streptococcus iniae , has been linked to skin abscesses and mortality in juvenile common dolphins, emphasising the potential severity of bacterial infections (Souter et al., 2021 ). Herpesvirus causes genital lesions, impacting the reproductive health of dolphins (Bento et al., 2019 ), while dolphin morbillivirus is associated with neurological and respiratory symptoms in dolphins, affecting their overall health (Bressem et al., 1998 ). Parasitic protozoans, such as Toxoplasma gondii , Giardia spp. and Cryptosporidium spp. have been reported among common dolphins, although their specific effects are yet to be fully understood (Forman et al., 2009 ; Reboredo- Fernández et al., 2014). Macroparasite infections have also been reported in common dolphins leading to morbidity and in some cases, mortality. Adult gastrointestinal nematodes, particularly Anisakis simplex (s.s.) and A. pegreffii , typically attach to the mucosa of the dolphin stomach, leading to the development of granulomatous ulcers, haemorrhages, and perforations of the stomach lining, which can result in peritonitis (Jaber et al., 2006 ; Motta et al., 2008 ). Other nematodes such as Halocercus lagenorhynchi can cause lung lesions, and high burdens can impair lung function (Tomo et al., 2010 ) while nematodes of the genus Crassicauda are associated with verminous prostatitis, causing inflammation of the prostate gland (Suárez-Santana et al., 2018 ). The trematode Campulla palliata can cause severe inflammation and damage to the bile ducts and liver tissue (Cordes and O’Hara, 1979 ), while another trematode, Pholeter gastrophilus , is associated with granulomatous gastritis, characterised by the formation of nodular granulomas in the stomach lining (Jaber et al., 2006 ). To better understand the pressures faced by marine mammals, it is necessary to gain more information on their parasite burdens and the factors affecting infection levels. Here we examine parasite burden and infection intensity in the stomachs of common dolphins that stranded along the Irish coastline. We investigate the relationships between parasite infection and dolphin sex, total body length, nutritional status, and year of stranding. We also assess the presence of granulomas on the stomach lining in relation to parasite infection intensity. Materials and methods Sample acquisition Stranded common dolphins, Delphinus delphis , were collected from the Irish coastline between 2017 and 2019 (Fig. 1 ). Dolphin carcasses were subsequently necropsied at the Regional Veterinary Laboratory located in Cork, following established post-mortem procedures (see Levesque et al., 2021 for further information). Information on date and location of stranding, sex and total body length (cm) were recorded (Levesque et al., 2021 ). The decomposition status of each carcass was categorised as: extremely fresh, slight decomposition, moderate decomposition and advanced decomposition (Levesque et al., 2021 ). Upon necropsy, the nutritional status of each individual was assessed by viewing the flanks around the dorsal fin and assessing whether they exhibited a convex, flat or concave profile. Nutritional status was categorised as good, moderate, or poor (Levesque et al., 2021 ) and stomachs were then frozen at -20℃ until further analysis. For the present study focusing on parasite burdens in common dolphin stomachs, the presence or absence of stomach ulcers was also recorded. Recovery and identification of gastrointestinal parasites Thawed stomachs were dissected, washed into a tray using 99% ethanol and all parasites were recovered using forceps. Parasites were collected from all three stomach compartments (forestomach, glandular stomach and pyloric stomach) (Harrison et al., 1970 ) and preserved in 70% ethanol. All parasites recovered were nematodes, and individuals were identified morphologically as larval stages (L3, L4s) or adults (Davey, 1971 ; Grabda, 1976 ) using light microscopy, focusing mainly on size differences. Representatives of the three different life stages were cleared in lactophenol and examined using a light microscope (Olympus BX51) to identify sex. Sexually mature adults were identified based on morphological features (Grabda, 1976 ; Gomes et al., 2021 ). To assess the parasite burden, parasites were individually counted. However, due to heavy infections in 24 individuals (35% of dolphin samples), subsampling was required, with subsequent estimation of parasites. For these individuals, parasite samples were weighed (Adventurer SL Precision balance) (ethanol wet weight) and three subsamples accounting for 10% of the total wet weight, were isolated and individual parasites were counted (Ugland et al., 2004 ). Total parasite burden per dolphin stomach (N) was calculated following the equation below for each subsample, and the average N of the three subsamples was calculated and used as the parasite burden measure for these 24 dolphin individuals. N = (W/w)*n Where, W = Wet weight of the parasites in the full stomach sample w = Wet weight of the parasites in each subsample n = Number of parasites in each subsample Data analysis Prevalence, mean abundance and mean intensity of parasite burdens were calculated using Web-based Quantitative Parasitology (QPweb) software (Reiczigel et al., 2019 ). Prevalence refers to the proportion of hosts infected with a specific parasite (Bush et al., 1997 ), whereas mean abundance refers to the average number of individuals of a specific parasite per host individual including uninfected hosts (Bush et al., 1997 ). Mean intensity refers to the average number of individuals of a particular parasite per infected host (Bush et al., 1997 ). The 95% confidence interval (CI) for prevalence was presented as Clopper-Pearson intervals and for mean abundance and mean intensity, CIs were based on 10,000 bootstrap replications (BCa method). All statistical analyses were performed using R software for statistical computing (version 4.3.1) (R Core Team, 2023 ) in Posit (previously RStudio) IDE (version 2023.06.1 + 524) (Posit Team, 2023 ). Initial data exploration was carried out to identify the spread and distribution of data and outliers, as well as the collinearity of predictor variables (Zuur et al., 2010 ). Two extreme values for parasite burden were identified as outliers and removed from further analysis. A generalised linear model (GLM) was constructed using parasite burden as the response variable, with dolphin sex, length, and nutritional status, and year of stranding as predictors. Body length data were centered prior to the analysis so that the intercept is representative of a common dolphin of average length (Ten Doeschate et al., 2017 ). Average body length was calculated to be 190.25 cm based on the 69 individuals for which length was recorded. Since parasite burden was highly skewed, non-continuous, and the variance (36,779,883) was greater than the mean (4264.11), standard linear regression models were not suitable for these over-dispersed count data. Parasites are characteristically aggregated across their host populations, and burden generally follows a negative binomial distribution (Grenfell et al., 1995 ; Shaw et al., 1998 ), hence, assuming the negative binomial error structure, the glm.nb function from the R MASS package (Venables and Ripley, 2002 ), was used. To assess the potential influence of parasite burden on the presence of gastric ulcers a Wilcoxon two-sample test was carried out. Parasite biomass was also calculated as a measure of the parasite burden in common dolphins and a generalised linear model (GLM) was constructed using parasite biomass as the response variable, with common dolphin sex, length, and nutritional status, and year of stranding as predictors (Supplementary Table S1 ). To explore the biomass scaling relationship between parasite biomass and host biomass a log-log linear regression was carried out (Brown et al., 2004 ; Poulin and George -Nascimento, 2007). The rate of conversion of resources into growth for eukaryotic organisms is predicted to have a slope of 0.75 on a log-log biomass plot when corrected for operating temperature (Brown et al., 2004 ). If host resources are converted into parasite biomass similar to host growth, we would expect a slope of 0.75 on a log-log plot of parasite biomass and host mass (Poulin and George-Nascimento, 2007 ). If the slope is smaller host resources are not converted into parasite biomass as efficiently as they are for host growth, while if the slope is larger host resources are converted into parasite biomass at a higher rate than for host growth. Results Of the 69 common dolphins collected between 2017 to 2019, 32 were female and 37 were male. Regarding decomposition state 36.2% of dolphin carcasses were extremely fresh, 36.2% were slightly decomposed, 27.5% were moderately decomposed and no dolphins of an advanced decomposed state were reported. Of the 69 dolphin stomachs analysed, parasites were recovered from 65 and together contained an estimated total of 319,344 anisakid nematodes, of the genus Anisakis , with burden per dolphin ranging from 2 to 24,892 parasites (Fig. 2 ). No other parasites were recorded within the stomachs. Sexually mature adults were identified by the presence of a clearly visible zigzag tubiform testis, ventrally bent tail, spicules near the anus in males and sinuous ovary ducts, uteri, and ova in the uteri in females where larval forms had no obvious sexual features (Fig. 3 ; Grabda, 1976 ; Gomes et al., 2021 ). In addition to adult nematodes, both L3 and L4 larval stages of Anisakis spp. were observed. Anisakis prevalence was 94% (95% confidence interval (CI): 85–98%), mean abundance was 4,630 worms (95% CI: 3,330 to 6,280), and mean intensity was 4,910 worms (95% CI: 3,520 to 6,640). Analysis of dolphin health and parasite burden The results showed that dolphin nutritional status and total body length, as well as year of stranding, were significantly associated with parasite burden (model output shown in Supplementary Table S2). No significant interactions were detected. Lower parasite burdens were observed in common dolphins that stranded in 2019 compared to those from 2017 (z= -2.76, p = 0.006; Fig. 4 a), while no significant difference was found between 2017 and 2018 (z= -0.80, p = 0.422; Fig. 4 ). The intercept was changed, and the model was re-run to ensure all comparisons were obtained. A significant difference was found between 2019 and 2018 (z =-1.98, p = 0.047). Parasite burdens differed between dolphin nutritional states, with significantly higher parasite burdens observed in individuals in poor (z = 2.62, p = 0.009) and moderate nutritional status (z = 2.34, p = 0.019) compared to good nutritional status (Fig. 4 b). The model revealed a significant positive association between dolphin length and parasite burden (z = 3.78, p = < 0.001) (Supplementary Table S2) where longer dolphins harboured a higher number of parasites compared to smaller dolphins. There was no significant difference in Anisakis burden between male and female common dolphins (z= -1.20, p = 0.230; mean abundance: males 4,610, females 4,650). Overall, Anisakis prevalence in males (97.3%) was slightly higher than that for females (90.6%), whereas mean parasite intensity was slightly higher for females (5,130) than for males (4,740) but neither were significant. Ulcers were identified in 89.9% of common dolphins and no significant association between the presence of ulcers and parasite burden was found (W = 190.5, p = 0.605). Parasite biomass and parasite burden were highly positively correlated (r = 0.79, p < 0.001) while parasite biomass and host body weight were moderately positively correlated (r = 0.2, p < 0.056). The results of the GLM analysis on parasite biomass in relation to common dolphin sex, nutritional status, and length, and year of stranding showed that only nutritional status and body length were significantly associated with parasite biomass (Supplementary Table S1 ). Significantly higher parasite burdens were observed in individuals in poor nutritional status compared to good nutritional status (t = 2.73, p = 0.008) and a significant positive association between dolphin length and parasite biomass was observed where longer individuals harboured a higher parasite biomass compared to smaller dolphins (t = 3.19, p = < 0.002). On investigating the scaling of total parasite biomass with host body mass the log-log linear regression produced a slope of 1.57 (95% CI: 0.77–2.36) with considerable variation observed in the data. (Fig. 5 ) Discussion In the current study, 69 common dolphins were examined for gastrointestinal macroparasite infections. Of these, 65 were infected with gastrointestinal nematodes, Anisakis spp., identified morphologically to genus level. As morphology alone is not sufficient for species identification, (Ángeles- Hernández et al., 2020), follow up molecular work is required (Mattiucci et al., 2018 ). Only two species of Anisakis have been reported in common dolphins thus far, A. simplex (s.s.) and A. pegreffii (Mattiucci et al., 2018 ; Pons-Bordas et al., 2020 ), both of which exhibit different geographical distributions (Mattiucci et al., 2008 ). Anisakis simplex (s.s.) has a widespread distribution, between 35 °N and the Arctic circle, while A. pegreffii is mainly found between 30 °N and 55°S (Mattiucci et al., 1997 ). Based on this, and despite the potential for some overlap in Anisakis spp. distributions (Mattiucci et al., 2008 ), it is likely that, given the sampling region, the anisakids recovered in this study were A . simplex (s.s.). In addition, in a metabarcoding study examining the diet of the same common dolphins examined here A. simplex was detected in the genetic sequences obtained (unpubl data). Compared to previous studies, a higher prevalence (94%, 65/69) and higher mean abundance (4,630 worms) of Anisakis spp. was observed in common dolphins sampled in the current study. Gibson et al. ( 1998 ) reported a prevalence of 71.3% (72/101, 1990–1994) for common dolphins stranded in England and Wales, while Moloney ( 1994 ) reported a lower prevalence of 32% (6/19) and mean abundance of 498 worms for individuals stranded in Ireland during a similar three-year time period (1991–1993). All studies sampled stranded dolphins, which in part are comprised of animals in both poor health and also animals that died from acute anthropogenic interactions, i.e. fisheries interactions, and because stranding events are often biased towards sick or injured individuals (Coombs et al., 2019 ), the observed prevalence and burden of parasites may not be representative of the entire population. Nonetheless, differences in Anisakis spp. prevalence and burden between these studies of common dolphin may also be attributed to either a larger sample size within the current study compared to Moloney ( 1994 ), and/or an actual increase in dolphin parasite burden within the region. Within the current analysis, year of stranding, and dolphin nutritional status and total body length were found to be significantly associated with Anisakis burden. While parasite prevalence and burden in this study appears higher in comparison to earlier studies, within our study a declining trend in parasite burden and prevalence was observed between 2017 and 2019. Upon examining the cause of death assigned to the dolphins in this study an increase was observed in the number of trauma cases in 2019 and 2018, compared to 2017 (see data presented in Fig. 6 a as reported in Levesque et al. ( 2021 ). The parasite burden in dolphins that died from trauma was low in comparison to other causes of death (Fig. 6 b), although the values reported from dolphins that died of starvation were the lowest. It is possible that the decrease in parasite prevalence and burden values observed over the three years is a result of increased numbers of dolphins in the dataset that died from trauma, and which tended to have relatively low parasite burden (Fig. 6 b). Future marine mammal necropsy studies could include investigations of parasite burden in relation to cause of death to further identify any possible patterns. The life cycle of Anisakis spp. can involve various invertebrates as first intermediate host, larger invertebrates such as copepods and euphausiids, as well as smaller fish, as second intermediate hosts, predatory fish and cephalopods as transport or paratenic hosts (Køie, 2001 ), and cetaceans as definitive hosts (Mattiucci, 2018). A recent meta-analysis reported a significant increase of Anisakis spp. between 1978–2015 in the prey species of cetaceans (Mastick et al., 2024 ) suggesting an increasing risk of Anisakis spp. infection for cetacean hosts. Increasing parasite burden of Anisakis spp. have also been observed in marine fish and invertebrates, with a 283-fold increase reported over a 53-year period between 1962 to 2015 (Fiorenza et al., 2020 ). The authors suggest that these increases could be due to increases in definitive host populations, resulting from enhanced protection measures for marine mammals, providing an increasing host population to support larger Anisakis populations (Fiorenza et al., 2020 ). Furthermore, they propose climate change may increase the susceptibility of the hosts to parasitic diseases and that rising temperature can also expedite growth and shorten generation times in aquatic parasites (Fiorenza et al., 2020 ). Studies on the diet of common dolphins in the North-East Atlantic have shown they prey on a diverse range of fish and squid species (Pusineri et al., 2007 ; Brophy et al., 2009 ) which are known to harbour Anisakis (Mattiucci et al., 2008 ; Pierce et al., 2018 ; Cipriani et al., 2022), and as outlined here may be increasing. Taken together these factors may be contributing to the overall observed increase in parasite burden in the stomachs of common dolphins examined here. To gain a better understanding of the underlying mechanisms that are driving temporal variation in parasite prevalence and burden in common dolphins, detailed long-term temporal data on prey consumption and Anisakis prevalence in prey populations are required. The role of host condition and nutritional state in mediating parasite burden is emphasised by the significant association between nutritional status and parasite burden. Common dolphins in poor nutritional condition generally harboured a higher number of parasites, as did dolphins with moderate nutritional status, when compared to individuals in good nutritional status. It has been reported that gastrointestinal helminth infections negatively affect body condition of mammals (Shanebeck et al., 2022 ) and better nutritional condition is linked to more robust immunity among mammals (Brock et al., 2013 ). Hence common dolphins with better nutritional status could be generally healthier with a strong immune system and therefore more resistant to Anisakis infections, resulting in lower worm burden. It is possible that higher levels of parasite infections as reported here have influenced the overall decline in the nutritional health of individual dolphins, as reported in a morphometric analysis of the same dolphin samples compared to historical data (Albrecht et al, in review). Potential underlying causes of poor nutritional status among cetaceans are reduced prey availability (Raverty et al., 2020 ) and prey quality (Spitz et al., 2010 ), physiological conditions such as compromised health conditions due to chronic diseases, hormone imbalances leading to weight loss (Schwacke et al., 2013 ), increased metabolic demands due to gestation and lactation (Gómez-Campos et al., 2011 ) and various poor environmental conditions such as high and increasing anthropogenic noise (sonar, shipping) potentially causing disruptions in dolphins ability to forage (Wright et al., 2007 ). Upon examining the causes of death among the dolphins in this study, we observed that 82% of those that died from trauma were assigned either good or moderate nutritional status. Additionally, these dolphins had fewer parasites compared to those that died from other causes. These data suggest that dolphins that died from trauma may represent 'healthier' individuals. However, further investigations are required to uncover the patterns occurring in these dolphin-nematode interactions. Larger common dolphins were found to harbour a higher number of parasites compared to smaller individuals, with a significant positive association between body length and parasite burden. Larger animals may be able to host a greater number of parasites (Kamiya et al., 2014 ), and increased prey consumption due to an increase in nutritional demands during key life history stages (Rechsteiner et al., 2013 ) and diet differences between age-groups (Novak, 2023 ) may lead to higher infection rates. Similar findings regarding size have been reported in other studies examining parasite burden in cetaceans. For instance, anisakid burden was correlated with body length in both bottlenose ( T. truncatus ) and striped ( S. coeruleoalba ) dolphins inhabiting the Adriatic Sea (Blažeković et al., 2015 ), and longer and older long-finned pilot whales ( Globicephala melas ) sampled off the Faroe Islands tended to harbour a higher number of helminth parasites (Balbuena and Raga 1993 ; Bellay at al., 2020). Contrary to other work, Mateu et al. ( 2014 ) reported that the burden of helminth species ( Tetrabothrius forsteri, Trigonocotyle globicephalae and Strobilocephalus triangularis ) was not significantly associated with the body length of striped dolphins (n = 52) in the western Mediterranean Sea, and the authors suggest that this could be due to the low number of juvenile individuals in their sample. Anisakis burden was not influenced by the sex of an individual within the current study, and similar findings have been found in many other cetacean studies to date, for example bottlenose dolphins ( T. truncatus ) (Blažeković et al., 2015 ), striped dolphins ( S. coeruleoalba ) (Blažeković et al., 2015 ), beluga whales ( Delphinapterus leucas ) (Pufall et al., 2012 ), and franciscana ( Pontoporia blainvillei ) (Hoss et al., 2017 ). A lack of variation in parasite burden with host sex could be due to high similarity in the diets of male and female common dolphins (Brophy et al., 2009 , Novak, 2023 ). In contrast, through reviewing numerous studies, Poulin ( 1996 ) showed a higher prevalence of nematodes in male mammalian hosts compared to females which fits with the slight, but non-significant difference, observed in this study (males 92.3%, females 85.7%). Gastric ulcers were observed in many of the common dolphins examined in the current analysis, but their presence did not appear to be associated with parasite burden. Pons-Bordas et al. ( 2020 ) reported a significant increase in ulcerative lesions with Anisakis infection in common dolphins from the North-East Atlantic (Galician coast) when comparing data collected in the early 1990s to 2017–2018. The authors proposed that common dolphins could be particularly vulnerable to environmental stress, making them more susceptible to Anisakis infections, or that dolphins have encountered increased instances of Anisakis infections in recent years. They also reported that 86% of common dolphins had ulcers (Pons-Bordas et al., 2020 ), similar to the present study with 90% of common dolphins showing signs of gastric ulcers. To examine the relationship between ulceration and parasite burden Buhrmann et al. ( 2023 ) described a grading scale to diagnose gastric ulceration in bottlenose dolphins and future studies might consider evaluating the surface area of the stomach covered by ulcers as a more quantitative measure of ulceration. On examining the biomass scaling relationship, a slope of 1.57 slope in the log-log linear regression between parasite biomass and host biomass is twice that expected if parasites act in the same way as host tissues. As previously mentioned metabolic theory suggests a slope of 0.75 for the rate at which host resources are converted into host growth (Brown et al., 2004 ; Poulin and George-Nascimento, 2007 ), and so it appears that the anisakid nematodes occurring in the common dolphin stomachs examined here can acquire more host resources than expected for host growth rates. Conclusion The interaction between parasites and common dolphins is complex and may be influenced by variations in dolphin health, immunity, and the presence of other pathogens or disease processes, as well as a range of environmental factors including parasite infections in prey. Here we examined stomach parasite burden in common dolphins in relation to basic biological data gathered during necropsy. In the current analysis, year of stranding, and dolphin nutritional status and body length were found to be significantly associated with Anisakis burden while no variation was found between dolphin sex. Overall higher parasite prevalence was observed compared to previous studies although within the three-year dataset a trend towards decreasing parasite prevalence was observed which warrants ongoing studies. To further examine and assess the influence of these parasites on dolphin health future studies could involve investigations of potential interactions with co-occurring pathogens, the extent and type of tissue reaction and inflammation within organs, and detailed long-term temporal data on prey consumption and Anisakis prevalence in prey populations. Declarations Funding SSA was funded by an Erasmus Mundus scholarship within the International Master of Science in Marine Biological Resources. This research was part-funded through the Irish Research Council postgraduate fellowship (GOIPG/2021/378) awarded to SA. Dolphins were collected for post-mortem under the Marine Institute-EMFF funded Irish Vertebrate Necropsy Project (2017-2019), awarded to the Irish Whale and Dolphin Group in collaboration with the Regional Veterinary Laboratory, Cork and the Atlantic Technological University (Tender Reference Numbers: ITT17-024, ITT18-005, ITT18-050). Conflict of interest The authors declare no conflicts of interest. Ethical Approval All dolphins were recovered dead and necropsies were carried out at the Regional Veterinary Laboratory in Cork following best practice guidelines. Data Availability The authors do not have permission to share data. Author Contributions SSA, SM and KOD contributed to the study conception and design. SSA, SA, JOD, SB, MD and SL collected data. SSA and KOD analysed data. SSA produced the first draft of the manuscript and all authors commented on the manuscript. All authors read and approved the final manuscript. Acknowledgement The authors wish to thank volunteers of the Irish Whale and Dolphin Group for dolphin collection and assisting in necropsies. Special thanks to Atlantic Technological University (ATU) placement students for their assistance: Georgia Novak and Morena Gaudino with stomach dissections, Rebecca Kinsella and Ryan Lobley with parasite counting, and Orla Gosnell with database organisation. Thanks to Ailbhe Kavanagh at the Marine Institute for providing access to the stomach samples, and to ATU laboratory technicians Mary Veldon and John Kennedy for their assistance. References Aguirre AA, Tabor GM (2004) Introduction: Marine vertebrates as sentinels of marine ecosystem health. EcoHealth 1(3):236–238. https://doi.org/10.1007/s10393-004-0091-9 Ángeles-Hernández JC, Gómez-de Anda FR, Reyes-Rodríguez NE, Vega-Sánchez V, García-Reyna PB, Campos-Montiel RG, Calderón-Apodaca NL, Salgado- Miranda C, Zepeda-Velázquez AP (2020) Genera and species of the anisakidae family and their geographical distribution. Animals 10(12). https://doi.org/10.3390/ani10122374 Atkinson S, Dierauf L (2018) Stress and marine mammals. 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Int J Comp Psychol 20(2–3):274–316. https://doi.org/10.46867/ijcp.2007.20.02.01 Zuur AF, Ieno EN, Elphick CS (2010) A protocol for data exploration to avoid common statistical problems. Methods Ecol Evol 1:3–14. https://doi.org/10.1111/j.2041-210x.2009.00001.x Supplementary Files SupplementaryMaterial.docx Cite Share Download PDF Status: Published Journal Publication published 03 Apr, 2025 Read the published version in Marine Biology → Version 1 posted Editorial decision: Revise and Resubmit 28 Sep, 2024 Reviewers agreed at journal 19 Aug, 2024 Reviewers invited by journal 15 Aug, 2024 Editor assigned by journal 13 Aug, 2024 First submitted to journal 13 Aug, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4906242","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":340694519,"identity":"e3a0de21-41d0-44b0-a9bd-5be33fdd646e","order_by":0,"name":"H.A.S.S. Alwis","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABEUlEQVRIiWNgGAWjYPACZh4GdgaGAwwV/+RA3AMPiNLCDNJy5oAxWEsCEVrAiIGx7UBiA4iPT4t8e4+ZxM8d1jL8zTyGB3+23UmfH3b4IdAWOzndBuxaDM6cMZPsPZPOI3GYx+Awz7lnuRtvpxkAtSQbmx3AoUUid9sN3rbDPAyHeTccZihjzt04OwGk5UDiNhxa5Oe/3XbzL1CLPFDLwR9szOmGs9M/4NXCcIN3222QLQZALQd42g4nyEvn4LfF4Ez+99+ybek8hof5PxzmOZNmuEE6p+BAggFuv8i3H0s2fNtmbS93vC35448KG3n52embP3yosJPDpQWLvWCVBsQqB9vbQIrqUTAKRsEoGAkAAPxpaIYqhxK8AAAAAElFTkSuQmCC","orcid":"https://orcid.org/0009-0000-9760-6030","institution":"Atlantic Technological University","correspondingAuthor":true,"prefix":"","firstName":"H.A.S.S.","middleName":"","lastName":"Alwis","suffix":""},{"id":340694520,"identity":"f7097b0c-335c-4cb1-9c7a-c2789fa8ee4f","order_by":1,"name":"Sofia Albrecht","email":"","orcid":"","institution":"Atlantic Technological University","correspondingAuthor":false,"prefix":"","firstName":"Sofia","middleName":"","lastName":"Albrecht","suffix":""},{"id":340694521,"identity":"48ca8fad-5c3f-47e2-af6a-581f123b7dfe","order_by":2,"name":"Sinéad Murphy","email":"","orcid":"","institution":"Atlantic Technological University","correspondingAuthor":false,"prefix":"","firstName":"Sinéad","middleName":"","lastName":"Murphy","suffix":""},{"id":340694522,"identity":"f71d7170-bcb5-4cbe-8052-02393d8d824c","order_by":3,"name":"Jim O'Donovan","email":"","orcid":"","institution":"Department of Agriculture Fisheries and Food: Government of Ireland Department of Agriculture Food and the Marine","correspondingAuthor":false,"prefix":"","firstName":"Jim","middleName":"","lastName":"O'Donovan","suffix":""},{"id":340694523,"identity":"a6fc7ce4-f843-463d-b610-daba0d8ccdea","order_by":4,"name":"Simon Berrow","email":"","orcid":"","institution":"Irish Whale and Dolphin Group, Kilrush, Ireland","correspondingAuthor":false,"prefix":"","firstName":"Simon","middleName":"","lastName":"Berrow","suffix":""},{"id":340694524,"identity":"a930184c-0123-4858-80de-7657dbebe390","order_by":5,"name":"Mags Daly","email":"","orcid":"","institution":"Irish Whale and Dolphin Group, Kilrush, Ireland","correspondingAuthor":false,"prefix":"","firstName":"Mags","middleName":"","lastName":"Daly","suffix":""},{"id":340694525,"identity":"ba00e9a3-bb85-46c7-a109-def9631f1750","order_by":6,"name":"Stephanie Levesque","email":"","orcid":"","institution":"Irish Whale and Dolphin Group, Kilrush, Ireland","correspondingAuthor":false,"prefix":"","firstName":"Stephanie","middleName":"","lastName":"Levesque","suffix":""},{"id":340694526,"identity":"dd831909-ea3c-4cd5-91ff-8434c2d7a65a","order_by":7,"name":"Katie O'Dwyer","email":"","orcid":"","institution":"Atlantic Technological University","correspondingAuthor":false,"prefix":"","firstName":"Katie","middleName":"","lastName":"O'Dwyer","suffix":""}],"badges":[],"createdAt":"2024-08-13 10:15:39","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4906242/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4906242/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s00227-025-04620-9","type":"published","date":"2025-04-03T15:57:21+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":64328490,"identity":"27a9e6f3-7e9c-4770-86b4-ff1a38b4bfbf","added_by":"auto","created_at":"2024-09-11 17:31:19","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":61132,"visible":true,"origin":"","legend":"\u003cp\u003eStranding locations of common dolphins sampled for the current study (2017- 2019)\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4906242/v1/ca6a4284129ca77672928a39.jpg"},{"id":64328491,"identity":"aee25629-13b7-4473-a120-3ad216cdf51c","added_by":"auto","created_at":"2024-09-11 17:31:19","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":93600,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eAnisakis \u003c/em\u003espp. in the forestomach of a common dolphin (IWDG code: 2017-233; length: 205 cm; total parasite burden estimation: 20,962)\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4906242/v1/88b7f602f6fb683e05d4bf55.jpg"},{"id":64328710,"identity":"c30f1b2d-67c3-48b1-bc7f-c7df3214a48e","added_by":"auto","created_at":"2024-09-11 17:39:19","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":80440,"visible":true,"origin":"","legend":"\u003cp\u003eDifferent body parts of adult \u003cem\u003eAnisakis\u003c/em\u003espp. \u003cstrong\u003ea\u003c/strong\u003e Posterior end of female and male with the spicule (S); \u003cstrong\u003eb\u003c/strong\u003e Male posterior end with spicule (S) and unfertilised ova in female (UO); \u003cstrong\u003ec\u003c/strong\u003e Male posterior end with the cuticle (C), intestine (I), rectal gland (RG), rectum (R), and spicule (S); \u003cstrong\u003ed\u003c/strong\u003e Female body showing ova (O) and vulva (V); \u003cstrong\u003ee\u003c/strong\u003eMale body showing zigzag tubiform testis (T); \u003cstrong\u003ef\u003c/strong\u003e Female body showing sinuous ovary ducts (OD)\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4906242/v1/5e6be47a92ea64dda9eb2bc5.jpg"},{"id":64328492,"identity":"d295c61a-58a2-4b1f-af6e-c8cdb781cfdb","added_by":"auto","created_at":"2024-09-11 17:31:19","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":26649,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea\u003c/strong\u003e Boxplot displaying the number of parasites and year of stranding of common dolphins (2017 n= 20, 2018 n= 25, 2019 n =25). \u003cstrong\u003eb\u003c/strong\u003e Boxplot displaying the distribution of the number of parasites among different nutritional states of common dolphins (Poor n= 30, Moderate n =31, Good n=8). The box represents the interquartile range (IQR), with the horizontal line inside the box indicating the median. The whiskers extend to the minimum and maximum values within 1.5 times the IQR. The boxplots exclude the two previously detailed outliers\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4906242/v1/0535c76fbbb84d90f8b44cd7.jpg"},{"id":64328493,"identity":"c3d9682d-88b5-459a-a1a4-ba1c341b7429","added_by":"auto","created_at":"2024-09-11 17:31:19","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":24108,"visible":true,"origin":"","legend":"\u003cp\u003eRelationship between temperature-corrected average parasite biomass (in g) and host biomass (in kg). The trend line is depicted in red and the green shadow represents the 95% confidence interval. Pearson coefficient and p-value is shown in the graph\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4906242/v1/c32e86c643bf046f4a795331.jpg"},{"id":64328494,"identity":"d83e6f2d-c511-4ac5-8af6-fca2b5e7d56c","added_by":"auto","created_at":"2024-09-11 17:31:19","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":31919,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea\u003c/strong\u003e Stacked bar plot showing the number of dolphins assigned to different cause of death categories between 2017 – 2019 (Levesque et al., 2021). \u003cstrong\u003eb\u003c/strong\u003e Boxplot showing the parasite burden of dolphins within each of the four assigned causes of death. The box represents the interquartile range (IQR), with the horizontal line inside the box indicating the median. The whiskers extend to the minimum and maximum values within 1.5 times the IQR. The boxplots exclude the two previously detailed outliers\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4906242/v1/ee02f8a7edcd7914d49d910d.jpg"},{"id":80082008,"identity":"347c2fe8-9c01-46fc-923c-de8dca422381","added_by":"auto","created_at":"2025-04-07 16:05:36","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":990493,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4906242/v1/aa97e9ae-0ad9-469e-ba21-1c13cdf7c3f3.pdf"},{"id":64328496,"identity":"d52d0ffc-f075-4cbf-addb-abd11eb8051e","added_by":"auto","created_at":"2024-09-11 17:31:19","extension":"docx","order_by":10,"title":"","display":"","copyAsset":false,"role":"supplement","size":21534,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryMaterial.docx","url":"https://assets-eu.researchsquare.com/files/rs-4906242/v1/07bdeffc7ca005054ddf6877.docx"}],"financialInterests":"","formattedTitle":"\u003cp\u003eStomach parasite burden and health status of stranded common dolphins, \u003cem\u003eDelphinus delphis\u003c/em\u003e\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eParasites are an integral part of natural ecosystems, influencing ecosystem functioning and enhancing biodiversity, while occurring in virtually all free-living organisms (Marcogliese, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). However, parasites can be a natural stressor on their hosts as they display an obligate physical association to obtain critical resources (Stewart and Schnitzer, \u003cspan citationid=\"CR85\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Infection with parasites can have substantial effects on the health of host organisms, by causing diseases, impairing growth and reproduction, and even leading to mortality (Dailey and Walker, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e1978\u003c/span\u003e; Geraci et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e1978\u003c/span\u003e; Bressem et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e1998\u003c/span\u003e; Fauquier et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). Their impacts extend to population levels influencing population dynamics and community structure (Geraci et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e1978\u003c/span\u003e; Terracciano et al., \u003cspan citationid=\"CR89\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) demonstrating the important ecological role they occupy.\u003c/p\u003e \u003cp\u003eThe transmission of parasites from intermediate hosts to definitive hosts, such as marine mammals, typically occurs trophically through the food web (Luque and Poulin, \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). Marine mammals occupy important ecological roles as top predators in marine ecosystems and thereby can affect ecosystem function and structure (Nelms et al., \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). These animals are considered sentinel species or indicators of their environs, and can provide early warnings of potential changes to their ecosystem (Aguirre and Tabor, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Bossart, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Furthermore, marine mammals can be used as indicators of human health (O\u0026rsquo;Brien et al., 1993).\u003c/p\u003e \u003cp\u003eUp to certain thresholds in parasite burden, the consequences of parasitic infections may manifest as localised reactions within the host, rather than systemic reactions. For instance, the embedding of the proboscis of the acanthocephalan \u003cem\u003eBolbosoma\u003c/em\u003e into the intestinal wall in the fin whale, \u003cem\u003eBalaenoptera physalus\u003c/em\u003e, leads to localised reactions in the host (Santoro et al., \u003cspan citationid=\"CR79\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). These local reactions are unlikely to have a substantial impact on the overall health and survival of the host, enabling some levels of tolerance. However, more severe reactions, such as granulomas, ulcers and haemorrhages in response to parasites, such as the nematodes \u003cem\u003eAnisakis\u003c/em\u003e and \u003cem\u003ePseudoterranova\u003c/em\u003e, can have greater effects on host health (Jaber et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Di Azevedo et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Pons-Bordas et al., \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). In extreme cases, severe infections can cause debilitation and death (Geraci and Aubin, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e1987\u003c/span\u003e; Stroud and Roffe, \u003cspan citationid=\"CR86\" class=\"CitationRef\"\u003e1979\u003c/span\u003e; Terracciano et al., \u003cspan citationid=\"CR89\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). For instance, parasitosis, such as cerebral nasitremosis caused by trematodes in the genus \u003cem\u003eNasitrema\u003c/em\u003e may lead to inflammation and death in brain tissue and has resulted in strandings of striped dolphins (\u003cem\u003eStenella coeruleoalba\u003c/em\u003e) (Dailey and Walker, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e1978\u003c/span\u003e; O\u0026rsquo;Shea et al., \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e1991\u003c/span\u003e). Some studies have reported the influence of parasites on marine mammal population dynamics with parasites affecting the survival and reproduction of the host. It is suggested that transplacental lungworm infections in stillborn bottlenose dolphins (\u003cem\u003eTursiops truncatus\u003c/em\u003e) leading to increased neonatal mortality could affect recruitment (Fauquier et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2009\u003c/span\u003e), although the effect of such infections on neonatal survival has yet to be fully discovered. Geraci et al. (\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e1978\u003c/span\u003e) reported that mastitis in Atlantic white-sided dolphins (\u003cem\u003eLagenorhynchus acutus\u003c/em\u003e), caused by the nematode \u003cem\u003eCrassicauda grampicola\u003c/em\u003e, affected the quality and quantity of milk production, compromising the reproductive success of the pod studied.\u003c/p\u003e \u003cp\u003eShort-beaked common dolphins, \u003cem\u003eDelphinus delphis\u003c/em\u003e, have a global distribution encompassing temperate and tropical waters. (Perrin, \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). They are one of the most abundant cetacean species in the North-East Atlantic, with one population reported to date in the region, ranging from waters off Macaronesia and North-West Africa to Norway (Murphy et al., \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). A recent abundance estimate reported 439,212 dolphins (coefficient of variation (CV)\u0026thinsp;=\u0026thinsp;0.18; 95% confidence interval (CI) 309,153\u0026ndash;623,987) for the European continental shelf and adjacent waters, excluding the Irish EEZ, for the summer of 2022 (Gilles et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Within Irish waters, common dolphins are present year-round, and are the most frequently reported species stranded on coastlines (McGovern et al., \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Levesque and Berrow, \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2022\u003c/span\u003e, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe species faces numerous anthropogenic stressors across the North-East Atlantic, including by-catch in fisheries, persistent pollutants, underwater noise, marine debris and wildlife tourism (Atkinson and Dierauf, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Lusher et al., \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Murphy et al., \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Among natural stressors are competition for resources, environmental changes, including shifts in ocean temperatures resulting in changes in prey availability and/or quality, and alterations in habitat conditions, parasites and disease, and predator attacks, all of which can impose stressful conditions with sublethal or lethal consequences (Murphy et al., \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Marcogliese and Pietrock, \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Murphy et al., \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Atkinson and Dierauf, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Murphy et al., \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Methion and L\u0026oacute;pez, \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). The severity of parasitic infections in marine mammals is dependent on factors involved in the host-parasite interaction such as the host species, health status of the host, type(s) of parasite, parasite burden, and co-occurrence of other anthropogenic or natural stressors (Bull et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Raga et al., \u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Cable et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2017\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eAmong the parasites affecting common dolphins, various microparasites have been documented. For instance, the bacteria \u003cem\u003eHelicobacter\u003c/em\u003e spp., associated with gastritis and gastric ulcers, can impact their digestive system (Harper et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2000\u003c/span\u003e). Another bacterium, \u003cem\u003eStreptococcus iniae\u003c/em\u003e, has been linked to skin abscesses and mortality in juvenile common dolphins, emphasising the potential severity of bacterial infections (Souter et al., \u003cspan citationid=\"CR83\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Herpesvirus causes genital lesions, impacting the reproductive health of dolphins (Bento et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2019\u003c/span\u003e), while dolphin morbillivirus is associated with neurological and respiratory symptoms in dolphins, affecting their overall health (Bressem et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e1998\u003c/span\u003e). Parasitic protozoans, such as \u003cem\u003eToxoplasma gondii\u003c/em\u003e, \u003cem\u003eGiardia\u003c/em\u003e spp. and \u003cem\u003eCryptosporidium\u003c/em\u003e spp. have been reported among common dolphins, although their specific effects are yet to be fully understood (Forman et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Reboredo- Fern\u0026aacute;ndez et al., 2014).\u003c/p\u003e \u003cp\u003eMacroparasite infections have also been reported in common dolphins leading to morbidity and in some cases, mortality. Adult gastrointestinal nematodes, particularly \u003cem\u003eAnisakis simplex\u003c/em\u003e (s.s.) and \u003cem\u003eA. pegreffii\u003c/em\u003e, typically attach to the mucosa of the dolphin stomach, leading to the development of granulomatous ulcers, haemorrhages, and perforations of the stomach lining, which can result in peritonitis (Jaber et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Motta et al., \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Other nematodes such as \u003cem\u003eHalocercus lagenorhynchi\u003c/em\u003e can cause lung lesions, and high burdens can impair lung function (Tomo et al., \u003cspan citationid=\"CR90\" class=\"CitationRef\"\u003e2010\u003c/span\u003e) while nematodes of the genus \u003cem\u003eCrassicauda\u003c/em\u003e are associated with verminous prostatitis, causing inflammation of the prostate gland (Su\u0026aacute;rez-Santana et al., \u003cspan citationid=\"CR87\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). The trematode \u003cem\u003eCampulla palliata\u003c/em\u003e can cause severe inflammation and damage to the bile ducts and liver tissue (Cordes and O\u0026rsquo;Hara, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e1979\u003c/span\u003e), while another trematode, \u003cem\u003ePholeter gastrophilus\u003c/em\u003e, is associated with granulomatous gastritis, characterised by the formation of nodular granulomas in the stomach lining (Jaber et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2006\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eTo better understand the pressures faced by marine mammals, it is necessary to gain more information on their parasite burdens and the factors affecting infection levels. Here we examine parasite burden and infection intensity in the stomachs of common dolphins that stranded along the Irish coastline. We investigate the relationships between parasite infection and dolphin sex, total body length, nutritional status, and year of stranding. We also assess the presence of granulomas on the stomach lining in relation to parasite infection intensity.\u003c/p\u003e"},{"header":"Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eSample acquisition\u003c/h2\u003e \u003cp\u003eStranded common dolphins, \u003cem\u003eDelphinus delphis\u003c/em\u003e, were collected from the Irish coastline between 2017 and 2019 (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Dolphin carcasses were subsequently necropsied at the Regional Veterinary Laboratory located in Cork, following established post-mortem procedures (see Levesque et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2021\u003c/span\u003e for further information). Information on date and location of stranding, sex and total body length (cm) were recorded (Levesque et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). The decomposition status of each carcass was categorised as: extremely fresh, slight decomposition, moderate decomposition and advanced decomposition (Levesque et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eUpon necropsy, the nutritional status of each individual was assessed by viewing the flanks around the dorsal fin and assessing whether they exhibited a convex, flat or concave profile. Nutritional status was categorised as good, moderate, or poor (Levesque et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2021\u003c/span\u003e) and stomachs were then frozen at -20℃ until further analysis. For the present study focusing on parasite burdens in common dolphin stomachs, the presence or absence of stomach ulcers was also recorded.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003eRecovery and identification of gastrointestinal parasites\u003c/h2\u003e \u003cp\u003eThawed stomachs were dissected, washed into a tray using 99% ethanol and all parasites were recovered using forceps. Parasites were collected from all three stomach compartments (forestomach, glandular stomach and pyloric stomach) (Harrison et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e1970\u003c/span\u003e) and preserved in 70% ethanol. All parasites recovered were nematodes, and individuals were identified morphologically as larval stages (L3, L4s) or adults (Davey, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1971\u003c/span\u003e; Grabda, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e1976\u003c/span\u003e) using light microscopy, focusing mainly on size differences. Representatives of the three different life stages were cleared in lactophenol and examined using a light microscope (Olympus BX51) to identify sex. Sexually mature adults were identified based on morphological features (Grabda, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e1976\u003c/span\u003e; Gomes et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eTo assess the parasite burden, parasites were individually counted. However, due to heavy infections in 24 individuals (35% of dolphin samples), subsampling was required, with subsequent estimation of parasites. For these individuals, parasite samples were weighed (Adventurer SL Precision balance) (ethanol wet weight) and three subsamples accounting for 10% of the total wet weight, were isolated and individual parasites were counted (Ugland et al., \u003cspan citationid=\"CR91\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). Total parasite burden per dolphin stomach (N) was calculated following the equation below for each subsample, and the average N of the three subsamples was calculated and used as the parasite burden measure for these 24 dolphin individuals.\u003c/p\u003e \u003cp\u003eN = (W/w)*n\u003c/p\u003e \u003cp\u003eWhere,\u003c/p\u003e \u003cp\u003eW\u0026thinsp;=\u0026thinsp;Wet weight of the parasites in the full stomach sample\u003c/p\u003e \u003cp\u003ew\u0026thinsp;=\u0026thinsp;Wet weight of the parasites in each subsample\u003c/p\u003e \u003cp\u003en\u0026thinsp;=\u0026thinsp;Number of parasites in each subsample\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003eData analysis\u003c/h2\u003e \u003cp\u003ePrevalence, mean abundance and mean intensity of parasite burdens were calculated using Web-based Quantitative Parasitology (QPweb) software (Reiczigel et al., \u003cspan citationid=\"CR78\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Prevalence refers to the proportion of hosts infected with a specific parasite (Bush et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e1997\u003c/span\u003e), whereas mean abundance refers to the average number of individuals of a specific parasite per host individual including uninfected hosts (Bush et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e1997\u003c/span\u003e). Mean intensity refers to the average number of individuals of a particular parasite per infected host (Bush et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e1997\u003c/span\u003e). The 95% confidence interval (CI) for prevalence was presented as Clopper-Pearson intervals and for mean abundance and mean intensity, CIs were based on 10,000 bootstrap replications (BCa method).\u003c/p\u003e \u003cp\u003eAll statistical analyses were performed using R software for statistical computing (version 4.3.1) (R Core Team, \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) in Posit (previously RStudio) IDE (version 2023.06.1\u0026thinsp;+\u0026thinsp;524) (Posit Team, \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Initial data exploration was carried out to identify the spread and distribution of data and outliers, as well as the collinearity of predictor variables (Zuur et al., \u003cspan citationid=\"CR94\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). Two extreme values for parasite burden were identified as outliers and removed from further analysis. A generalised linear model (GLM) was constructed using parasite burden as the response variable, with dolphin sex, length, and nutritional status, and year of stranding as predictors. Body length data were centered prior to the analysis so that the intercept is representative of a common dolphin of average length (Ten Doeschate et al., \u003cspan citationid=\"CR88\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Average body length was calculated to be 190.25 cm based on the 69 individuals for which length was recorded. Since parasite burden was highly skewed, non-continuous, and the variance (36,779,883) was greater than the mean (4264.11), standard linear regression models were not suitable for these over-dispersed count data. Parasites are characteristically aggregated across their host populations, and burden generally follows a negative binomial distribution (Grenfell et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e1995\u003c/span\u003e; Shaw et al., \u003cspan citationid=\"CR82\" class=\"CitationRef\"\u003e1998\u003c/span\u003e), hence, assuming the negative binomial error structure, the glm.nb function from the R \u003cem\u003eMASS\u003c/em\u003e package (Venables and Ripley, \u003cspan citationid=\"CR92\" class=\"CitationRef\"\u003e2002\u003c/span\u003e), was used. To assess the potential influence of parasite burden on the presence of gastric ulcers a Wilcoxon two-sample test was carried out.\u003c/p\u003e \u003cp\u003eParasite biomass was also calculated as a measure of the parasite burden in common dolphins and a generalised linear model (GLM) was constructed using parasite biomass as the response variable, with common dolphin sex, length, and nutritional status, and year of stranding as predictors (Supplementary Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). To explore the biomass scaling relationship between parasite biomass and host biomass a log-log linear regression was carried out (Brown et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Poulin and George -Nascimento, 2007). The rate of conversion of resources into growth for eukaryotic organisms is predicted to have a slope of 0.75 on a log-log biomass plot when corrected for operating temperature (Brown et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). If host resources are converted into parasite biomass similar to host growth, we would expect a slope of 0.75 on a log-log plot of parasite biomass and host mass (Poulin and George-Nascimento, \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). If the slope is smaller host resources are not converted into parasite biomass as efficiently as they are for host growth, while if the slope is larger host resources are converted into parasite biomass at a higher rate than for host growth.\u003c/p\u003e \u003c/div\u003e"},{"header":"Results","content":"\u003cp\u003eOf the 69 common dolphins collected between 2017 to 2019, 32 were female and 37 were male. Regarding decomposition state 36.2% of dolphin carcasses were extremely fresh, 36.2% were slightly decomposed, 27.5% were moderately decomposed and no dolphins of an advanced decomposed state were reported. Of the 69 dolphin stomachs analysed, parasites were recovered from 65 and together contained an estimated total of 319,344 anisakid nematodes, of the genus \u003cem\u003eAnisakis\u003c/em\u003e, with burden per dolphin ranging from 2 to 24,892 parasites (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). No other parasites were recorded within the stomachs. Sexually mature adults were identified by the presence of a clearly visible zigzag tubiform testis, ventrally bent tail, spicules near the anus in males and sinuous ovary ducts, uteri, and ova in the uteri in females where larval forms had no obvious sexual features (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e; Grabda, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e1976\u003c/span\u003e; Gomes et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn addition to adult nematodes, both L3 and L4 larval stages of \u003cem\u003eAnisakis\u003c/em\u003e spp. were observed. \u003cem\u003eAnisakis\u003c/em\u003e prevalence was 94% (95% confidence interval (CI): 85\u0026ndash;98%), mean abundance was 4,630 worms (95% CI: 3,330 to 6,280), and mean intensity was 4,910 worms (95% CI: 3,520 to 6,640).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eAnalysis of dolphin health and parasite burden\u003c/h2\u003e \u003cp\u003eThe results showed that dolphin nutritional status and total body length, as well as year of stranding, were significantly associated with parasite burden (model output shown in Supplementary Table S2). No significant interactions were detected. Lower parasite burdens were observed in common dolphins that stranded in 2019 compared to those from 2017 (z= -2.76, p\u0026thinsp;=\u0026thinsp;0.006; Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea), while no significant difference was found between 2017 and 2018 (z= -0.80, p\u0026thinsp;=\u0026thinsp;0.422; Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). The intercept was changed, and the model was re-run to ensure all comparisons were obtained. A significant difference was found between 2019 and 2018 (z =-1.98, p\u0026thinsp;=\u0026thinsp;0.047). Parasite burdens differed between dolphin nutritional states, with significantly higher parasite burdens observed in individuals in poor (z\u0026thinsp;=\u0026thinsp;2.62, p\u0026thinsp;=\u0026thinsp;0.009) and moderate nutritional status (z\u0026thinsp;=\u0026thinsp;2.34, p\u0026thinsp;=\u0026thinsp;0.019) compared to good nutritional status (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe model revealed a significant positive association between dolphin length and parasite burden (z\u0026thinsp;=\u0026thinsp;3.78, p\u0026thinsp;=\u0026thinsp;\u0026lt;\u0026thinsp;0.001) (Supplementary Table S2) where longer dolphins harboured a higher number of parasites compared to smaller dolphins. There was no significant difference in \u003cem\u003eAnisakis\u003c/em\u003e burden between male and female common dolphins (z= -1.20, p\u0026thinsp;=\u0026thinsp;0.230; mean abundance: males 4,610, females 4,650). Overall, \u003cem\u003eAnisakis\u003c/em\u003e prevalence in males (97.3%) was slightly higher than that for females (90.6%), whereas mean parasite intensity was slightly higher for females (5,130) than for males (4,740) but neither were significant. Ulcers were identified in 89.9% of common dolphins and no significant association between the presence of ulcers and parasite burden was found (W\u0026thinsp;=\u0026thinsp;190.5, p\u0026thinsp;=\u0026thinsp;0.605).\u003c/p\u003e \u003cp\u003eParasite biomass and parasite burden were highly positively correlated (r\u0026thinsp;=\u0026thinsp;0.79, p\u0026thinsp;\u003cem\u003e\u0026lt;\u003c/em\u003e\u0026thinsp;0.001) while parasite biomass and host body weight were moderately positively correlated (r\u0026thinsp;=\u0026thinsp;0.2, p\u0026thinsp;\u003cem\u003e\u0026lt;\u003c/em\u003e\u0026thinsp;0.056). The results of the GLM analysis on parasite biomass in relation to common dolphin sex, nutritional status, and length, and year of stranding showed that only nutritional status and body length were significantly associated with parasite biomass (Supplementary Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). Significantly higher parasite burdens were observed in individuals in poor nutritional status compared to good nutritional status (t\u0026thinsp;=\u0026thinsp;2.73, p\u0026thinsp;=\u0026thinsp;0.008) and a significant positive association between dolphin length and parasite biomass was observed where longer individuals harboured a higher parasite biomass compared to smaller dolphins (t\u0026thinsp;=\u0026thinsp;3.19, p\u0026thinsp;=\u0026thinsp;\u0026lt;\u0026thinsp;0.002).\u003c/p\u003e \u003cp\u003eOn investigating the scaling of total parasite biomass with host body mass the log-log linear regression produced a slope of 1.57 (95% CI: 0.77\u0026ndash;2.36) with considerable variation observed in the data. (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e)\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eIn the current study, 69 common dolphins were examined for gastrointestinal macroparasite infections. Of these, 65 were infected with gastrointestinal nematodes, \u003cem\u003eAnisakis\u003c/em\u003e spp., identified morphologically to genus level. As morphology alone is not sufficient for species identification, (\u0026Aacute;ngeles- Hern\u0026aacute;ndez et al., 2020), follow up molecular work is required (Mattiucci et al., \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Only two species of \u003cem\u003eAnisakis\u003c/em\u003e have been reported in common dolphins thus far, \u003cem\u003eA. simplex\u003c/em\u003e (s.s.) and \u003cem\u003eA. pegreffii\u003c/em\u003e (Mattiucci et al., \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Pons-Bordas et al., \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), both of which exhibit different geographical distributions (Mattiucci et al., \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). \u003cem\u003eAnisakis simplex\u003c/em\u003e (s.s.) has a widespread distribution, between 35 \u0026deg;N and the Arctic circle, while \u003cem\u003eA. pegreffii\u003c/em\u003e is mainly found between 30 \u0026deg;N and 55\u0026deg;S (Mattiucci et al., \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e1997\u003c/span\u003e). Based on this, and despite the potential for some overlap in \u003cem\u003eAnisakis\u003c/em\u003e spp. distributions (Mattiucci et al., \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2008\u003c/span\u003e), it is likely that, given the sampling region, the anisakids recovered in this study were \u003cem\u003eA\u003c/em\u003e. \u003cem\u003esimplex\u003c/em\u003e (s.s.). In addition, in a metabarcoding study examining the diet of the same common dolphins examined here \u003cem\u003eA. simplex\u003c/em\u003e was detected in the genetic sequences obtained (unpubl data).\u003c/p\u003e \u003cp\u003eCompared to previous studies, a higher prevalence (94%, 65/69) and higher mean abundance (4,630 worms) of \u003cem\u003eAnisakis\u003c/em\u003e spp. was observed in common dolphins sampled in the current study. Gibson et al. (\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e1998\u003c/span\u003e) reported a prevalence of 71.3% (72/101, 1990\u0026ndash;1994) for common dolphins stranded in England and Wales, while Moloney (\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e1994\u003c/span\u003e) reported a lower prevalence of 32% (6/19) and mean abundance of 498 worms for individuals stranded in Ireland during a similar three-year time period (1991\u0026ndash;1993). All studies sampled stranded dolphins, which in part are comprised of animals in both poor health and also animals that died from acute anthropogenic interactions, i.e. fisheries interactions, and because stranding events are often biased towards sick or injured individuals (Coombs et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2019\u003c/span\u003e), the observed prevalence and burden of parasites may not be representative of the entire population. Nonetheless, differences in \u003cem\u003eAnisakis\u003c/em\u003e spp. prevalence and burden between these studies of common dolphin may also be attributed to either a larger sample size within the current study compared to Moloney (\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e1994\u003c/span\u003e), and/or an actual increase in dolphin parasite burden within the region. Within the current analysis, year of stranding, and dolphin nutritional status and total body length were found to be significantly associated with \u003cem\u003eAnisakis\u003c/em\u003e burden. While parasite prevalence and burden in this study appears higher in comparison to earlier studies, within our study a declining trend in parasite burden and prevalence was observed between 2017 and 2019. Upon examining the cause of death assigned to the dolphins in this study an increase was observed in the number of trauma cases in 2019 and 2018, compared to 2017 (see data presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea as reported in Levesque et al. (\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). The parasite burden in dolphins that died from trauma was low in comparison to other causes of death (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb), although the values reported from dolphins that died of starvation were the lowest. It is possible that the decrease in parasite prevalence and burden values observed over the three years is a result of increased numbers of dolphins in the dataset that died from trauma, and which tended to have relatively low parasite burden (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb). Future marine mammal necropsy studies could include investigations of parasite burden in relation to cause of death to further identify any possible patterns.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe life cycle of \u003cem\u003eAnisakis\u003c/em\u003e spp. can involve various invertebrates as first intermediate host, larger invertebrates such as copepods and euphausiids, as well as smaller fish, as second intermediate hosts, predatory fish and cephalopods as transport or paratenic hosts (K\u0026oslash;ie, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2001\u003c/span\u003e), and cetaceans as definitive hosts (Mattiucci, 2018). A recent meta-analysis reported a significant increase of \u003cem\u003eAnisakis\u003c/em\u003e spp. between 1978\u0026ndash;2015 in the prey species of cetaceans (Mastick et al., \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2024\u003c/span\u003e) suggesting an increasing risk of \u003cem\u003eAnisakis\u003c/em\u003e spp. infection for cetacean hosts. Increasing parasite burden of \u003cem\u003eAnisakis\u003c/em\u003e spp. have also been observed in marine fish and invertebrates, with a 283-fold increase reported over a 53-year period between 1962 to 2015 (Fiorenza et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The authors suggest that these increases could be due to increases in definitive host populations, resulting from enhanced protection measures for marine mammals, providing an increasing host population to support larger \u003cem\u003eAnisakis\u003c/em\u003e populations (Fiorenza et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Furthermore, they propose climate change may increase the susceptibility of the hosts to parasitic diseases and that rising temperature can also expedite growth and shorten generation times in aquatic parasites (Fiorenza et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Studies on the diet of common dolphins in the North-East Atlantic have shown they prey on a diverse range of fish and squid species (Pusineri et al., \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Brophy et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2009\u003c/span\u003e) which are known to harbour \u003cem\u003eAnisakis\u003c/em\u003e (Mattiucci et al., \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Pierce et al., \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Cipriani et al., 2022), and as outlined here may be increasing. Taken together these factors may be contributing to the overall observed increase in parasite burden in the stomachs of common dolphins examined here. To gain a better understanding of the underlying mechanisms that are driving temporal variation in parasite prevalence and burden in common dolphins, detailed long-term temporal data on prey consumption and \u003cem\u003eAnisakis\u003c/em\u003e prevalence in prey populations are required.\u003c/p\u003e \u003cp\u003eThe role of host condition and nutritional state in mediating parasite burden is emphasised by the significant association between nutritional status and parasite burden. Common dolphins in poor nutritional condition generally harboured a higher number of parasites, as did dolphins with moderate nutritional status, when compared to individuals in good nutritional status. It has been reported that gastrointestinal helminth infections negatively affect body condition of mammals (Shanebeck et al., \u003cspan citationid=\"CR81\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) and better nutritional condition is linked to more robust immunity among mammals (Brock et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Hence common dolphins with better nutritional status could be generally healthier with a strong immune system and therefore more resistant to \u003cem\u003eAnisakis\u003c/em\u003e infections, resulting in lower worm burden. It is possible that higher levels of parasite infections as reported here have influenced the overall decline in the nutritional health of individual dolphins, as reported in a morphometric analysis of the same dolphin samples compared to historical data (Albrecht et al, in review). Potential underlying causes of poor nutritional status among cetaceans are reduced prey availability (Raverty et al., \u003cspan citationid=\"CR74\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) and prey quality (Spitz et al., \u003cspan citationid=\"CR84\" class=\"CitationRef\"\u003e2010\u003c/span\u003e), physiological conditions such as compromised health conditions due to chronic diseases, hormone imbalances leading to weight loss (Schwacke et al., \u003cspan citationid=\"CR80\" class=\"CitationRef\"\u003e2013\u003c/span\u003e), increased metabolic demands due to gestation and lactation (G\u0026oacute;mez-Campos et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2011\u003c/span\u003e) and various poor environmental conditions such as high and increasing anthropogenic noise (sonar, shipping) potentially causing disruptions in dolphins ability to forage (Wright et al., \u003cspan citationid=\"CR93\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). Upon examining the causes of death among the dolphins in this study, we observed that 82% of those that died from trauma were assigned either good or moderate nutritional status. Additionally, these dolphins had fewer parasites compared to those that died from other causes. These data suggest that dolphins that died from trauma may represent 'healthier' individuals. However, further investigations are required to uncover the patterns occurring in these dolphin-nematode interactions.\u003c/p\u003e \u003cp\u003eLarger common dolphins were found to harbour a higher number of parasites compared to smaller individuals, with a significant positive association between body length and parasite burden. Larger animals may be able to host a greater number of parasites (Kamiya et al., \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2014\u003c/span\u003e), and increased prey consumption due to an increase in nutritional demands during key life history stages (Rechsteiner et al., \u003cspan citationid=\"CR77\" class=\"CitationRef\"\u003e2013\u003c/span\u003e) and diet differences between age-groups (Novak, \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) may lead to higher infection rates. Similar findings regarding size have been reported in other studies examining parasite burden in cetaceans. For instance, anisakid burden was correlated with body length in both bottlenose (\u003cem\u003eT. truncatus\u003c/em\u003e) and striped (\u003cem\u003eS. coeruleoalba\u003c/em\u003e) dolphins inhabiting the Adriatic Sea (Blažeković et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2015\u003c/span\u003e), and longer and older long-finned pilot whales (\u003cem\u003eGlobicephala melas\u003c/em\u003e) sampled off the Faroe Islands tended to harbour a higher number of helminth parasites (Balbuena and Raga \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e1993\u003c/span\u003e; Bellay at al., 2020). Contrary to other work, Mateu et al. (\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2014\u003c/span\u003e) reported that the burden of helminth species (\u003cem\u003eTetrabothrius forsteri, Trigonocotyle globicephalae\u003c/em\u003e and \u003cem\u003eStrobilocephalus triangularis\u003c/em\u003e) was not significantly associated with the body length of striped dolphins (n\u0026thinsp;=\u0026thinsp;52) in the western Mediterranean Sea, and the authors suggest that this could be due to the low number of juvenile individuals in their sample.\u003c/p\u003e \u003cp\u003e \u003cem\u003eAnisakis\u003c/em\u003e burden was not influenced by the sex of an individual within the current study, and similar findings have been found in many other cetacean studies to date, for example bottlenose dolphins (\u003cem\u003eT. truncatus\u003c/em\u003e) (Blažeković et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2015\u003c/span\u003e), striped dolphins (\u003cem\u003eS. coeruleoalba\u003c/em\u003e) (Blažeković et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2015\u003c/span\u003e), beluga whales (\u003cem\u003eDelphinapterus leucas\u003c/em\u003e) (Pufall et al., \u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e2012\u003c/span\u003e), and franciscana (\u003cem\u003ePontoporia blainvillei\u003c/em\u003e) (Hoss et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). A lack of variation in parasite burden with host sex could be due to high similarity in the diets of male and female common dolphins (Brophy et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2009\u003c/span\u003e, Novak, \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). In contrast, through reviewing numerous studies, Poulin (\u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e1996\u003c/span\u003e) showed a higher prevalence of nematodes in male mammalian hosts compared to females which fits with the slight, but non-significant difference, observed in this study (males 92.3%, females 85.7%).\u003c/p\u003e \u003cp\u003eGastric ulcers were observed in many of the common dolphins examined in the current analysis, but their presence did not appear to be associated with parasite burden. Pons-Bordas et al. (\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) reported a significant increase in ulcerative lesions with \u003cem\u003eAnisakis\u003c/em\u003e infection in common dolphins from the North-East Atlantic (Galician coast) when comparing data collected in the early 1990s to 2017\u0026ndash;2018. The authors proposed that common dolphins could be particularly vulnerable to environmental stress, making them more susceptible to \u003cem\u003eAnisakis\u003c/em\u003e infections, or that dolphins have encountered increased instances of \u003cem\u003eAnisakis\u003c/em\u003e infections in recent years. They also reported that 86% of common dolphins had ulcers (Pons-Bordas et al., \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), similar to the present study with 90% of common dolphins showing signs of gastric ulcers. To examine the relationship between ulceration and parasite burden Buhrmann et al. (\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) described a grading scale to diagnose gastric ulceration in bottlenose dolphins and future studies might consider evaluating the surface area of the stomach covered by ulcers as a more quantitative measure of ulceration.\u003c/p\u003e \u003cp\u003eOn examining the biomass scaling relationship, a slope of 1.57 slope in the log-log linear regression between parasite biomass and host biomass is twice that expected if parasites act in the same way as host tissues. As previously mentioned metabolic theory suggests a slope of 0.75 for the rate at which host resources are converted into host growth (Brown et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Poulin and George-Nascimento, \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e2007\u003c/span\u003e), and so it appears that the anisakid nematodes occurring in the common dolphin stomachs examined here can acquire more host resources than expected for host growth rates.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThe interaction between parasites and common dolphins is complex and may be influenced by variations in dolphin health, immunity, and the presence of other pathogens or disease processes, as well as a range of environmental factors including parasite infections in prey. Here we examined stomach parasite burden in common dolphins in relation to basic biological data gathered during necropsy. In the current analysis, year of stranding, and dolphin nutritional status and body length were found to be significantly associated with \u003cem\u003eAnisakis\u003c/em\u003e burden while no variation was found between dolphin sex. Overall higher parasite prevalence was observed compared to previous studies although within the three-year dataset a trend towards decreasing parasite prevalence was observed which warrants ongoing studies. To further examine and assess the influence of these parasites on dolphin health future studies could involve investigations of potential interactions with co-occurring pathogens, the extent and type of tissue reaction and inflammation within organs, and detailed long-term temporal data on prey consumption and \u003cem\u003eAnisakis\u003c/em\u003e prevalence in prey populations.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eFunding\u0026nbsp;\u003c/h2\u003e\n\u003cp\u003eSSA was funded by an Erasmus Mundus scholarship within the International Master of Science in Marine Biological Resources. This research was part-funded through the Irish Research Council postgraduate fellowship (GOIPG/2021/378) awarded to SA. Dolphins were collected for post-mortem under the Marine Institute-EMFF funded Irish Vertebrate Necropsy Project (2017-2019), awarded to the Irish Whale and Dolphin Group in collaboration with the Regional Veterinary Laboratory, Cork and the Atlantic Technological University (Tender Reference Numbers: ITT17-024, ITT18-005, ITT18-050).\u003c/p\u003e\n\u003ch2\u003eConflict of interest\u003c/h2\u003e\n\u003cp\u003e\u0026nbsp;The authors declare no conflicts of interest.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical Approval\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll dolphins were recovered dead and necropsies were carried out at the Regional Veterinary Laboratory in Cork following best practice guidelines.\u003c/p\u003e\n\u003ch1\u003eData Availability\u003c/h1\u003e\n\u003cp\u003eThe authors do not have permission to share data.\u003c/p\u003e\n\u003ch2\u003eAuthor Contributions\u003c/h2\u003e\n\u003cp\u003eSSA, SM and KOD contributed to the study conception and design. SSA, SA, JOD, SB, MD and SL collected data. SSA and KOD analysed data. SSA produced the first draft of the manuscript and all authors commented on the manuscript. All authors read and approved the final manuscript. \u003c/p\u003e\n\u003ch2\u003eAcknowledgement\u003c/h2\u003e\n\u003cp\u003eThe authors wish to thank volunteers of the Irish Whale and Dolphin Group for dolphin collection and assisting in necropsies. Special thanks to Atlantic Technological University (ATU) placement students for their assistance: Georgia Novak and Morena Gaudino with stomach dissections, Rebecca Kinsella and Ryan Lobley with parasite counting, and Orla Gosnell with database organisation. Thanks to Ailbhe Kavanagh at the Marine Institute for providing access to the stomach samples, and to ATU laboratory technicians Mary Veldon and John Kennedy for their assistance.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAguirre AA, Tabor GM (2004) Introduction: Marine vertebrates as sentinels of marine ecosystem health. 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Methods Ecol Evol 1:3\u0026ndash;14. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1111/j.2041-210x.2009.00001.x\u003c/span\u003e\u003cspan address=\"10.1111/j.2041-210x.2009.00001.x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e "}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"marine-biology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"mabi","sideBox":"Learn more about [Marine Biology](https://www.springer.com/journal/227)","snPcode":"227","submissionUrl":"https://submission.nature.com/new-submission/227/3","title":"Marine Biology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Anisakis, cetacean, North-East Atlantic, strandings, marine mammal health","lastPublishedDoi":"10.21203/rs.3.rs-4906242/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4906242/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eShort-beaked common dolphins are important top predators in marine ecosystems, and inevitably they encounter a range of pressures in their environment. Parasite infections pose one such possible pressure and examining parasite burdens of common dolphins, and any possible impacts of infection, enables us to better understand these pressures. Here we examined the stomach parasite burden of 69 stranded common dolphins collected along the Irish coastline between 2017\u0026ndash;2019. After isolating the parasites from the stomachs, all were identified as nematodes belonging to the genus \u003cem\u003eAnisakis\u003c/em\u003e. Additional to adult nematodes, L3 and L4 larval stages were observed. A total of 319,344 anisakid specimens were estimated from all stomach compartments of the animals. Parasite prevalence was 94%, mean abundance was 4,630 worms, and mean intensity was 4,910 worms. A generalised linear model with negative binomial error structure revealed that dolphin body length, nutritional status and year of stranding were significantly associated with parasite burden. The results presented highlight the high parasite burdens that may be carried by common dolphins and their potential interaction with health parameters such as nutritional status. To gain a comprehensive overview it is important to include parasite infection investigations along with other parameters when evaluating the health status of marine mammals.\u003c/p\u003e","manuscriptTitle":"Stomach parasite burden and health status of stranded common dolphins, Delphinus delphis","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-09-11 17:31:15","doi":"10.21203/rs.3.rs-4906242/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revise and Resubmit","date":"2024-09-28T13:25:48+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2024-08-19T08:30:11+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-08-15T12:34:40+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-08-14T03:22:31+00:00","index":"","fulltext":""},{"type":"submitted","content":"Marine Biology","date":"2024-08-13T06:14:27+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"marine-biology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"mabi","sideBox":"Learn more about [Marine Biology](https://www.springer.com/journal/227)","snPcode":"227","submissionUrl":"https://submission.nature.com/new-submission/227/3","title":"Marine Biology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"3d1d48ed-b268-4976-8e57-82ab7a8cda5d","owner":[],"postedDate":"September 11th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-04-07T16:00:14+00:00","versionOfRecord":{"articleIdentity":"rs-4906242","link":"https://doi.org/10.1007/s00227-025-04620-9","journal":{"identity":"marine-biology","isVorOnly":false,"title":"Marine Biology"},"publishedOn":"2025-04-03 15:57:21","publishedOnDateReadable":"April 3rd, 2025"},"versionCreatedAt":"2024-09-11 17:31:15","video":"","vorDoi":"10.1007/s00227-025-04620-9","vorDoiUrl":"https://doi.org/10.1007/s00227-025-04620-9","workflowStages":[]},"version":"v1","identity":"rs-4906242","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4906242","identity":"rs-4906242","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}