Emerging and Re-Emerging Fish Diseases and Pathogens in an Environment of the Expanding Aquaculture in Zambia

The growth of Zambia’s aquaculture sector has brought significant economic opportunities while also introducing challenges related to fish health management. This study investigated the prevalence and distribution of Epizootic Ulcerative Syndrome (EUS), Lactococcosis, and Tilapia Lake Virus (TiLV) across wild and farmed fish populations in Zambia. A total of 349 fish samples were collected from seven districts, encompassing eight fish species. EUS was detected in six wild fish species, including Tilapia rendalli (10.3%), Oreochromis macrochir (11.7%), and Oreochromis andersonii (7.2%). Its prevalence varied geographically, with Mongu (13.7%) and Shang’ombo (10.4%) exhibiting the highest positivity rates. Histopathological findings revealed lesions pathognomonic for EUS, confirming the impact of environmental conditions, such as low pH, on the pathogen’s proliferation. Importantly, no EUS cases were detected in farmed fish. Lactococcosis was identified exclusively in Oreochromis niloticus from Siavonga, with a high prevalence of 68.4%. This localized outbreak highlights the role of environmental stressors, such as high stocking densities in cage culture systems, in promoting pathogen proliferation. Other surveyed locations and species showed no evidence of Lactococcosis, suggesting effective biosecurity measures in these regions. No TiLV genome was detected, confirming Zambia’s TiLV-free status at the time of the study. This underscores the importance of ongoing surveillance and biosecurity to prevent the introduction of this globally significant pathogen, which poses severe risks to tilapia farming worldwide. These findings emphasize the need for enhanced disease monitoring, targeted health interventions, and improved management practices to support the sustainable development of Zambia’s aquaculture sector.