Faecal egg counts and bulk milk Anti-O. ostertagi antibody testing in pasture based dairy cattle of Victoria, Australia

Background: International research has suggested that anti Ostertagia ostertagi antibody optical density ratio (ODR) of vat milk can indicate the level of milk production benefit producers will get from applying effective anthelmintic treatments to lactating dairy cows. ODR is an indirect antibody ELISA assessment of the magnitude of antibody production specific to O. ostertagi . In Australian pastoral dairy systems, the worm challenge season can be prolonged with high rainfall temperate climates where lactating cattle and young stock are grazing year-round. The objective of this study was to look for a correlation between ODR and faecal egg count (FEC) or the presence of parasite eggs in faeces in corresponding cohorts. Methods: Fourteen commercial dairy herds in south-west Victoria (Australia) were sampled in the study. Faecal samples were collected from recently calved cows (less than 30 days in milk) for individual FEC (sensitive to 2.5 eggs per gram of faeces) Fifteen primiparous and 15 multiparous animals of representative ages were sampled. On the same day, milk vat samples were collected during the calving period for ODR. Farm management practices including recent and routine anthelmintic use was recorded for each property and each cohort of animals. Results: Bulk milk ODR was not related to measurable FEC in corresponding herds. Bulk milk ODR was also not related to the proportion of animals in each corresponding cohort with detected parasite eggs in faeces. Bulk milk ODR was strongly correlated with days since most recent anthelmintic application (r 2 =0.73). Conclusion: Anti- O. ostertagi ODR is a good measure of likely antibody presence against O. ostertagi . However, in pastoral systems in Australia where worms are present on pasture for sustained periods, possibly year-round, it may not be a good measure of current parasite load. ODR should not be used as the sole determinant of anthelmintic application in Victorian pastoral dairy herds.