Prevalence and seasonality of bulk milk antibodies against Dictyocaulus viviparus and Ostertagia ostertagi in Irish pasture-based dairy herds

•Management factors were associated with Ostertagia ostertagi and Dictyocaulus viviparus.•A prevalence of over 98% for O. ostertagi, while 62.8% of dairy herds had antibodies to D. viviparus.•Bulk tank milk results for O. ostertagi showed increasing antibodies as the year progressed.•D. viviparus antibodies were found highest towards the end and at the start of the grazing period.•D. viviparus and O. ostertagi were widespread on dairy farms, potentially affecting productivity.

Infections with Dictyocaulus viviparus and Ostertagia ostertagi nematode parasites are of importance to bovine health and production in temperate areas across the world. Losses due to these parasites in dairy herds can be considerable due to decreased milk productivity and fertility. However, information on current epidemiological patterns in Irish dairy herds is limited. Bulk milk samples were collected from a total of 319 dairy farms across the Republic of Ireland. The D. viviparus samples were tested with an ELISA based on recombinant major sperm protein, while the O. ostertagi samples were tested with an ELISA based on crude saline extract, whole worm O. ostertagi antigen. Management data were collected from the farms using a questionnaire. Logistic regression was used to find significant associations between the presence of antibodies against D. viviparus and O. ostertagi and management factors. The overall prevalence of D. viviparus infection was 62.8%, while over 98% of herds had antibodies to O. ostertagi at the specified cut-off. Both D. viviparus and O. ostertagi antibodies were highest in November, which could be explained by the accumulated uptake of larvae through the grazing season. In herds of farmers that dosed their in-calf heifers with anthelmintics were significantly more likely to be positive for antibodies against D. viviparus infection. This study highlights that both D. viviparus and O. ostertagi infections are widespread in dairy herds in Ireland throughout the grazing season.