Exposure to Ostertagia ostertagi in Swedish dairy cattle: Insights from a 20-year comparison based on BTM-ELISA and questionnaire analysis

Gastrointestinal parasites, especially Ostertagia ostertagi - the stomach worm - are known to affect the health and productivity of cattle. This study investigates the level of O. ostertagi antibodies on Swedish dairy farms and aims to investigate differences in the parasitic exposure over 20 years. Bulk-tank milk (BTM) samples were taken from 509 farms across Sweden, and the results were analysed along a north-south gradient in relation to three main country regions. Antibody levels against the stomach worm were quantified using a standardised and validated enzyme linked immunosorbent assay (ELISA) kit for the detection of antibodies in milk samples and compared with those obtained from similar samples from 2005/6. A questionnaire was also sent to farmers to update information on location, herd size, management practices including anthelmintic control measures. The results of the two studies (i.e. 2024 and 2005/6) were visualised for comparison. Statistical analysis of BTM-antibodies revealed significantly higher levels in the two northernmost regions (Norrland and Svealand) in 2024, while the increase in the southernmost region (Götaland) was not significant. There were also differences in husbandry practices: in 2024 herds were larger, the use of pour-on solutions was more common, and the grazing season was slightly longer, while in 2005/6 the heifers were housed later and dewormed less frequently. In this study, the BTM-ELISA test is used as a non-invasive and cost-effective method for mapping parasite hotspots and provides insight into the dynamics of O. ostertagi exposure over the years.