Characterising anthelmintic resistance to benzimidazoles and macrocyclic lactones in gastrointestinal nematodes of dairy cattle.

Anthelmintic resistance in gastrointestinal nematodes (GIN) of cattle is of increasing concern, but the number of studies monitoring anthelmintic efficacy is insufficient to provide a representative picture. In this study, resistance to macrocyclic lactones (ML) and benzimidazoles (BZ) in GINs from 14 Scottish dairy farms (four non-organic, 10 organic) was evaluated using multiple approaches. The faecal egg count reduction test (FECRT) remains the primary tool for evaluating anthelmintic resistance in the field and was used on the four non-organic farms. Differing methodologies and recent guideline updates complicate the interpretation of FECRT results across studies, as alternative statistical frameworks produce varying confidence intervals that can alter conclusions. Nonetheless, resistance to BZs and MLs in Ostertagia ostertagiwas consistently detected with all analytical approaches on 3/3 and 4/4 non-organic farms, respectively. The egg hatch test, combined with nematode differentiation, was used to assess BZ resistance across all 14 farms. The 95% effective concentration values observed at both the population and species levels, especially for O. ostertagi (EC95: 0.045-1.294 μg/ml), were consistent with BZ resistance on 13/14 farms. Mixed amplicon sequencing was applied to 10 populations from seven farms, including both pre- and post-treatment FECRT samples. Resistance-associated polymorphisms in the β-tubulin isotype-1 gene were detected on 5/7 farms and were present at over 25% frequency in O. ostertagi, Cooperia oncophora, and Trichostrongylus spp. populations.The presence of resistance to both BZs and MLs on 3/4 non-organic farms and higher than expected EC values for BZ on 13/14 farms, showing reduced efficacy of BZ across nearly all farms, including organic, underscoring that anthelmintic resistance is not confined to a single management system. This finding emphasizes the need for sustainable parasite control across all production systems to prevent further development and spread of resistance. Although these data originate from Scotland, the high level of animal movement across the UK suggests that the findings are likely relevant to grazing systems throughout much of the country.