Increasing resistance to multiple anthelmintic classes in nematodes including Haemonchus contortus on sheep farms in Southwest England.

Abstract

Background: Anthelmintic resistance (AR) in gastrointestinal nematodes (GIN) is increasing globally and farmers are encouraged to adopt sustainable control measures. Haemonchus contortus is increasingly reported in the UK, potentially complicating effective GIN control. Methods: Faecal egg count reduction tests (FECRT) were conducted on 13 farms in North Devon, England, UK in 2016. Relative abundance of H. contortus was quantified using peanut agglutinin (PNA) staining and used to estimate faecal egg count reduction percentages (FECR%) using the egg Counts R package. Results: On average, farms had GIN resistance to three anthelmintic classes. No farms had susceptibility to all anthelmintics tested. AR was more prevalent in 2016 than on the same farms in 2013. Haemonchus contortus was present on 85% of the farms tested and comprised on average 6% (0-52%) of GIN eggs before treatment. Resistance or suspected resistance was observed in this species to all anthelmintics tested, on different farms. Conclusion: The results demonstrate diversity of AR profiles on farms, progression of anthelmintic resistance within a 3-year period, and challenges detecting AR in mixed species infections. Where possible, interpretation of mixed-species FECRT should take into account the relative abundance of species pre- and post-treatment to identify pragmatic treatment options targeting individual genera.