Increase in anthelmintic resistance in sheep flocks in Ireland

Abstract Gastrointestinal nematode (GIN) infection is widespread in grazing lambs in Ireland, with heavy infections resulting in ill-thrift and ill-health. Control is currently dependent on the use of broad-spectrum anthelmintics. However, both anthelmintic resistance (AR) and treatment failure have been reported in Irish flocks. Most recent reports have determined the rate of treatment failure. Therefore, to ascertain the level of AR on sheep farms in Ireland 77 faecal egg count reduction tests (FECRT) were carried out on 24 farms between 2018 and 2023 with the test repeated on 3 farms in another year. Resistance was widespread, with all farms showing evidence of resistance to benzimidazoles. For levamisole, 50% of farms evidenced susceptibility and 42% resistance; the remaining 2 farms had conflicting results in different years. For the macrocyclic lactone, ivermectin, susceptibility was found on 25% of farms and resistance on 71%. The remaining farm had conflicting results in different years. Moxidectin was tested on 3 farms and all 3 showed evidence of resistance. For a small number of tests larvae were identified to genus level pre- and post-treatment. The predominant genus post-treatment was Teladorsagia circumcincta although Trichostrongylus spp. and Cooperia spp. were also identified. Overall, AR in GIN was widespread on sheep farms in Ireland and the adoption of sustainable parasite control practices that reduces reliance on anthelmintics is imperative. The finding of conflicting results in different years highlights the importance of regular monitoring for AR and the need to incorporate GIN species identification into the FECRT to enable species-specific efficacy calculation.