Automated larval motility assays reveal links between eprinomectin treatment failure and drug resistance in Haemonchus contortus

Small ruminants are frequently infected with gastrointestinal nematode parasites (GIN) such as the highly pathogenic Haemonchus contortus, which severely impact animal health, welfare and production performance. The increasing prevalence of clinical anthelmintic resistance poses a global threat to effective parasite control and the productivity of livestock farming. This includes resistance to eprinomectin (EPR), the only anthelmintic currently approved for use in dairy production with a zero-withdrawal period. This study aims to link EPR therapeutic failure against H. contortus in dairy sheep farms in southwestern France with drug potency, as determined via the larval motility phenotype in an in vitro test. Six field isolates (four EPR-resistant and two EPR-susceptible isolates) were collected from dairy sheep farms, where EPR efficacy was assessed using the faecal egg count reduction test (FECRT). In addition, two laboratory isolates, known for their EPR-susceptible status, were used. We simultaneously evaluated the effects of ivermectin (IVM), moxidectin (MOX), EPR, and levamisole on larval stage development and motility, comparing putative EPR-resistant isolates with those expected to be EPR-susceptible. The automated motility assay effectively distinguished EPR-susceptible isolates from EPR-resistant isolates, with IC₅₀ values between 0.29 and 0.48 µM for susceptible isolates and between 8.16 and 32.03 µM for resistant isolates, revealing that isolates from farms with EPR treatment failure presented high resistance factors for EPR, ranging from 17 to 101. Our results reveal the sensitivity, reliability, and reproducibility of the motility test in monitoring the response of H. contortus to eprinomectin, and it may be used to detect H. contortus resistance to eprinomectin on farms. This paves the way for improving diagnostics and treatments for helminth infections in dairy sheep farms.