Eprinomectin ‘pour-on’ and the shedding of Haemonchus contortus eggs in experimentally infected goats

First report of multiple resistance to eprinomectin and benzimidazole in Haemonchus contortus on a dairy goat farm in France

Pour-on eprinomectin was recently registered for lactating small ruminants. Given the high prevalence of benzimidazole resistance in gastrointestinal nematodes in dairy goats, many farmers use eprinomectin exclusively to treat their animals. On a French dairy goat farm, a veterinary practitioner noted a poor response to two types of eprinomectin treatment (pour-on application and injectable formulation). Therefore, we evaluated the efficacy of both formulations of eprinomectin, as well as moxidectin and fenbendazole, using the fecal egg count reduction test (FECRT) according to the World Association for the Advancement of Veterinary Parasitology (WAAVP) guidelines. Nematode species were identified at days 0 and post-treatment days 14 after bulk larval cultures, by morphology and real-time PCR. Plasma concentrations of eprinomectin were analyzed by high-performance liquid chromatography (HPLC) at post-treatment days 2 and 5 in the eprinomectin-treated groups. Egg count reductions were poor in animals treated with topical (-16.7%; 95% CI:[-237; 59]) or subcutaneous (21.5%; 95% CI:[-126; 73]) eprinomectin, and with fenbendazole (-5.8%; 95% CI:[-205; 63]). Haemonchus contortus was the main species identified by morphology and by real-time PCR before and after treatment. The plasma concentrations of eprinomectin were determined in all eprinomectin-treated animals and were above 2 ng/ml at post-treatment day 2, indicating that the lack of effect was not due to low exposure of the worms to the drug. Interestingly, moxidectin remained effective in all infected animals. This is the first report of multiple resistance to eprinomectin and benzimidazole in H. contortus on a French dairy goat farm with moxidectin as a relevant alternative.