Efficacy of oral moxidectin gel against benzimidazole-resistant cyathostomes in horses both naturally and artificially infected with a field population

The efficacy of orally administered moxidectin was evaluated against benzimidazole-resistant populations of cyathostomes. The mares used belonged to a herd in which the presence of benzimidazole-resistant small strongyles had been previously demonstrated. First, the mares harboring resistant cyathostomes were identified by fenbendazole treatment and egg hatch assays. The fecal samples containing the small strongyle population with the highest LC50 (0.375 mu g/ml) were cultured in order to prepare infective doses. Thirty-six (36) mares were each experimentally infected with 10,000 resistant third stage larvae. Fifty-seven (57) days later, 30 mares were selected and randomly allocated to three groups of ten according to their fecal epg counts. One group remained untreated, one group received fenbendazole (7.5 mg/kg of body weight) and one group was treated with moxidectin 2 % equine gel (0.4 mg/kg), Samples of feces were taken just before treatment and ten days after. Fenbendazole reduced the average number of cyathostome eggs by 40.7 %; moxidectin reduced this number by 100 %, Compared to the control group, the efficacies were 100 % and 0 % for moxidectin and fenbendazole, respectively. No adverse reactions to either of the drugs were observed.