Investigation of Anthelmintic Resistance and De-Worming Regimens in Horses

Parasite resistance has emerged as a health dilemma in the livestock industry as many classes of parasites are becoming resistant to dewormers. Parasitism is very costly to the equine industry as it affects the performance and health of horses. Resistance has been documented for every type of equine anthelmintic class. A series of three studies was conducted to determine the efficacy of different deworming regimens. In Study 1, three fenbendazole deworming regimens were given to assess efficacy as determined by fecal egg reduction and to test for possible anthelmintic resistance in a herd that was administered only fenbendazole for 18 months prior. Resistance to fenbendazole was documented in this population based on fecal egg count reduction (FECR) tests and DrenchRite assays. Significant differences in FECR were observed between the larvicidal treatment and 5 mg/kg treatment on both day 42 versus 28 (P < 0.05) and day 72 versus 56 (P = 0.10). When analyzed by age of young horses (< 2 years of age) versus mature mares (> 2 years of age), there was a significant difference on both day 42 versus 28 and day 72 versus 56 (P < 0.05). In Study two, the efficacy of moxidectin, ivermectrin, and the larvicidial dose of fenbendazole were studied in this herd of known resistance. Mean FECR tested on day 14 post treatment was 99.91%, 98.73%, and 89.07% respectively, for the three treatment groups. Moxidectin and ivermectin groups were significantly different from the fenbendazole group (P < 0.05). When analyzed by age however, there were no differences between treatment groups in the mature mares (> 2 years of age). However, the moxidectin and ivermectin were significantly different from the fenbendazole group in the young horses (< 2 years of age) (P < 0.05). Study three determined the efficacy of a year-long rotation on breaking fenbendazole resistance in a field setting. A farm wide study showed a quarterly rotation of pyrantel pamoate (June), ivermectin (September), a larvicidal dose of fenbendazole (December), and moxidectin (March) broke resistance based on fecal egg count reduction tests. The mean fecal egg count reductions for the treatments, tested on day 7 post treatment, were 95.86%, 100%, 97.84%, and 100% respectively.