Multiple-species resistance to avermectin anthelmintics on beef cattle farms in Georgia, USA.

Anthelmintic resistance (AR) in gastrointestinal nematodes (GIN) is a worldwide problem in all livestock production systems. Several prevalence studies performed internationally demonstrate that AR in cattle parasites is a growing problem; however, there are no published studies on prevalence in the United States (USA). In this study, we performed fecal egg count reduction tests (FECRT) on twelve cow-calf farms in Georgia, USA. On all farms the efficacy of eprinomectin pour-on (Eprinex®) and doramectin injectable (Dectomax®) was tested individually, as well as a combination treatment of doramectin injectable with oral fenbendazole (Safe-guard®). Fenbendazole was also tested individually on 7 farms, and long-acting eprinomectin (LongRange®) on one farm. FECRTs were performed, analyzed and interpreted following the 2023 WAAVP guidelines. Coprocultures were conducted to assess genus-specific efficacy. Overall, 100 % (12/12) and 75 % (9/12) of farms had resistance to at least one of the avermectin drugs tested in one or more GIN species, respectively. FECR ranged from 14.0 % to 96.8 % (mean=61.81 %) and 8.9-97.9 % (mean=66.87 %) for eprinomectin and doramectin, respectively; this difference was not significant (P = 0.924). In contrast, FECR for fenbendazole was high on all 7 farms tested, with just one farm demonstrating a low level of resistance. The combination treatment of doramectin plus fenbendazole was effective on all 12 farms (mean FECR=98.75 %), being significantly higher than for eprinomectin or doramectin (P≤0.001). Where coproculture data were sufficient to evaluate genus-specific reduction, resistance to Cooperia spp., Haemonchus spp. and Ostertagia spp. was present on 11/12 (92 %), 6/8 (75 %), and 4/10 farms (40 %), respectively. These data demonstrate that resistance to avermectin drugs is highly prevalent in Cooperia, Haemonchus, and Ostertagia on beef farms in Georgia. Though these data represent just one southern state, given the high level of movement of cattle in the USA, AR in GIN of cattle is likely to be highly prevalent throughout not only the southern USA, but across much of the country.