Prevalence of anthelmintic resistance on sheep and goat farms in the southeastern United States

Objective-To determine prevalence of anthelmintic resistance on sheep and goat farms in the southeastern United States. Design-Cross-sectional study. Animals-Sheep and goats from 46 farms in 8 southern states, Puerto Rico, and St Croix in the US Virgin Islands. Procedures-Parasite eggs were isolated from fecal samples, and susceptibility to benzimidazole, imidathiazole, and avermectin-milbemycin anthelmintics was evaluated with a commercial larval development assay. Results-Haemonchus contortus was the most common parasite on 44 of 46 farms; Trichostrongylus colubriformis was the second most commonly identified parasite. Haemonchus contortus from 45 (98%), 25 (54%), 35 (76%), and 11 (24%) farms were resistant to benzimidazole, levannisole, ivermectin, and moxidectin, respectively. Resistance to all 3 classes of anthelmintics was detected on 22 (48%) farms, and resistance to all 3 classes plus moxidectin was detected on 8 farms (17%). Conclusions and Clinical Relevance-Findings provided strong evidence that anthelmintic resistance is a serious problem on small ruminant farms throughout the southeastern United States. Owing to the frequent movement of animals among regions, the prevalence of resistance in other regions of the United States is likely to also be high. Consequently, testing of parasite eggs for anthelmintic resistance should be a routine part of parasite management on small ruminant farms. (J Am Vet Med Assoc 2008,233:1913-1919)