Equine Cyathostomin Resistance to Fenbendazole in Texas Horse Facilities

Cyathostome resistance to the benzimidazole fenbendazole (FBZ) and other anthelmintic medication has been documented worldwide. Parasite resistance to anthelmintic medication is of great concern to the anthelmintic industry and to horse owners. The present study examines the efficacy of FBZ anthelmintic treatments in horse herds from ranches in four different geographical locations within Texas. In addition, the load reduction method was compared with the traditional fecal egg count reduction test to determine the incidence of parasite resistance to FBZ. Four ranches in different areas of Texas were surveyed to determine cyathostome resistance to FBZ. Two of the four ranches had young (aged ≤2 years) and older animals (aged >2 years). The number of animals with parasites varied widely between the study's locations. Differences were observed in both the fecal egg count reduction test and load reduction method across the four study sites (P < .001). Cyathostome resistance to FBZ seemed to be prevalent in three of the four ranches, whereas FBZ was highly efficacious against cyathostomes on one ranche. There was a trend toward increased parasite resistance in the younger animals (P = .081). These results show the importance of testing anthelmintic medication effectiveness.