Detection of anthelmintic resistance in nematodes infecting dairy heifers in Puerto Rico

Abstract

The efficacy of the anthelmintic albendazole (ALB), doramectin (DOR) and ivermectin (IVM) were evaluated in Holstein and Holstein crossbreed dairy heifers using the fecal egg count reduction test (FECRT). Dairy heifers (n=420; ~6 months of age; BW = 149 ± 39.6 kg) were evaluated in 21 dairy farms. All heifers were required to be on pasture and not have been exposed to anthelmintic treatment for a minimum of 3 months. At each dairy farm, a priori, groups of at least 18 heifers were randomly divided to be treated with IVM, or DOR, or ALB following manufacturer's instructions. Fecal samples were collected directly from the rectum 7 days before anthelmintic treatment (d-7), at the time of the treatment (d0), and 14 days after treatment (d14). Only heifers that had an average FEC ³ 100 eggs per gram of feces (average of FEC from day d-7 and d0) were considered in the FECRT. The FECRT was determined by the modified McMaster technique. Of the three-anthelmintic used, ALB (n=142) proved to be the most effective (P < 0.0005) with an efficacy of 67.47% ± 8.43. The efficacy of DOR and IVM did not differ (P = 0.8713). While DOR (n = 130) exhibited a efficacy of 25.50% ± 18.00, IVM (n = 148) exhibited a efficacy of -8.13% ± 19.89 due to an increase in FEC after treatment. Dairy farmers should implement integrated methods of nematode control to preserve the effectiveness of currently available anthelmintics.