Case Study: An assessment of anthelmintic resistance through in vivo fecal egg count reduction test and in vitro egg hatch test on small ruminant farms in the southcentral United States1

An in vivo fecal egg count reduction (FECR) test was conducted on 5 farms in the southcentral United States participating in an animal resistance selection project to assess internal parasite resistance to anthelmintics. Seventy-six Kiko does on farm G1, 54 Spanish does (G2), 37 Katahdin sheep (S1), 61 Dorper ewes (S2), and 80 St. Croix sheep (S3) were randomly allocated within farm to control and 3 classes of anthelmintics. After determining initial fecal egg count, recommended doses of anthelmintics were given and fecal egg count was assessed 7 to 8 d later. Resistance to eprinomectin was detected on all farms, with FECR <63%. There was no levamisole resistance on sheep farms (FECR >95%). There was resistance to albendazole on 4 farms (FECR <95%). An egg hatch test was conducted to evaluate resistance to albendazole using composite fecal samples from untreated animals of G1, S1, S2, and S3 farms as well as control eggs from susceptible larvae. Final concentrations of albendazole were 0.00005, 0.0005, 0.005, 0.05, 0.5, and 2.0 μg/mL. After 48 h of incubation at 25°C, numbers of unhatched eggs and larvae per well were counted. The hatched percentage of susceptible larvae was 96% in the control wells. Drug concentration affected (P < 0.01) the percentage of unhatched eggs for S2 and S3, whereas values were similar (P > 0.10) for G1 and S1. In conclusion, resistance to common anthelmintics varied considerably among farms and products, suggesting a need for such testing rather than general treatment recommendations.