Field test data on small strongyles in evaluation of activity of fenbendazole given once a day for 5 consecutive days to thoroughbred yearlings on two farms in Kentucky in 2002 and 2003.

Fenbendazole (FBZ) suspension was administered intraorally at the dose rate of 7.0-10.3 mg/kg once a day for 5 consecutive days to 58 thoroughbred yearlings on two farms in central Kentucky in April, 2002. The average dose rates of drug given to groups of colts and fillies on each farm were 7.8-8.5 mg/kg. Only 3 of the yearlings had negative counts of strongyle eggs per gram of feces (EPGs) after treatment which was at 8.4, 8.7, or 9.4 mg/kg; the pretreatment EPG counts were low (10-30). Reduction of EPG values at the highest dose rates was 0% (at 9.5 mg/kg) and 78% (at 10.3 mg/kg). This study was repeated in April, 2003 in 38 thoroughbred yearlings on one of the two same farms used in the 2002 research, but all horses were treated at the same dose rate (10 mg/kg) of FBZ paste once daily for 5 consecutive days. Only 1 of these yearlings had a negative EPG count after treatment, but this value was also negative before treatment. Reductions of EPG counts after treatment ranged from 0% to 85% (mean =22%) for the colts and from 0 to 63% (mean =14%) for the fillies. Examination of cultures of fecal samples from these yearlings revealed that only small strongyle larvae were present. There was obvious FBZ-resistance of the small strongyles in yearlings on both farms at the dose rates used.