Multiple anthelmintic resistance in an Ontario sheep flock

GLAUSER, JOHN; FERNÁNDEZ, ALICIA SILVINA; MENZIES, PAULA; PEREGRINE, ANDREW

Jornada; Summer Leadership and Research Program; 2007

Gastrointestinal nematodes (GIN) in sheep cause anemia, edema, and diarrhea, with associated weight loss, poor milk and wool production, and reduced fertility. Broad-spectrum anthelmintics reduce GIN levels, but anthelmintic resistance (AR) has become a worldwide problem. Under dosing and/or excessively frequent dosing are two means of selection for AR. There are signs of AR in Canada but no study has proven its existence. A farm with a history of poor response to ivermectin and albendazole was selected for a fecal egg count reduction test. Sixty ewes were divided into 4 anthelmintic treatment groups (ivermectin, albendazole, moxidectin, and levamisole) and a control group. A fecal sample was taken from each ewe at day 0 and day 14 posttreatment. A modified McMaster technique was used to determine the level of GIN eggs per gram (epg) of feces in each sample. The average pre- and post-treatment epg, respectively, for each group, along with percentage reductions and upper 95% confidence intervals (CI) were as follows: ivermectin, 3172 and 1565 [51.5% (77% CI)]; albendazole, 4517 and 1676 [48.0% (75% CI)]; moxidectin, 4588 and 2 [(99.9% (100% CI)]; levamisole, 4865 and 2 [99.9% (100% CI)]; control, 5550 and 3200. AR is confirmed when the epg reduction is less than 95% and the 95% CI is less than 90%. The results therefore showed that this flock is infected with GIN that are resistant to both ivermectin and albendazole. This is the first reported case of AR in Canada and demonstrates the presence of multiple resistances.