A field study on the efficacy of doramectin against strongyles and its egg reappearance period in horses

Abstract

The aim of the present study was to determine the efficacy and the so-called "egg reappearance period" (ERP) of doramectin in horses naturally infected with strongyles during a period of 34 weeks. A group of yearlings of 10 animals was treated intramuscularly with doramectin at a dose rate of 0.2 mg/kg bodyweight (BW) at the begin of the grazing season. To obtain comparable data, another group of yearlings (n = 10) was treated orally with ivermectin at a dose rate of 0.2 mg/kg BW. Individual faecal samples were examined for strongyle egg counts per gram of faeces (EPG) in two-week intervals. Twelve weeks later, a second treatment was given in both groups with the respective anthelmintic followed by a third treatment when the group mean egg count reached ≥ 200 EPG. The efficacy of doramectin was ≥ 96 % and that of ivermectin 100 %, based on the mean egg counts two weeks post treatments (wpt). The highest and the lowest extensity of the efficacy (average values) for doramectin were 90 % and 41 % two and ten wpt, respectively, whereas these values for ivermectin differed from 100 % (two wpt) to 24.3 % (eight wpt). The ERP was found to be 10 and 8 weeks for doramectin and ivermectin, respectively.