Efficacy of a new long-acting formulation of ivermectin and other injectable avermectins against induced Psoroptes ovis infestations in cattle.

Two studies describe the therapeutic and protective efficacy of commercially available, injectable avermectins against Psoroptes ovis infestations. In each study, six untreated calves were compared to groups of six calves each treated with ivermectin long-acting injectable (LAI) 3.15% w/v solution, generic long-acting (LA) ivermectin 1% w/v solution or 1% w/v doramectin solution. Treatments were carried out according to the manufacturer's instructions at 1 ml/50 kg body weight. Live mites in skin scrapings were counted prior to treatment and at weekly intervals for 8 weeks thereafter in the therapeutic study, or at 15, 21 and 28 days after P. ovis challenge which occurred 28 days post-treatment in the study to compare protective efficacy. P. ovis infested calves treated with ivermectin LAI had significantly ( P<0.05) fewer mites than did the untreated controls from 1 week after treatment. They also had significantly fewer mites at 7 and 8 weeks after treatment than did the calves treated with generic LA ivermectin and at 8 weeks after treatment than the calves treated with doramectin. No mites were counted on the animals treated with ivermectin LAI from 4 weeks after treatment through to the end of the study; however, five of the six calves from each of the groups treated with generic LA ivermectin or doramectin were infested with P. ovis at 8 weeks after treatment. No P. ovis mites were recorded on the calves treated prophylactically with ivermectin LAI. In contrast, five or all of the six calves treated prophylactically with generic LA ivermectin or doramectin, respectively, were positive for P. ovis mites at the end of the study. Calves treated prophylactically with ivermectin LAI gained significantly more weight than the untreated controls.