Efficacy of ivermectin and moxidectin injection against larvae of Wohlfahrtia magnifica (Diptera: Sarcophagidae) in sheep.

The therapeutic efficacies of ivermectin (Ivomec injection, Merck Sharp & Dohme B.V.) and moxidectin (Cydectin 1% injection, American Cyanamid Company) were evaluated in sheep naturally infested with larvae of Wohlfahrtia magnifica. Sheep were randomly allocated to one of the 2 groups, each consisting of 19 animals. Sheep in one group received ivermectin and those in the other, moxidectin by subcutaneous injection at a dose of 0.2 mg/kg body weight. Evaluation was performed at 19, 24, 28, 39, 43, 48, 52, 63, 67, 72, 87, 96, 120, 144 and 168 h after treatment. At 144 and 168 h post-treatment, late third-instar larvae were collected from wounds of four sheep in both groups and from untreated, infested sheep. These larvae were reared in the laboratory to assess adult emergence. Neither ivermectin nor moxidectin was effective as a rapid acting treatment or as a long-term, or even short-term, prophylactic. Despite the treatment, 30-40% of sheep had live larvae at every evaluation. Although larvae disappeared from the wounds of some sheep in both groups after the treatment, the wounds in these animals failed to recover and were reinfested by larvae of W. magnifica. On day 7 post-treatment the trial had to be finished because the majority of treated sheep were severely infested by Wohlfahrtia maggots. The average number of infested sheep in the two groups and the number of adults that were produced from larvae collected from treated sheep indicate that ivermectin and moxidectin did not differ significantly in efficacy.