Effects of drug residues in the faeces of cattle treated with injectable formulations of ivermectin and moxidectin on larvae of the bush fly,Musca vetustissimaand the house fly,Musca domestica

Abstract

To assess the toxicity of residues of ivermectin and moxidectin in cattle faeces collected at intervals after treatment. Replicated bioassays of faeces using larvae of the bush fly, Musca vetustissima and the house fly, Musca domestica. Two groups of five Murray Grey x Aberdeen Angus steers were treated with injectable formulations of ivermectin and moxidectin respectively. A third group was used as an untreated control. Newly emerged fly larvae were reared in the dung of treated animals. Drug residues in faeces collected 3 to 35 days after treatment with an injectable formulation of moxidectin had no significant effect on the survival of larvae of M vetustissima. Similarly, faeces dropped up to seven days after treatment caused no significant reduction in larval survival in M domestica. In day 2 dung, residues of moxidectin delayed development of M vetustissima larvae, but had no effect on their survival. In contrast, ivermectin-treated steers, produced dung that inhibited larval development of both M vetustissima and M domestica for 7 to 14 days after treatment. Significant reductions in survival of M vetustissima larvae occurred in dung collected on days 21 and 28 after treatment, but by day 35 survival did not differ from that in control dung. Excreted faecal residues of moxidectin are relatively innocuous to larvae of both M vetustissima and M domestica. Those of ivermectin inhibit survival for 7 to 14 days after treatment and are likely to have adverse effects on non-target organisms.