Persistent activity of doramectin and ivermectin in the prevention of cutaneous myiasis in cattle experimentally infested with Cochliomyia hominivorax

A study was conducted to evaluate the activity of a single administration o f doramectin or ivermectin against severe, induced infestations of Cochliom yia hominivorax. Twenty-four Holstein bull calves were allocated to four gr oups of six animals each and treated either with saline, doramectin 1%, or either one of two formulations of ivermectin 1% at a dose rate of 200 mu g/ kg. On Day 12 after treatment, each calf was anesthetized and two wounds we re created on the left side of the shoulder and rump of each calf and 2 h l ater, each wound was implanted with 100 newly hatched larvae of C. hominivo rax. On Day 15 after treatment, the procedure was repeated on the right sid e of each calf. Wounds were examined daily for 5 days and evidence of live larvae was recorded. Doramectin provided reduction in myiasis of 90.9 and 8 3.3% at 12 and 15 days after treatment, respectively, compared to the salin e control treatment (P < 0.0001). In contrast, then were no significant dif ferences in the number of calves with myiasis between those treated with ei ther of the ivermectin formulations and the saline control. (C)2000 Elsevie r Science B.V. All rights reserved.