PROPHYLACTIC AND PERSISTENT EFFICACY OF DORAMECTIN AGAINST COCHLIOMYIA-HOMINIVORAX IN CATTLE

Two prophylactic studies and one persistent efficacy study were conduc ted in Brazil to evaluate one injection of doramectin at a dose rate o f 200 mug kg-1 against induced infestations of the New World screwworm , Cochliomyia hominivorax. In each prophylactic study, 12 calves were allocated to two groups of six animals. Six calves were treated with d oramectin and six with saline. Two hours after treatment, animals were anesthetized and received two incisions, one in the shoulder and one in the rump. Each incision was then infested with 50 first instar larv ae. In the persistent efficacy study, 24 calves were allocated to six groups (TI-T6) of four animals each. On the day of treatment, three gr oups (T1, T3 and T5) were treated with saline and three groups (T2, T4 and T6) with doramectin. All animals were anesthetized and received f our incisions, one in the shoulder and one in the rump of each side. E ach incision was then infested with 30 first instar larvae of C homini vorax with the following schedule: T1 and T2 at Day 3, T3 and T4 at Da y 7, and T5 and T6 at Day 14 post-treatment. Calves were observed dail y and incisions evaluated for 8 days post-infection. Larvae that compl eted development and exited the wounds were collected and incubated to evaluate viability. In all experiments, doramectin was 100% efficacio us in preventing screwworm infestation caused by field isolates of C h ominivorax. First instar larvae were eliminated from the incisions of doramectin-treated calves between 24 and 48 h post-treatment, and no t hird instar were recovered at any time. As a consequence, healing star ted at 24 h and was completed at 96-120 h post-infection. Screwworm in festations and viable third-instar larvae developed in all the incisio ns in non-treated calves, and healing did not begin until larvae had c ompleted their normal life cycle and exited the wounds. The persistent efficacy of a single injection of doramectin extended beyond 14 days.