THERAPEUTIC AND PERSISTENT EFFICACY OF DORAMECTIN AGAINST DERMATOBIA-HOMINIS IN CATTLE

One therapeutic and one persistent efficacy study were conducted in Brazil to evaluate doramectin at a dose rate of 200 mug/kg-1 against induced infestations of the tropical warble-fly, Dermatobia hominis. Doramectin was very effective in both the treatment of established infestations and also in the prevention of damage caused by the parasite. In the therapeutic trial, 12 calves were infested along the dorsal line with 25 first instar larvae of recent field isolates of D. hominis but in one calf nodules did not develop. Twenty-four days later animals were allocated to two groups on the basis of the number of parasite nodules present. Six calves were treated with doramectin, and five received saline solution. Animals were examined daily for 11 days post-treatment and the number of nodules mapped and recorded. Larvae that completed development were collected and incubated to evaluate viability. In the persistent efficacy study, 24 calves were allocated to six groups (T1-T6) of four animals each. On the day of treatment, three groups (T1, T3 and T5) were treated with saline and three groups (T2, T4 and T6) with doramectin. At 21 days, 28 days and 35 days post-treatment, 25 first instar D. hominis larvae were seeded along the dorsal line of each calf of T1 and T2, T3 and T4, and T5 and T6, respectively. Animals were examined daily for 18 days and the number of nodules mapped and recorded 6, 12 and 18 days post-infestation. In the therapeutic efficacy study, parasitic nodules in the doramectin-treated animals were reduced by 74% (P<0.05) at 48 h post-treatment, and efficacy reached 100% at 6 days post-treatment. In the saline-treated calves, parasitic larvae remained inside the nodules and completed their normal larval development. Sixty-five percent of the larvae that emerged from the nodules of control animals developed into adult flies. The persistent efficacy of a single injection of doramectin extended beyond 35 days, and no parasitic nodules developed in the treated calves at any time. By contrast, all saline-treated calves developed nodules with presence of viable larvae recorded at 6, 12 and 18 days post-infestation.