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

One therapeutic and one persistent efficacy study were conducted in Br azil to evaluate doramectin at a dose rate of 200 mug/kg-1 against ind uced infestations of the tropical warble-fly, Dermatobia hominis. Dora mectin was very effective in both the treatment of established infesta tions and also in the prevention of damage caused by the parasite. In the therapeutic trial, 12 calves were infested along the dorsal line w ith 25 first instar larvae of recent field isolates of D. hominis but in one calf nodules did not develop. Twenty-four days later animals we re allocated to two groups on the basis of the number of parasite nodu les present. Six calves were treated with doramectin, and five receive d saline solution. Animals were examined daily for 11 days post-treatm ent and the number of nodules mapped and recorded. Larvae that complet ed 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 (T 1, 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 ea ch 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 r ecorded 6, 12 and 18 days post-infestation. In the therapeutic efficac y study, parasitic nodules in the doramectin-treated animals were redu ced 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 larv ae remained inside the nodules and completed their normal larval devel opment. Sixty-five percent of the larvae that emerged from the nodules of control animals developed into adult flies. The persistent efficac y of a single injection of doramectin extended beyond 35 days, and no parasitic nodules developed in the treated calves at any time. By cont rast, all saline-treated calves developed nodules with presence of via ble larvae recorded at 6, 12 and 18 days post-infestation.