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 micrograms/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.(ABSTRACT TRUNCATED AT 250 WORDS)