THERAPEUTIC AND PERSISTENT EFFICACY OF DORAMECTIN AGAINST BOOPHILUS-MICROPLUS 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 single host tick, Boophilus microplus. Dorame ctin was highly effective in eliminating established tick populations from cattle and also in preventing infestation by the parasite. In the therapeutic study, 12 calves were infested three times a week along t he dorsal line with 2500 recently hatched larvae, for a total of 11 ti mes before treatment. Animals were allocated to two groups on the basi s of uniformity of established engorged tick burdens. Six calves were treated with doramectin and six received saline solution. From Day - 3 to Day 21 post-treatment, individual collections of detached engorged female ticks were made from each calf. In the persistent efficacy stu dy, 12 calves were allocated to two groups of six animals. Six calves were treated with doramectin and six received saline solution. From Da y 1 to Day 17 post-treatment, each animal was infested three times a w eek along the dorsal line with 2500 recently hatched Boophilus micropl us larvae, for a total of nine times. From Day 18 to Day 42 post-treat ment, daily collections of detached engorged female ticks were made fr om individual animals. In the therapeutic study, efficacy (reduction o f collected engorged female ticks) progressed from 51% at 24 h post-tr eatment (p.t.) to at least 99% at 4 days p.t., and reached 100% at 8 d ays p.t. With the exception of one tick that did not lay eggs, recover ed from one animal at 11 days p.t., no more ticks were recovered from doramectin-treated calves for the duration of the experiment. For the first 6 days after treatment, only a few detached engorged ticks were collected from treated animals, and their oviposition and hatchability declined rapidly. In the persistent efficacy study, doramectin treatm ent was highly efficacious in preventing the establishment of Boophilu s microplus populations for 20 days after the first ticks completed th eir cycle in the non-treated group. The oviposition and hatchability o f the few ticks that completed their life cycle in the doramectin grou p were severely reduced.