Therapeutic and persistent efficacy of doramectin against Boophilus microplus 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 single host tick, Boophilus microplus. Doramectin 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 the dorsal line with 2500 recently hatched larvae, for a total of 11 times before treatment. Animals were allocated to two groups on the basis 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 study, 12 calves were allocated to two groups of six animals. Six calves were treated with doramectin and six received saline solution. From Day 1 to Day 17 post-treatment, each animal was infested three times a week along the dorsal line with 2500 recently hatched Boophilus microplus larvae, for a total of nine times. From Day 18 to Day 42 post-treatment, daily collections of detached engorged female ticks were made from individual animals. In the therapeutic study, efficacy (reduction of collected engorged female ticks) progressed from 51% at 24 h post-treatment (p.t.) to at least 99% at 4 days p.t., and reached 100% at 8 days p.t. With the exception of one tick that did not lay eggs, recovered 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 treatment was highly efficacious in preventing the establishment of Boophilus microplus populations for 20 days after the first ticks completed their cycle in the non-treated group. The oviposition and hatchability of the few ticks that completed their life cycle in the doramectin group were severely reduced.