EFFICACY OF INJECTABLE DORAMECTIN AGAINST NATURAL BOOPHILUS-MICROPLUSINFESTATIONS IN CATTLE

Three experiments were conducted in Latin America-1 in Brazil, 1 in Ve nezuela, and 1 in Argentina-using a common protocol to investigate the efficacy of a single sc injection of doramectin (200 mu g/kg of body weight) for treatment and control of Boophilus microplus infestations in grazing cattle. In each experiment, 2 groups of 20 to 27 cattle wer e allocated on the basis of tick burdens present 24 hours before treat ment to a treated or control group, which received saline solution (1 ml/50 kg). Al cattle were injected in the mid-dorsal cervical region a nd examined on treatment day and on posttreatment days (PTD) 8, 14, 21 , and 28. On each observation day, the number of engorged and semiengo rged ticks on half of each animal was counted and recorded. Doramectin was highly efficacious in removing tick burdens and controlling reest ablishment of B microplus field conditions of continuous reinfestation . Compared with tick counts in the control group, doramectin-treated c attle began to eliminate ticks after treatment and efficacy improved o n subsequent observation days. In Brazil, efficacy was 91% at PTD 8 an d 14, and increased to 99% or higher at PTD 21 and 28. In Venezuela, e fficacy was 100% at PTD 8, 14, and 21 and 92% at PTD 28. In Argentina, efficacy progressed from 94.5% at PTD 8 to 99 and 98.5% at PTD 21 and 28, respectively. The reduction of the number of ticks in the doramec tin-treated cattle was significant (P < 0.05) at each observation day. None of the doramectin-treated cattle had clinical signs of adverse r eactions to treatment in any of the 3 experiments.