Efficacy of injectable doramectin against natural Boophilus microplus infestations in cattle.

Three experiments were conducted in Latin America--1 in Brazil, 1 in Venezuela, and 1 in Argentina--using a common protocol to investigate the efficacy of a single SC injection of doramectin (200 micrograms/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 were allocated on the basis of tick burdens present 24 hours before treatment to a treated or control group, which received saline solution (1 ml/50 kg). All cattle were injected in the mid-dorsal cervical region and examined on treatment day and on posttreatment days (PTD) 8, 14, 21, and 28. On each observation day, the number of engorged and semiengorged ticks on half of each animals was counted and recorded. Doramectin was highly efficacious in removing tick burdens and controlling reestablishment of B microplus under field conditions of continuous reinfestation. Compared with tick counts in the control group, doramectin-treated cattle began to eliminate ticks after treatment and efficacy improved on subsequent observation days. In Brazil, efficacy was 91% at PTD 8 and 14, and increased to 99% or higher at PTD 21 and 28. In Venezuela, efficacy 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 doramectin-treated cattle was significant (P < 0.05) at each observation day.(ABSTRACT TRUNCATED AT 250 WORDS)