The efficacy of a single treatment with a pour-on application or a subcutaneous injection of the macrocyclic lactone endectocide, doramectin, was evaluated in separate trials on Hereford heifers infested with Boophilus microplus (Canestrini). Significantly fewer ticks per calf were recovered from both groups of treated animals than from the complimentary untreated calves. The mean weights of engorged females and egg masses from both pour-on–treated and injectable-treated calves were also significantly smaller than the complimentary variables for the two groups of untreated calves. Among the treated groups, the mean weight of females from calves treated with the subcutaneous injection was 55% less than females from cattle that received the pour-on treatment and the weights of egg masses were 71% lighter than those from the pour-on–treated group. The estimates of percentage control for the two treatments were 88.6 for the pour-on formulation and a notably higher 99.8 for the injectable formulation. To obtain estimates of the effect of the treatments on the parasitic stages of the tick, cattle were infested with B. microplus larvae at three weekly intervals beginning 18 d pretreatment to ensure that, on the day of treatment, ticks in all three parasitic stages (adult, nymph, larva) would be on the cattle. The effect of the treatments on each parasitic stage was estimated by partitioning detached females into three groups by noting in which of the three 7-d intervals after detachment of engorged females began that detachment occurred. There was no difference for either the pour-on or injectable in the effect of the specific treatment on each parasitic stage. The persistent efficacy of the pour-on treatment against larvae placed on the hosts 1 wk after treatment was zero. The persistent efficacy of the injectable treatment ranged from 100 to 82.1% (mean, 93.7%) against the larvae placed on calves the first 3 wk after treatment and was still 44% against the fourth weekly posttreatment infestation. The injectable doramectin is a potential alternative to the coumaphos product now used as a precautionary treatment at USDA, Veterinary Services, Livestock Import Stations, for cattle exported from Mexico.
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