Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/9/2003
Publication Date: 5/10/2004
Citation: George, J.E., Davey, R.B. 2004. The therapeutic and persistent efficacy of a single application of doramectin applied either as a pour-on or injection to cattle infested with Boophilus microplus (Acari:Ixodidae). Journal of Medical Entomology. 41(3):402-407. Interpretive Summary: The effectiveness of a single treatment with of two different formulations of the systemic drug doramectin was determined by infesting Hereford heifer calves with larvae of the southern cattle tick, Boophilus microplus. One of the formulations was a "pour-on" that is an oily liquid applied by pouring the product along the mid-line of an animal's back. The second formulation, an injectable product, was administered to tick-infested cattle as a subcutaneous injection. Prior to the administration of either one of the treatments, calves were infested three times with tick larvae by placing vials of larvae on hosts on 18, 11, and 4 days prior to the date on which animals were treated. Fewer ticks per calf were recovered from both groups of treated animals than from the complimentary untreated calves. Both treatments also caused the average weight of engorged females to be significantly lower than the average weight of the females from each of the complimentary untreated groups of calves. Among the treated groups of calves the average weight of engorged females from the injectable-treated group was 55% lower than for females that detached from calves treated with the pour-on. Also, the masses of eggs deposited by engorged females from the animals that received the injection of doramectin were 71% lighter that the masses of eggs produced by ticks from calves treated with the pour-on form of doramectin. 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. Because cattle were infested with tick larvae three times at weekly intervals before the doramectin treatment was applied, calves had ticks in three different parasitic stages (adult, nymphs, and larvae) on them when they were treated. It was possible to estimate the effect of the treatment on each of the parasitic stages by recording when during the first 21 days post-treatment engorged females detached. Engorged females that detached days 1-7 post-treatment were adults, ones that detached days 8-14 were nymphs, and those that detached on days 15-21 were larvae on the day hosts were treated. Both treatments had an equal effect on each parasitic stage. By infesting cattle with tick larvae weekly after the day on which the treatment was applied it was possible to estimate the degree to which residues of doramectin lethal to tick larvae persisted on the cattle. The pour-on formulation did not have any effect on even the larvae placed on treated hosts one week after they were treated. The persistent efficacy of the injectable treatment ranged from 100 to 82.1% (mean = 93.7%) against the larvae placed on calves the first three weeks post-treatment and was still 44% against the fourth post-treatment 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 into the United States.
Technical Abstract: The efficacies of a single treatment with a pour-on application or a subcutaneous injection of the macrocyclic lactone endectocide doramectin were evaluated in separate trials on Hereford heifers infested with Boophilus microplus (Canestrini). 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 lower 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 for 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 days pre-treatment to insure 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 could be estimated by partitioning detached females by noting in which of the three 7-day 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 one wk post-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 three wks post-treatment and was still 44% against the fourth post-treatment 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