When Hereford heifers infested with Boophilus annulatus (Say) were treated with a single Ivomec SR Bolus, the concentration of ivermectin in the serum of the treated cattle reached a maximum of 8.8 ± 0.9 ppb at 2 wk posttreatment. The single bolus treatment resulted in 84.4% control of standard engorging B. annulatus females on treated cattle over the 20-wk trial. Although fewer engorged ticks were collected from the sentinel heifers exposed in the treated pasture than those in the control pasture at weeks 4, 10, and 16 posttreatment, none of the differences was statistically significant. Each exposure of sentinel cattle found free-living ticks in both the treated and control pastures, indicating the infestation was not eliminated by the treatment. When the trial was repeated using two Ivomec SR Boluses/heifer, the concentration of ivermectin in the serum of the treated cattle reached a maximum level of 31.2 ± 3.9 ppb at week 13 posttreatment. The use of two boluses/heifer resulted in 99.6% control of standard engorging B. annulatus females over the 20-wk trial. No ticks were found on sentinels placed in the treated pasture after week 9 posttreatment, an indication that the treatment had eliminated the free-living population in the treated pasture. From these studies, we conclude that a single Ivomec SR Bolus is incapable of sufficient control of B. annulatus to meet the rigid requirements of the Cattle Fever Tick Eradication Program in South Texas. Although two boluses per animal did eliminate the ticks from treated heifers and the pasture they were in, the treatment would not be sufficiently efficacious for mature cattle (>400 kg) for it to be useful in the program.
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Vol. 94 • No. 6