Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 28, 2006
Publication Date: March 1, 2007
Citation: Davey, R.B., Miller, J.A., George, J.E., Klavons, J.A. 2007. Efficacy of a single doramectin injection against adult female Boophilus microplus (Acari: Ixodidae) in the final stages of engorgement before detachment. Journal of Medical Entomology. 44(2):277-282. Interpretive Summary: Survival of any female Boophilus ticks following treatment of infested cattle with an acaricide is a concern to the U.S. Boophilus Eradication Program because it could result in dispersal of viable ticks to tick-free areas. Though previous studies showed that injectable doramectin provided 99% control against ticks that were infested on cattle for 18 days or less, the level of control against ticks infested on cattle for more than 18 days when treatment occurred was not known. Thus, doramectin was administered to cattle infested with B. microplus at 18, 19, and 20 days after ticks were infested to determine efficacy. Even though serum doramectin levels in treated cattle were above lethal limits within 24 hours of treatment, substantial numbers of ticks were still recovered from cattle treated at 19 of 20 day after infestation. Only calves treated at 18 days after infestation produced a level of control (>99%) high enough for use in the U.S. Boophilus Eradication Program. Daily levels of control were always >99% in cattle treated at 18 days after infestation, whereas control ranged from 22.6 to 85.6% on the first 2 days of female detachment in cattle treated at 19 or 20 days after infestation. Thus, it was evident that doramectin treatment applied to ticks infested on cattle for more than 18 days posed a substantial risk of causing an outbreak of ticks in tick-free areas.
Technical Abstract: Efficacy of injectable doramectin at 200 µg/kg was evaluated against female Boophilus microplus (Canestrini) in the late stages of parasitic development. Within 24 h after treatment, serum doramectin levels in cattle remained above lethal limits (5-8 ppb), ranging from 9.7 to 36.6 ppb. However, substantially higher tick numbers recovered from cattle treated at 19 or 20 d post-infestation suggested that many of these females were already far enough along in development that they did not receive a lethal quantity of blood. Results suggested that the 18 d post-infestation treatment had a substantially greater (though not significant) adverse impact on survival and reproductive capacity than treatment at either 19 or 20 d post-infestation. Only calves treated at 18 d post-infestation produced the requisite level of control (99%) considered acceptable in the U.S. Boophilus Eradication Program. Daily analysis demonstrated that treatment at 18 d post-infestation provided >99% control on each day of the evaluation. However, treatment at 19 or 20 d post-infestation provided control that ranged from 22.6 to 85.6% on the first 2 d of female detachment, and >99% control was not achieved until Day 5 of female detachment. Thus, doramectin treatment applied to cattle that have been infested with ticks for >18 d poses a substantial risk of creating an outbreak of ticks in tick-free areas.