Efficacy of selamectin against experimentally induced tick (Rhipicephalus sanguineus and Dermacentor variabilis) infestations on dogs

Seven controlled studies were conducted to investigate the efficacy of sela mectin against weekly infestations of dogs with Rhipicephalus sanguineus an d Dermacentor variabilis. Treatments (selamectin or vehicle alone) were app lied topically at weekly, 2-week, or monthly intervals or in a "Monthly Plu s" regimen (monthly treatment with an additional treatment at 14 days after the first treatment). Selamectin was supplied in unit dose tubes designed to deliver a minimum dosage of 6 mg kg(-1). The studies ranged in duration from 37 to 90 days. Fifty adult ticks (+/-2) were applied approximately wee kly, and tick counts were performed 3, 4, and 5 days after each infestation . The efficacy of selamectin was expressed as the percentage reduction in g eometric mean tick counts on selamectin-treated dogs compared with those fo r dogs treated with the vehicle alone (negative-control). In one study, the engorgement of Dermacentor variabilis was assessed by weighing ticks after removal on the fifth day after each infestation. Weekly and 2-week interva l treatments with selamectin provided efficacies against R. sanguineus of > 89% across the entire study periods, with 100% efficacy being achieved from 21 days after the first dose and thereafter (study duration, 37 days for t he weekly regimen and 34 days for the 2-week interval regimen). D. variabil is also was well controlled by the 2-week interval treatment regimen, with >96% efficacy being achieved from 21 days after the first treatment and the reafter until the end of the study (study duration: 90 days). In five of si x studies incorporating three treatments at monthly intervals, the percenta ge reduction in R. sanguineus and D. variabilis counts 5 days after infesta tion ranged from 90 to 100% in the second and third months after treatment began. In the sixth study, reductions of greater than or equal to 95% in D. variabilis counts 5 days after infestation were achieved for 2 weeks after each treatment in the second and third months. For the Monthly Plus regime n, from the second treatment (day 14) onwards, selamectin achieved 83-100% reductions in R. sanguineus and D. variabilis counts 3 days after infestati on, and 94-100% reductions 5 days after infestation in three of the four st udies. In the fourth study, selamectin demonstrated good efficacy against D . variabilis for 2 weeks after each treatment. In all seven studies, the co unts from the selamectin-treated dogs were significantly (P less than or eq ual to 0.018) lower than those from the vehicle-treated dogs on 77 of the 8 0 assessments made 5 days after infestation. Selamectin also significantly (P less than or equal to 0.0105) reduced engorgement of female D. variabili s. These studies demonstrated that selamectin, administered topically to th e skin in a single spot at a minimum dosage of 6 mg kg(-1) at monthly inter vals, was effective in the control of experimentally induced R. sanguineus and D. variabilis infestations on dogs. (C) 2000 Elsevier Science B.V. All rights reserved.