The efficacy of selamectin in the treatment of naturally acquired infestations of Sarcoptes scabiei on dogs

Selamectin, a novel avermectin, was evaluated for its effect on naturally occurring infestations of Sarcoptes scabiei in 42 dogs. In two controlled and masked laboratory studies conducted in the USA and Italy, infested dogs received treatment with either selamectin (6 mg kg −1; range: 6–12 mg kg −1) or the vehicle only (negative control). Treatments were administered topically to the skin on each animal’s back at the base of the neck in front of the scapulae. Study day 0 was defined as the first day of treatment administration. Dogs were treated on days 0 and 30, and efficacy was assessed by counting viable mites recovered from skin scrapings performed on each dog on days 14, 29 or 30, 44, and 60, and by categorising the clinical signs of canine scabies on the same days. Percentage reductions in geometric mean mite counts for selamectin, compared with vehicle, on days 14, 29 or 30, 44, and 60 were ≥98.1, ≥93.5, 100, and 100%, respectively. Analysis of variance, confirmed by Savage Scores, showed that ln(mite counts+1) values for selamectin-treated dogs were significantly lower ( P≤0.0391) than those for vehicle-treated dogs on all post-treatment assessment days. Clinical signs of scabies were markedly reduced in selamectin-treated dogs, compared with vehicle-treated dogs. Topical administration to the skin in a single spot of a single unit dose of selamectin, or of two unit doses given 1 month apart, each providing at least the recommended minimum dosage of 6 mg kg −1, was highly effective against naturally acquired infestations of S. scabiei in dogs, reducing mite counts by >93% (single dose) and 100% (two doses).