Prevention of experimentally induced heartworm (Dirofilaria immitis) infections in dogs and cats with a single topical application of selamectin

In a series of six controlled studies (four in dogs, two in cats), heartwor m-free dogs and cats were inoculated with Dirofilaria immitis larvae (L-3) prior to topical treatment with the novel avermectin selamectin or a negati ve control containing inert formulation ingredients (vehicle). Selamectin a nd negative-control treatments were administered topically to the skin at t he base of the neck in front of the scapulae. In dogs, selamectin was appli ed topically at dosages of 3 or 6 mg kg(-1) at 30 days post-inoculation (PI ), or of 3 or 6 mg kg(-1) at 45 days PI, or of 6 mg kg(-1) at 60 days PI. C ats were treated topically with unit doses providing a minimum dosage of 6 mg kg(-1) selamectin at 30 days PI, Of the animals that were treated 30 day s PI, some dogs were bathed with water or shampoo between 2 and 96 h after treatment, and some cats were bathed with shampoo at 24 h after treatment. Between 140 and 199 days PI, the animals were euthanized and examined for a dult D. immitis. Adult heartworms developed in all control dogs (geometric mean count, 18.7 worms) and in 88% of control cats (geometric mean count, 2 .1 worms). Selamectin was 100% effective in preventing heartworm developmen t in dogs when administered as a single topical dose of 3 or 6 mg kg(-1) at 30 days after infection, 3 or 6 mg kg(-1) at 45 days after infection, or 6 mg kg(-1) at 60 days after infection. Selamectin was 100% effective agains t heartworm infections in cats when administered as a single topical unit d ose of 6 mg kg(-1). Bathing with water or shampoo between 2 and 96 h after treatment did not reduce the efficacy of selamectin as a heartworm prophyla ctic in dogs. Likewise, bathing with shampoo at 24 h after treatment did no t reduce the efficacy of selamectin in cats. These studies demonstrated tha t, at the recommended dosage and treatment interval, a single topical admin istration of selamectin was 100% effective in preventing the development of D. immitis in dogs and cats. (C) 2000 Elsevier Science B.V. All rights res erved.