In a series of six controlled studies (four in dogs, two in cats), heartworm-free dogs and cats were inoculated with Dirofilaria immitis larvae (L(3)) prior to topical treatment with the novel
avermectin selamectin or a negative control containing inert formulation ingredients (vehicle).
Selamectin and negative-control treatments were administered topically to the skin at the base of the neck in front of the scapulae. In dogs,
selamectin was applied topically at dosages of 3 or 6mgkg(-1) at 30 days post-inoculation (PI), or of 3 or 6mgkg(-1) at 45 days PI, or of 6mgkg(-1) at 60 days PI. Cats were treated topically with unit doses providing a minimum dosage of 6mgkg(-1)
selamectin at 30 days PI. Of the animals that were treated 30 days PI, some dogs were bathed with water or shampoo between 2 and 96h
after treatment, and some cats were bathed with shampoo at 24h
after treatment. Between 140 and 199 days PI, the animals were euthanized and examined for adult 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 development in dogs when administered as a single topical dose of 3 or 6mgkg(-1) at 30 days after
infection, 3 or 6mgkg(-1) at 45 days after
infection, or 6mgkg(-1) at 60 days after
infection.
Selamectin was 100% effective against heartworm
infections in cats when administered as a single topical unit dose of 6mgkg(-1). Bathing with water or shampoo between 2 and 96h
after treatment did not reduce the efficacy of
selamectin as a heartworm prophylactic in dogs. Likewise, bathing with shampoo at 24h
after treatment did not reduce the efficacy of
selamectin in cats. These studies demonstrated that, at the recommended dosage and treatment interval, a single
topical administration of
selamectin was 100% effective in preventing the development of D. immitis in dogs and cats.