The efficacy of
selamectin against experimentally induced and naturally acquired
infections of adult ascarids (Toxocara cati) and adult hookworms (Ancylostoma tubaeforme) was evaluated in five controlled studies in cats. Two studies evaluated the efficacy of
selamectin against both ascarid (natural or induced) and hookworm (induced)
infections; two studies evaluated the efficacy of
selamectin against single natural
infections of T. cati or A. tubaeforme; and the fifth study evaluated the efficacy of
selamectin against induced
infections of A. tubaeforme. Cats received
selamectin topically in unit doses designed to deliver a minimum of 6mgkg(-1). Treatments were applied to the skin on each animal's back at the base of the neck in front of the scapulae. For experimentally induced
infections, cats were inoculated orally with approximately 500 embryonated eggs of T. cati 56 days prior to treatment and/or approximately 150-250 larvae (L(3)) of A. tubaeforme 30 or 42 days prior to treatment. For both induced and naturally acquired
infections, cats were allocated randomly to treatments (6-12 cats per treatment) on the basis of fecal egg counts to receive either
selamectin or a vehicle containing the inert formulation ingredients. In all studies, adult worm counts were performed at necropsy 14 days after the last treatment administration. Against T. cati, a single application of
selamectin provided a 100% reduction in the geometric mean number of adult worms for both experimentally induced and naturally acquired
infections. Against A. tubaeforme, a single administration of
selamectin provided a 99.4% reduction in the geometric mean number of adult worms in cats with natural
infections, and an 84.7-99.7% reduction in adult worms in cats with induced
infections. Two doses of
selamectin administered at monthly intervals provided a 91.9% reduction in the geometric mean number of adult A. tubaeforme worms in cats with experimentally induced
infections. The geometric mean numbers of adult worms (T. cati and A. tubaeforme) from
selamectin-treated cats were significantly (P< or =0.0018) lower than for vehicle-treated cats in all studies. Thus, a single topical unit dosage providing a minimum dosage of 6mgkg(-1)
selamectin was highly effective in the treatment of naturally acquired and experimentally induced
infections of T. cati and A. tubaeforme in cats.