Anthelmintic efficacy of avermectin B1a and dihydroavermectin B1a against ovine gastrointestinal nematodes in 1977.

In a critical study, 40 female mixed-breed lambs with experimental infections of Cooperia spp, Trichostrongylus colubriformis, Ostertagia circumcincta, T axei, and Haemonchus contortus were allocated to 8 groups in 1977. Groups 1 and 5 served as controls. Groups 2 to 4 were treated orally with avermectin B1a at dosage levels of 25, 50, and 100 micrograms/kg of body weight, respectively. Groups 6 to 8 were given oral dihydroavermectin B1a at dosage levels of 50, 100, and 200 micrograms/kg, respectively. The lambs were euthanatized and necropsied 6 days after treatment. The compounds were greater than 99% effective at all dosage levels.