Efficacy of avermectin B1a for treatment of experimentally induced nematode infections in cattle.

Two formulations of avermectin B1a, C-076 and MK-933, were examined for anthelmintic activity in two trials, each using 20 calves with experimentally produced nematode infections. In one trial, C-076 was given orally, and in the other trial, MK-933 was given by injection at dosages of 50, 100, and 200 micrograms/kg of body weight. Treated calves were held for 7 to 8 days and then, together with nontreated controls, were euthanatized and were examined for nematodes in their gastrointestinal tracts and lungs. Efficacy of C-076 approached 100% for Dictyocaulus viviparus, Ostertagia ostertagi, Trichostrongylus axei, T colubriformis, and Oesophagostomum radiatum at 50 micrograms/kg. A dosage level of 100 micrograms/kg was required to achieve 97% efficacy against Cooperia punctata. Efficacy of MK-933 at 50 micrograms/kg was variable, but at 100 micrograms/kg, it approached 100% for D viviparus, O ostertagi, Haemonchus placei, T axei, and T longispicularis. Activity against C oncophora was less, reaching 80% efficacy at 200 micrograms/kg.