Effect of fenbendazole and ivermectin on development of strongylate nematode eggs and larvae in calf feces.

Thirty-nine weaned steer calves (mean weight 284 kg) were maintained under dry-lot conditions and assigned (based on fecal nematode egg count) to one of three treatment groups of 13 animals each as follows: control (no treatment), fenbendazole (5 mg kg-1), and ivermectin (0.2 mg kg-1). Fecal samples were collected 12 h before treatment, at treatment, and 12, 24, 48 and 72 h after treatment for determination of nematode eggs per gram, and (after culture) infective larvae per gram and population distribution. The effect of treatment on egg development was observed in feces collected 12 and 24 h after treatment. There was essentially no difference in efficacy, based on egg counts, of fenbendazole and ivermectin. Egg count was reduced 100% by both anthelmintics at 72 h after treatment. Viability, based on percent of eggs reaching the infective larval stage, of developing stages at 12, 24, and 48 h after fenbendazole treatment was 0.1%, 1.1%, and 0%; after ivermectin treatment the corresponding values were 23.7%, 30.1%, and 28.6%, respectively. Fenbendazole treatment resulted in little or no development of eggs and/or larvae in feces deposited 12 and 24 h after treatment, whereas development proceeded normally (compared with the control group) in ivermectin treated feces. Population distribution of infective larvae was predominantly Haemonchus and Cooperia with some Ostertagia and Oesophagostomum.