Variable Efficacy of Benzimidazole Anthelmintics Against Inhibited Larvae of Ostertagia Ostertagi

Variable efficacy of benzimidazole anthelmintics (albendazole, fenbendazole, and oxfendazole) against inhibited Ostertagia ostertagi larvae has been reported in the literature. Efficacies at manufacturer's recomended dosages for the three anthelmintics, respectively, were 18.6%-84.9%, 61.5%-97 .5%, and 33.5%-93.6%. Respective efficacies for dosages lower than recommended were 30.8%-86.6%, 0.0%-97.5%, and 0.0%-85.8%. Respective efficacies for dosages higher than recommended were 84.9%, 92.0%-99.0%, and 78.8%-95.0%. One of the hypotheses for variable efficacy is a difference in larval metabolic activity during the inhibition season (i.e. when metabolic activity is high efficacy is high and vice versa). This hypothesis was tested in a critical evaluation using oxfendazole. Forty-eight steer calves commenced grazing 10 acres of pasture in November. In the months of March, May, July, and September, 12 animals were removed and placed on concrete. After 3 weeks, 6 animals were treated intraruminally with oxfendazole ( 4.5 mg/kg) and 6 were left as untreated controls. Seven days after treatment, the steers were slaughtered and gastrointestinal nematodes were enumerated and identified. Results of this hypotheses-test trial and some implications for beef practitioners will be discussed in this presentation.