Further studies on the efficacy of fenbendazole against inhibited larvae of Ostertagia ostertagi

Abstract

Fenbendazole, at low dosage levels, was evaluated for its efficacy against inhibited early fourth stage larvae of O. ostertagi in naturally infected, yearling beef cattle. The mean number of inhibited larvae in 10 untreated control cattle was 51, 500. At a dosage level of 1 mg/kg (pelleted fenbendazole)/day in feed over 5 days in 10 cattle, the percentage reduction in comparison to controls was: adults, 85.9%; developing stages, 79.0%; inhibited early fourth stage, 47.6%; other abomasal genera (Haemonchus spp. and Trichostrongylus axei, 100%). At a dosage level of 5 mg/kg (10% liquid suspension) administered as an oral drench to 10 cattle, the percentage reduction in comparison to controls was: adults, 96.9%; developing stages, 82.9%; inhibited early fourth stage, 74.7%; other genera, 98.6%. Efficacy against parasite genera of the intestinal tract (primarily Cooperia spp.) was in excess of 94% at both dosage levels. Some variability of efficacy was observed, in that large numbers of surviving inhibited O. ostertagi larvae were recovered from 2 or 3 animals in treated groups.