In vitro and in vivo studies on the ovicidal activity of fenbendazole.

The in vitro ovicidal activity of fenbendazole against eggs of Ostertagia ostertagi, Haemonchus contortus and Trichostrongylus colubriformis was observed using different concentrations of active principle. The optimum concentration for ovicidal activity is evidently 0.5 ppm fenbendazole. A further increase in concentration results in a decrease in activity which is probably due to physical causes. Ovicidal activity of the product is assessed by the proportion of non embryonated eggs as well as embryonated eggs which are not capable of hatching. By this method of evaluation a higher ovicidal activity is recorded. Ten h after treating sheep with fenbendazole (dose 5 mg/kg) a large number of eggs whose development was inhibited, as well as atypical eggs were detectable in the faeces. Twelve h after treatment, the capacity for development of the eggs was largely suppressed.