Morphological changes in trichostrongylid eggs after treatment with fenbendazole

The influence of daily doses of 0.05-0.1 mg fenbendazole/kg body weight on egg development was tested in twenty-seven lambs artificially infected with Haemonchus contortus plus Trichostrongylus colubriformis or Ostertagia circumcincta, as well as in two additional sheep naturally infected with Nematodirus spp. Atypical eggs were detected before a reduction in the number of eggs. Atypical eggs had blastomeres of different sizes and shapes; knot-like, crater-like or bubble-like. In isolated cases eggs with atypical first larvae were recognizable. Morphologically changes eggs occurred in the reproductive organs of female O. circumcincta worms. None, or only a very small number of third stage larvae, could be cultured from the faeces of treated sheep.