Efficacy of ivermectin against internal parasites of sheep.

Ivermectin in an oral formulation was evaluated by the non-parametric method against a wide range of endoparasites of sheep in 15 trials involving a total of 297 sheep. Ivermectin at 200 micrograms/kg was more than 80% effective in more than 80% of the treated animals (i.e. Class A) against induced infestations of 3rd and 4th stage larvae and adults of Chabertia ovina, Dictyocaulus filaria, Haemonchus contortus (including a benzimidazole-resistant strain), Nematodirus spathiger, Oesophagostomum columbianum, Ostertagia circumcincta (including a benzimidazole-resistant isolate), and Trichostrongylus colubriformis; an "A" class was also obtained against the 4th stage larvae and adults of Gaigeria pachyscelis and Strongyloides papillosis and natural infestations of adult Trichuris spp. Efficacy against the 3rd stage larvae of G. pachyscelis and S. papillosus fell to Class B (i.e. more than 60% effective in more than 60% of the treated animals). Ivermectin completely eliminated Oestrus ovis larvae (all 3 instars). In 2 additional trials the efficacy of concurrent oral treatment with ivermectin at 200 micrograms/kg and rafoxanide suspension at 7,5 mg/kg was evaluated against the 3rd stage larvae of N. spathiger, O. columbianum and S. papillosus and 56 day-old Fasciola gigantica. Concurrent medication did not adversely affect the individual efficacies of either ivermectin or rafoxanide.