Efficacy of closantel, albendazole and levamisole on an ivermectin resistant strain of Haemonchus contortus in sheep.

Following evidence of reduced efficacy of oral ivermectin in a field population of Haemonchus contortus on a sheep farm in Kenya, this strain of the parasite was submitted to a controlled anthelmintic test. One hundred and twenty worm-free lambs were randomly split into two groups of 60. Each lamb in the first group was infected with 5000 third stage larvae (L3) of the suspected resistant strain the remaining 60 lambs were each infected with 5000 (L3) of a H. contortus strain of known susceptibility to the major groups of anthelmintic compounds used in sheep. On day 28 post-infection, each group was subdivided according to egg counts and at random into six sub-groups of ten lambs, each of which was treated with closantel at 5.0 mg kg-1, albendazole at 5.0 mg kg-1, levamisole at 7.5 mg kg-1, oral ivermectin at 0.2 mg kg-1 or injectable (Inj.) ivermectin at 0.2 mg kg-1, or was left as an untreated control. At slaughter, 10 days later, all the anthelmintics had resulted in reduced worm burdens in animals infected with the susceptible strain (albendazole 99.0%, levamisole 99.5%, closantel, Inj. and oral ivermectin 100%). By contrast, in the lambs infected with the suspected resistant strain, closantel was 100% effective, but inj. ivermectin, oral ivermectin, albendazole and levamisole reduced worm counts by 47.6%, 24.2%, 38.5% and 41.4%, respectively. Anthelmintic resistance to the other chemical groups had been confirmed previously on this farm and although benzimidazoles and levamisole had not been used for the last 4 years, this strain was still highly resistant to albendazole and levamisole; closantel remained as the only drug evaluated which was still effective against this population of H. contortus.