Effect of development of resistance to benzimidazoles, salicylanilides and ivermectin on the pathogenicity and survival of Haemonchus contortus

Department of Veterinary Pharmacology, University of Glasgow Veterinary School, Bearsden.

Two groups of six lambs were infected either with an anthelmintic-susceptible strain of Haemonchus contortus or with a strain resistant to benzimidazoles, ivermectin and salicylanilides. The pathogenicity of the two strains of parasite was compared by monitoring the development of anaemia, changes in plasma proteins and abomasal damage in the two groups of lambs. There were no significant differences between the groups, suggesting that the development of resistance to several anthelmintics did not correlate with an increase in the pathogenicity of the parasites. Furthermore, the establishment rates and egg production of the susceptible and resistant parasites were similar. However, fewer of the eggs of the resistant parasites survived and developed at a variety of temperatures.