SELECTION OF HIGH LEVAMISOLE RESISTANCE IN HAEMONCHUS-CONTORTUS MONITORED WITH AN EGG-HATCH ASSAY

To investigate the characteristics of selection for levamisole resista nce in Haemonchus contortus, the consecutive nematode generations of a n in vivo selection were monitored with a newly developed egg-hatch as say. The in vice selection was started with a population not previousl y exposed to any anthelmintics (SHS). At first, the levamisole resista nce progressed gradually in successive nematode generations by treatin g sheep with increasing doses of levamisole, the initial dose being 1 mg kg(-1). Treatment with 5 mg kg(-1) levamisole resulted, however, in a steep increase of resistance. The selection was ended after six gen erations, since a level of 30 mg kg(-1) levamisole, which is not far f rom the toxic level for sheep, was reached. The final population, RHS6 , was studied in a controlled test. Treatment of RHS6-infected sheep w ith 30 mg kg(-1) levamisole caused an 80% decrease of faecal egg outpu t, and a reduction of 34% in worm numbers. It was remarkable that only the number of female adults was reduced. RHS6 showed a reduced viabil ity, but a fertility similar to the starting population SHS. (C) 1997 Australian Society for Parasitology. Published by Elsevier Science Ltd .