MODEL PREDICTIONS FOR ANTHELMINTIC RESISTANCE AMONGST HAEMONCHUS-CONTORTUS POPULATIONS IN SOUTHERN BRAZIL

A computer model developed to study Ostertagia circumcincta resistance to anthelmintics in UK sheep flocks has been adapted for use with Hae monchus contortus under southern Brazilian conditions. The model simul ates the effect of different anthelmintic control regimens on the year -to-year pattern of resistance in breeding ewes. The nematode control regimen most used by Brazilian sheep farmers was found to increase the frequency of genes which confer resistance from approximately 3% to 1 4% in an H. contortus population over a 20 year period. The effect of early versus late season anthelmintic treatment was investigated. This indicated that early season treatment would select for resistance rap idly, whereas late season treatments would not, owing to large numbers of untreated parasites accumulating at the beginning of the season. A model which can predict the development of anthelmintic resistance in parasites of ewes is a valuable tool in the understanding of the effe ct of different strategies on nematode control programmes and merits f urther consideration.