Amplified fragment length polymorphism analysis of genetic diversity of Haemonchus contortus during selection for drug resistance.

For the first time we used amplified fragment length polymorphism on individual nematode parasites to analyse the genetic diversity between and within isolates during consecutive stages of increased benzimidazole resistance and of increased levamisole resistance of Haemonchus contortus. The genetic diversity of the H. contortus genome turned out to be unusually high, within and between the isolates. The difference between individuals of an isolate could be as high as between individuals of two different mammalian species that do not interbreed. During benzimidazole selection the genetic constitution of the population was changed, but did not lead to a decrease in the genetic diversity. The selection for levamisole resistance resulted in a limited reduction of the genetic diversity only after the first selection step. The extensive genetic diversity apparently has allowed a fast and flexible response of H. contortus to drug selection as shown by the appearance of drug resistant isolates. This selection however has little or no effect on the extent of the genetic diversity of these resistant isolates. Implications for more sustainable control methods are discussed.