The heritability and specificity of responsiveness to infection with Haemonchus contortus in sheep.

High responder (HR) and low responder (LR) lines of sheep of the Hungarian Merino breed were selected by using faecal egg counts (epg) as parameter of responsiveness after double artificial Haemonchus contortus test infections. A total of 81 lambs born to assortative matings of HR rams with HR ewes and LR rams with LR ewes, as well as 28 control lambs from unselected parent sheep, were studied to estimate the heritability of responsiveness. A relatively high level of heritability (h2 = 0.49 +/- 0.17) was observed. A comparative assessment of responder classification carried out by two subsequent and distinct H. contortus and Trichostrongylus colubriformis infections revealed a remarkable degree of agreement: identical results in 69.7% and marginal differences in 23.2% of 99 animals. These results suggest that by selecting sheep for high responsiveness to a certain species of nematodes, one can also achieve a substantial improvement of resistance against a wide range of other nematode species.