THE HERITABILITY AND SPECIFICITY OF RESPONSIVENESS TO INFECTION WITH HAEMONCHUS-CONTORTUS IN SHEEP

High responder (KR) and low responder (LR) lines of sheep of the Hunga rian Merino breed were selected by using faecal egg counts (epg) as pa rameter of responsiveness after double artificial Haemonchus contortus test infections. A total of 81 lambs born to assortative matings of H R rams with HR ewes and LR rams with LR ewes, as well as 28 control la mbs from unselected parent sheep, were studied to estimate the heritab ility of responsiveness. A relatively high level of heritability (h(2) = 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 d egree of agreement: identical results in 69.7% and marginal difference s in 23.2% of 99 animals. These results suggest that by selecting shee p for high responsiveness to a certain species of nematodes, one can a lso achieve a substantial improvement of resistance against a wide ran ge of other nematode species.