Unusual susceptibility of sheep selected for resistance to internal parasites

Two experiments were conducted to investigate the expression of genetic resistance to gastrointestinal nematodes infections of sheep. Merino lambs of parasite- resistant and random-bred genetic background were infected with nematodes of the genera Trichostrongylus and Haemonchus. In both experiments, and contrary to expectation, some groups of genetically resistant lambs had higher faecal egg counts than their random-bred counterparts. These unusual findings may be explained by the high levels of infection to which the lambs were exposed. Although these results are restricted to lambs in one family derived from a single highly resistant ram, we suggest that interactions between genotype and the level and nature of exposure to nematode infection require further investigation