Genetic resistance of sheep to Haemonchus contortus

To test the hypothesis that the response of sheep to primary infections with Haemonchus contortus is genetically determined, Scottish Blackface and Finn Dorset wethers of known haemoglobin type were each infected with 350 third-stage larvae/kg bodyweight. Resistance to the subsequent disease was monitored and compared by haematological, biochemical and radioisotopic methods, and resistance to parasite development and establishment by measurements of egg output and of worm recovery 32 days after infection. The results obtained demonstrated that sheep with HbA developed less severe clinical and pathophysiological disturbances, passed fewer eggs and harboured fewer worms at necropsy than animals with HbB, and that Scottish Blackfaces exhibited similar advantages over Finn Dorsets with the same haemoglobin type. Since variations in disease severity as judged by pathophysiological changes were broadly in line with parasite numbers, it appears that genetic resistance operates at its level of parasite establishment, which in turn is controlled by the immune response elicited. The mechanism of the immune response was not determined but appeared to act only against 4th and 5th stage larvae since no differences were found in the metabolic activities of individual adult worms within any of the experimental groups. A subsequent experiment, designed to examine the response of sheep to heavier infection (1400 larvae/kg), failed to reveal any correlation between haemoglobin type and worm establishment. It is suggested that this may be a reflection of a delayed immune response arising from exposure to excessive amounts of antigen. (author)