Variation in resistance to haemonchosis: selection of female sheep resistant to Haemonchus contortus

Seventy female lambs (6-7 months old) which were exposed to natural infecti ons of Haemonchus contortus were designated as responders or nonresponders on the basis of 10 weekly cumulative faecal egg counts. Selected responder and non-responder lambs were treated with ivermectin, housed separately and 6 weeks post-housing, seven lambs from each group were given a trickle inf ection of Haemonchus contortus at 1000 L-3 daily for 5 days per week up to 2 weeks and examined weekly for 10 weeks after first infection. Analysis of data revealed significantly lower mean faecal egg counts and non-significa ntly less weight loss in responder than non-responder lambs. Mean values of haemoglobin, packed cell volume, total serum protein and peripheral eosino phil counts were significantly higher in responders than non-responders. In contrast, serum pepsinogen concentration was significantly less in respond ers than in non-responders. At 10 weeks post-infection, there were fewer pa thological lesions and significantly lower worm burdens in responders than in nonresponders. These results demonstrate a distinct resistance in respon ders to Haemonchus contortus infection.