RESISTANCE OF EXOTIC AND DOMESTIC LAMBS TO EXPERIMENTAL-INFECTION WITH HAEMONCHUS-CONTORTUS

Resistance to primary and secondary infections with H. contortus was compared in St. Croix, 3/4 St. Croix, Florida Native, Barbados Blackbelly and domestic crossbred lambs. On the basis of worm counts, fetal egg counts and packed cell volumes, St. Croix and 3/4 St. Croix lambs were the most resistant to secondary infections, Florida Native and Barbados lambs were intermediate in resistance, and the domestic lambs were the least resistant. Breed differences were highly variable during primary infection. Breed differences were most pronounced prior to puberty, with domestic lambs largely highly susceptible to secondary infections. After puberty, domestic lambs were better able to resist secondary infections. Age apparently had little or no effect on the parasite resistance of exotic breeds. Sex differences were significant only after puberty when ewe lambs were more resistant to secondary infection than ram lambs. In 1 of the 3 experiments lambs of Hb genotype AB were more resistant to secondary infection than lambs of genotype BB in terms of fecal egg counts, packed cell volumes and the proportion of responded and nonresponder lambs but not in terms of worm counts, although subtle differences may not have been detected because of small numbers of lambs.