Abstract
Genetic selection for enhanced levels of protective antibody to specific nematode antigens may be a more user-friendly means of selecting animals for resistance to gastrointestinal nematodes than obtaining faecal samples and selecting on the basis of faecal egg counts. Saliva IgA antibody levels to the L3-specific surface glycan known as carbohydrate larval antigen were measured on six occasions over a 5month period in approximately 350 lambs. The carbohydrate larval antigen IgA response increased markedly with time as the lambs grazed on pasture naturally contaminated with nematode parasite larvae. The monthly loge transformed carbohydrate larval antigen IgA levels were moderately heritable at all samplings, with a combined value of 0.28±0.10 and a repeatability of 0.35±0.03. The genetic correlations between all samplings were high (0.86), suggesting that testing for a carbohydrate larval antigen IgA response could be carried out at any time in the 5months post-weaning. The transformed carbohydrate larval antigen IgA levels were genetically and phenotypically correlated negatively with loge transformed (faecal egg count+50), averaging −0.57±0.20 and −0.12±0.03 (P<0.05), respectively. The correlations between carbohydrate larval antigen IgA and breech-soiling (dag score) never reached significance. However, genetic correlations between carbohydrate larval antigen IgA and live weight were always positive and significantly so, especially at the beginning and end of the trial, indicating that carbohydrate larval antigen IgA production may be an important genetic determinant of growth rate for lambs experiencing a larval challenge. The data suggest that the ideal time to sample for a carbohydrate larval antigen IgA response and maximise selection for lowered faecal egg count and increased live-weight would be in the first 2months after weaning.
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