Genetic and phenotypic relationships between carbohydrate larval antigen (CarLA) IgA, parasite resistance and productivity in serial samples taken from lambs after weaning

•IgA response in saliva to carbohydrate larval antigen (CarLA) increased markedly in lambs grazing pasture.•The response was heritable and negatively correlated with faecal egg counts.•CarLA IgA may be an important genetic determinant of growth rate for lambs. 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