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Serum levels of antibody (Ab) to the larval (L3) stage of the internal parasite Trichostrongylus colubriformis were measured in 1577 lambs, the progeny of 64 rams. The lambs were part of an experiment to evaluate sires from a genetic selection project, breeding for Low or High faecal egg counts (FEC). Blood samples for Ab were taken from December, weaning, at 3 months of age until 17 months of age. Serial blood samples were also taken from 66 lambs in the selection lines themselves, to monitor Ab to T. colubriformis and three other nematode species. Lambs in both experiments were exposed to a natural parasite challenge on pasture, following anthelmintic treatment at weaning. Subsequent cycles of natural challenge and anthelmintic treatment followed. From the progeny test data, using an animal-model restricted maximum likelihood program, heritability estimates for log e (Ab) at weaning, 4–5, 6–7 and 8–17 months of age were 0.18±0.08, 0.10±0.05, 0.25±0.05 and 0.23±0.08 respectively, and corresponding repeatability estimates (apart from weaning) were 0.24±0.04, 0.32±0.03 and 0.23±0.07. From the selection line data, least squares means for the Low FEC line were 49% higher than in the High FEC line for anti- T. colubriformis , monthly range, 24% to 73%, and the same trends were found for antibodies to T. axei , T. vitrinus and H. contortus . These results show that heritable and repeatable measurements of Ab concentration are found in parasitised lambs from about three months of age onwards, and that genetic selection for Low FEC is positively associated with antibody levels in lambs postweaning. |
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