Means and heritabilities of concentrations of antibody to Trichostrongyluscolubriformis and other nematode parasites in lambs from three to seventeen months of age

Serum levels of antibody (Ab) to the larval (L3) stage of the internal para site Trichostrongylus colubriformis were measured in 1577 lambs, the progen y of 64 rams. The lambs were pan 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 ag e until 17 months of age. Serial blood samples were also taken from 66 lamb s in the selection lines themselves, to monitor Ab to T. colubriformis and three other nematode species. Lambs in both experiments were exposed to a n atural parasite challenge on pasture, following anthelmintic treatment at w eaning. Subsequent cycles of natural challenge and anthelmintic treatment f ollowed. From the progeny test data, using an animal-model restricted maxim um likelihood program, heritability estimates for log, (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. F rom 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 ran ge, 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 repeatab le measurements of Ab concentration are found in parasitised lambs from abo ut three months of age onwards, and that genetic selection for Low FEC is p ositively associated with antibody levels in lambs postweaning. (C) 1999 El sevier Science Ireland Ltd. All rights reserved.