Genetic factors affecting antibody responses to four species of nematode parasite in Romney ewe lambs.

Serum levels of antibodies (Ab) and immunoglobulin G1 (IgG1) to the larval (L3) stage of the internal parasites Cooperia curticei and Trichostrongylus colubriformis and levels of Ab to the L3 stages of Haemonchus contortus and Ostertagia circumcincta were determined in 1432 Romney ewe lambs which were born on one farm in 1990 and 1991 and were the progeny of 63 rams. The objectives were to estimate heritabilities of, and genetic correlations among, the serum concentrations in newly weaned lambs under commercial conditions and to estimate genetic correlations of Ab and IgG1 with production traits. Lambs were exposed to a natural parasite challenge on pasture, following an anthelmintic drench at weaning. Blood and faecal samples from 4- to 6-month-old lambs were then taken when the mean faecal nematode egg count of a monitor group reached 800-1500 eggs g-1. Heritabilities for the serum levels of the four Abs ranged from 0.25 +/- 0.05 to 0.37 +/- 0.06. Heritabilities for the level of IgG1 developed against C. curticei was 0.19 +/- 0.04 and against T. colubriformis, 0.18 +/- 0.05. Genetic correlations between Abs for the 4 species were high, averaging 0.84, and between the two IgG1s it was 0.82. The genetic correlations between Ab or IgG1 levels and weight or gain traits were negative (for the 6 significant values out of 18), with yearling fleece weight positive (for the 2 significant values out of 6), whilst those with loge faecal egg count were all negative (average -0.15).(ABSTRACT TRUNCATED AT 250 WORDS)