Genetic and phenotypic relationships among Trichostrongylus colubriformis-specific immunoglobulin E, anti-Trichostrongylus colubriformis antibody, immunoglobulin G(1), faecal egg count and body weight traits in grazing Romney lambs

Variation in total and Trichostrongylus colubriformis-specific immunoglobul in E (IgE) was investigated in four- and six-month old Romney lambs reared on pasture, where they were exposed to natural challenge with nematode para sites. The lambs, which were from an experimental progeny test flock (n = 6 4 sires), were born in spring over three years and weaned at an average age of three months (December). Data from the flock were analysed to obtain he ritability estimates for IgE and genetic correlations between IgE and other immunological, parasitological and production traits. In addition, correla ted responses of IgE to long-term genetic selection for high, control or lo w faecal nematode egg count (FEC) were investigated in a set of experimenta l breeding lines. Repeatabilities and heritabilities of IgE traits in the p rogeny-test flock were similar to those calculated previously for anti-T. c olubriformis antibody (Ab), immunoglobulin G(1) (IgG(1)) and FEC (generally > 0.30). Genetic correlations of the log, transformed IgE traits (January and March samples) with log, (FEC + 100) on three sampling occasions were a ll negative (-0.22 to -0.37). However, positive correlations (0.17 and 0.43 ) were found between log, IgE and dag score, indicating a tendency for ther e to be more severe breech soiling in lambs with elevated serum IgE. In the 1995 and 1996 selection-line lamb crops (after 17 and 18 years of selectiv e breeding for high or low FEC), both total and T. colubriformis-specific I gE levels were higher (by between 59 and 103%) in the low than in the high line (P < 0.001). IgE levels in the controls were intermediate between thos e of the high and low lines but closer to those of the low line. The result s are in line with other evidence which suggests that greater genetic resis tance to gastrointestinal nematode infection in Romney lambs is associated with an increase in activity of the T(H)2 arm of the immune system which me diates inflammatory responses against multicellular parasites. (C) 1999 Els evier Science BN. All rights reserved.