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

Abstract

Variation in total and Trichostrongylus colubriformis-specific immunoglobulin E (IgE) was investigated in four- and six-month old Romney lambs reared on pasture, where they were exposed to natural challenge with nematode parasites. The lambs, which were from an experimental progeny test flock ( n=64 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 heritability estimates for IgE and genetic correlations between IgE and other immunological, parasitological and production traits. In addition, correlated responses of IgE to long-term genetic selection for high, control or low faecal nematode egg count (FEC) were investigated in a set of experimental breeding lines. Repeatabilities and heritabilities of IgE traits in the progeny-test flock were similar to those calculated previously for anti- T. colubriformis antibody (Ab), immunoglobulin G 1 (IgG 1) and FEC (generally >0.30). Genetic correlations of the log e transformed IgE traits (January and March samples) with log e (FEC+100) on three sampling occasions were all negative (−0.22 to −0.37). However, positive correlations (0.17 and 0.43) were found between log e IgE and dag score, indicating a tendency for there 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 selective breeding for high or low FEC), both total and T. colubriformis-specific IgE 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 those of the high and low lines but closer to those of the low line. The results are in line with other evidence which suggests that greater genetic resistance to gastrointestinal nematode infection in Romney lambs is associated with an increase in activity of the T H2 arm of the immune system which mediates inflammatory responses against multicellular parasites.