Phenotypic and genetic analyses of parasitological, physiological and immunological indicator traits of gastrointestinal nematode infections in German Merino crossbred lambs

Most sheep have access to pasture but thus are exposed to gastrointestinal nematodes (GIN). One sustainable approach with long-term potential to counteract the currently increasing resistance of GIN towards anthelmintics is the breeding of sheep with better immunocompetence against GIN. The present study investigated parasitological, immunological and physiological indicator traits in a German sheep population consisting of Merino and crossbred lambs. The trial was conducted over two years under field conditions on three farms with two different grazing systems (extensive/intensive) per farm in southern Germany. After stable rearing, lambs were kept on pasture where blood and faecal samples were taken after approximately five weeks after natural infection. Depending on the trait, data of 383 to 1 015 lambs were available for phenotypic and genetic analyses. Estimated heritabilities were 0.15 ± 0.02 for FEC for Trichostrongylidae, 0.09 ± 0.02 for FEC for Nematodirus spp., 0.23 ± 0.03 for dag score and 0.13 ± 0.02 for faecal consistency score, whereas heritabilities for the total immunoglobulin A (IgA) concentration in plasma or faeces were 0.63 ± 0.04 or 0.11 ± 0.06, respectively. The heritability of the production trait average daily gain was 0.09 ± 0.02. Substantial phenotypic variation was observed in all traits. Except for total faecal IgA and average daily gain, heritabilities were significant and low to moderate, hence, indicating clear potential for breeding. The small sample size in combination with the varying infection environments did not allow clear inferences about the relationships between traits despite some significant correlations. However, this is necessary before valid breeding goals can be defined, whereby future studies might benefit from considering the course of infection and antigen-specific immune measures in large samples of purebred sheep. • Nematode infection indicators were analysed in naïve lambs under natural challenge. • Nematode infection indicators are heritable in German Merino and its crossbreeds. • Selection on parasitological, physiological and immunological indicators is possible. • Varying infection environments hamper the detection of relationships between traits.