Targeting improved resilience in Merino sheep - Correlations between immune competence and health and fitness traits.

ABSTRACT

Resilience can be defined as the ability of an animal to remain productive in the face of diverse environmental challenges. Several factors contribute to an animal's resilience including its ability to resist disease, cope with climatic extremes and respond to stressors. Immune competence, a proxy trait for general disease resistance, is expected to contribute to an animal's resilience. This research aimed to develop a practical method to assess immune competence in Merino sheep which would not restrict the future sale of tested animals, and to estimate genetic parameters associated with the novel trait. We also aimed to explore associations between immune competence and other industry-relevant disease resistance and fitness-related traits and to assess the ability of immune competence phenotypes to predict health outcomes. Here, the ability of Merino wethers (n = 1 339) to mount both an antibody-mediated and cell-mediated immune response was used to define their immune competence phenotype. For that purpose, antigens in a commercial vaccine were administered at the commencement of weaning and their responses were assessed. Univariate sire models were used to estimate variance components and heritabilities for immune competence and its component traits. Bivariate sire models were used to estimate genetic correlations between immune competence and a range of disease resistance and fitness-related traits. The heritability of immune competence and its component traits, antibody-mediated immune response and cell-mediated immune response were estimated at 0.49 ±â€¯0.14, 0.52 ±â€¯0.14 and 0.36 ±â€¯0.11, respectively. Immune competence was favourably genetically correlated with breech flystrike incidence (-0.44 ±â€¯0.39), worm egg count (-0.19 ±â€¯0.23), dag score (-0.26 ±â€¯0.31) and fitness compromise (-0.35 ±â€¯0.24) but not fleece rot (0.17 ±â€¯0.23). Results suggest that selection for immune competence has the potential to improve the resilience of Merino sheep; however, due to the large standard errors associated with correlation estimates reported here, further studies will be required in larger populations to validate associations between immune competence and disease resistance and fitness traits in Australian Merino sheep.