Salivary IgA: A biomarker for resistance to Teladorsagia circumcincta and a new estimated breeding value

Teladorsagia circumcincta is the dominant nematode of sheep in cool, temperate climates. Faecal nematode egg counts (FEC) are widely used to identify the intensity of infection and as a measure of host resistance to nematodes. However due to density-dependent effects on worm fecundity the relationship between FEC and worm burden is not linear. In addition collecting FEC data is challenging on a practical level and there is a need for more reliable markers of resistance. There are two major known mechanisms of immunity to T. circumcincta: IgE against third stage larvae (L3), which inhibits larval establishment and IgA against fourth stage larvae (L4), which inhibits parasite growth. We measured salivary IgA responses in over 5000 animals against L3 antigen by Enzyme Linked Immunosorbent Assay (ELISA). Antigen-specific IgA levels were negatively correlated with FEC (r=-0.26, SE=0.02) and were heritable (h2 = 0.16, SE=0.04) indicating that they can be used to identify resistant animals suitable for inclusion in selective breeding programs. Ecological theory predicts that a trade-off between immunity and host-growth will exist due to competing energetic needs. Antigen-specific IgA responses were not negatively correlated with muscle deposition such that the expected trade-off between growth and immunity was not apparent. Our analyses indicate that selection for antigen-specific IgA is possible without impacting on the production traits for the Lleyn breed.