Heritabilities Of Iga And Ige Activities Against Teladorsagia And Trichostrongylus L-3 Larval Antigens Correlated With Traits For Faecal Worm Egg Count, Health And Productivity In Merino Sheep

Low faecal worm-egg count (FEC) has been used as a phenotypic trait for breeding sheep for resistance to nematode infection. Application of this trait has resulted in a major reduction in worm-egg output in faeces, but in certain environments some of the resistant sheep develop what is believed to be a hypersensitivity-associated diarrhoea, resulting in soiling of the breech area (dags). To avoid breeding sheep with a propensity to scour, it, therefore, seems logical to combine selection for low FEC with selection for a trait based on the immune response to worm infection. We, therefore, investigated the relationships between immunoglobulin A (IgA) and immunoglobulin E (IgE) activities against L-3 larval antigens of Teladorsagia circumcincta and Trichostrongylus colubriformis and measured their inheritance and their genetic relationships with FEC, dag traits and measures of production. Total plasma IgE concentrations were also determined and included in the study. We used 762 progeny from 23 Merino sires selected for resistance to helminths (the Rylington Flock') and to breech flystrike. Total plasma IgE and IgE activities against T. circumcincta were negatively correlated with liveweight, longissimus dorsi muscle depth, and fleece weight, but positively correlated with severity of dags, softer faeces and back-fat depth. The IgA activity against T. circumcincta was negatively correlated with liveweight and clean fleece weight, but positively correlated with back-fat depth and faecal consistency score. The IgA activity against T. colubriformis was not correlated with any measure, except the breeding value for dags around yearling age. The heritability for the IgA activity against T. circumcincta and T. colubriformis was 0.13 and 0.08 respectively. The heritability for both IgE activity against T. circumcincta and total plasma IgE concentration was 0.39. We conclude that it is feasible to combine FEC and dag traits with an immune trait (in this case, helminth-specific IgE activity and total plasma IgE concentration) and, thus, genetically select sheep for resistance to both helminths and diarrhoea. However, under natural worm-challenge conditions, blood sampling protocols need to developed, on the basis of animal age, season and environment, for optimum estimation of the immune trait and its relationships with other traits.