Some relationships between age, immune responsiveness and resistance to parasites in ruminants.

In the Australian livestock industries, susceptibility to infectious diseases is generally greater in young than in mature ruminants. The increased susceptibility is manifest as respiratory and intestinal infections (viral and bacterial) of calves, as well as fleece rot, flystrike and, especially, gastrointestinal parasitic infestations of young sheep. Lower resistance to infectious disease in young ruminants appears to be due largely to immunological hyporesponsiveness, and is not simply a consequence of their not having been exposed sufficiently to pathogens to develop active immunity. Young sheep have significantly lower proportions of CD4+ and CD8+ lymphocytes, but similar proportions of T19+ and B lymphocytes in blood, lymph and skin compared with mature sheep. Blood lymphocytes from young sheep produce less interferon-gamma in culture and young sheep invariably mount smaller antibody responses than do mature animals. Taken together, these findings begin to explain why young ruminants are more susceptible to infectious diseases in general, and to gastrointestinal parasites in particular, when compared to mature animals. Haematological markers of disease resistance, the prevalence of non-selected diseases and immune responses to vaccination were examined in the internal parasite-resistance flocks in Armidale NSW and the fleece rot/flystrike selection flocks at Trangie NSW. Any programme that seeks to improve resistance to parasitic or any other disease should have the capacity to make contemporary measurements of resistance to other diseases which are important in, or threaten, the production system.