Immunity to abomasal parasites in lambs
The parasitic nematodes Teladorsagia (Ostertagia) circumcincta and Haemmonchus contortus are two of the most important pathogens of sheep and goats worldwide. The purpose of the work described in this thesis was to identify the mechanism of resistance to these parasites in young lambs. Lambs infected with T. circumcincta are incapable of controlling their worm burdens. However, it appears that some are capable of controlling the growth and therefore the fecundity of adult female worms. Work described in chapter three shows that the most important mechanism controlling the growth and fecundity of this parasite is the local IgA response. 933 lambs were studied over 5 years. Faecal egg counts were performed on these lambs and 485 of these lambs were slaughtered and the average female worm lengths determined. Analysis showed a highly significant effect of parasite specific IgA on worm length. Those lambs with higher IgA response to fourth-stage larvae had on average shorter worms. This response was heritable. Thus genetic resistance to T. circumcincta acts by reducing worm fecundity and works through a parasite-specific IgA response. In addition, this response is sex related with male lambs having the poorest response and females the best. Not only is the quantity of IgA important in determining host resistance, but also the specificity. Chapter four details work done in investigating the antigen specificity of the IgA response to T. circumcincta.