Ruminant immunity to abomasal parasites
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The studies submitted herein have contributed to our understanding of ruminant immunology, host-parasite interactions during ruminant infection with nematode parasites, and potential vaccine strategies to combat parasitic gastroenteritis (PGE). PGE of sheep and cattle, caused by T. circumcincta and O. ostertagia respectively, is a major problem for the global farming industry both in terms of productivity and animal welfare. To date control of these parasites has relied on the use of anthelmintic drugs however the emergence of widespread anthelmintic resistance is driving the search for alternative methods of control. As ruminants do acquire immunity in the field, vaccination is one such alternative under investigation. The first three papers contributing to this thesis used modern immunological tools alongside a locally developed surgical technique to revisit a model of nematode infection in sheep, investigating the composition and kinetics of the ovine local immune response to infection with Teladorsagia circumcincta via cannulation of the efferent gastric lymph duct. A protective local secondary immune response was observed in sheep which had previously experienced infection with T. circumcincta, but was absent from naive sheep. This immune response consisted initially of a rise in TE and BE cell activity peaking at 3 and 5 days post challenge respectively, followed by a secondary parasiteEspecific IgA response from 5 days post challenge which correlated with stunting of parasite growth. Significant parasite loss occurred by 2 days post challenge, prior to detection of the secondary immune response, suggesting critical early events in the host-parasite interaction and the potential importance of larval antigens in these interactions. No difference was observed in either the manifestations of immunity, or the magnitude and quality of the immune response, between adult sheep and lambs. The fourth and fifth papers describe vaccine trials carried out in bovine and ovine hosts using detergent soluble proteins derived from 4th larval stage Ostertagia ostertagi and Teladorsagia circumcincta respectively as antigens. Substantial reduction in total faecal egg output of up to 85% was observed in the calf trials, but not in the sheep trials which attained a maximum reduction of 29% in total faecal egg output. The sixth paper is a transcriptomic study carried out using the Roche 454 sequencing platform to investigate the immediate responses of Teladorsagia circumcincta upon encountering ovine host tissue of either immune or naive status. Following larval exsheathing and 4 hours of exposure to either immune or naive abomasal environments the transcript level of several genes was observed to differ. Genes which were most upregulated in response to encountering the immune environment included a peptidyl-glycine alpha-amidating mono-oxygenase homologue and a small heat shock protein. The studies described herein represent a body of work carried out using up-to-date tools and technologies. The first three papers confirmed the existence of critical early events in the host-parasite interaction, pointing to the potential use of larval antigens as vaccine candidates described in the trials in papers 4 and 5, and leading to the in-depth transcriptomic analysis described in paper 6. Papers 4 and 5 demonstrated that while Teladorsagia circumcincta and Ostertagia ostertagi have similar life cycles and host-site predilection, and both the ovine and bovine host can develop immunity to incoming parasitic larvae in the field, important differences may exist in either the proteome of the fourth stage larvae and/or the nature of the host response. Paper 6 revealed that changes in T. circumcincta transcript levels in response to ovine-host immune status can be detected early in the host-parasite interaction.