Effects of abomasal nematode parasites in vivo and in vitro : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science at Massey University, Palmerston North, New Zealand
During abomasal parasitism, parietal cells are inhibited and may be lost, although the exact mechanism is not known. The present in vivo and in vitro experiments investigated whether the hypoacidity was linked to the host's inflammatory response and whether they are directly targeted by parasites. Pathophysiological consequences of abomasal parasitism, particularly the effects on parietal cells were studied in different immunological settings in highfleece weight lambs (HFW) and their unselected controls (C). Lambs were infected with a single dose of Ostertagia circumcincta L3 at the age of 6.5 months (Experiment 1) or 4.5 months (Experiment 2). The older lambs also received further doses of L3 weekly following primary infection. Lambs were killed on Day 94 or Day 8 or 28 respectively. Serum gastrin and pepsinogen concentrations, blood eosinophils, abomasal pH, paracortical cells in wool follicles (wool quality), FEC and worm burden were monitored. Antibody levels were measured in Experiment 1 and lymphocyte types in abomasal lymph nodes and food intake in Experiment 2. Tissue eosinophil and parietal cell counts, mucosal thickness and general histopathological changes in abomasal tissues from lambs in Experiment 2 were studied along with tissues collected from randomly bred lambs between five and 30 days after infection with O. circumcincta L3 or six to 72 hours after transplantation of adult O. circumcincta. HFW lambs exhibited resilience, but its expression differed with age. Paracortical cells in wool follicles were lower and increased later in both experiments, yet FEC was higher in older HFW lambs. The rise in abomasal pH was delayed and serum pepsinogen concentrations were higher in older HFW lambs. Generally, blood eosinophils tended to be higher in controls and cytokine responses suggested a lower Th2 response in HFW lambs. In Experiment 2, tissue eosinophils were lower in HFW sheep on Day 8. Resilience in HFW lambs appeared to be based on a down-regulated inflammatory response and the expression of resilience, which was dependent on full immune responsiveness with age. Additional bases for resilience may be the ability to delay gastric hypoacidity and a higher food intake before and during parasitism as seen in the younger HFW lambs. No conclusive answer could be given to the question of hypoacidity being secondary to inflammation, although there was a strong link between inflammation and raised abomasal pH associated with vacuolation and loss of parietal cells. Tissue eosinophils were correlated with abomasal pH in all lambs. Parietal cell loss coincided with hypoacidity and inflammation. Blood eosinophil levels did not correlate well with abomasal mucosal eosinohils. In tissues collected 30 hours after larval infection, eosinophils appeared chiefly in the tips of folds, but parietal cell numbers were not reduced. Evidence for a direct effect of parasite excretory/secretory products on epithelial cells was obtained in vitro using the HeLa cell test system for vacuolating activity (neutral red uptake). Adult products were more potent than L3 chemicals. These experiments support roles for both the host inflammatory response and parasite chemicals in inhibiting and damaging parietal cells.