Abstract
Parasitic nematode infections are still one of the major causes of production losses in temperate regions of the world, with the majority of these infections involving the intestinal lumen dwelling nematode Cooperia oncophora and its abomasal counterpart Ostertagia ostertagi. Although nematode parasitism is common in animals of all age classes,
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calves entering the first grazing season are the most susceptible age group. Therefore, most work on nematode infection in cattle refers to calves. In general, C. oncophora has not received much attention compared to other nematodes and this is mainly caused by the mild pathogenicity of the worm. In natural conditions, clinical parasitism as caused by C. oncophora is rarely seen. Infection has been associated with production losses but the use of effective anthelmintics (anti-worm drugs) during the last decades has reduced its clinical importance. Increasing concerns over the presence of drug residues in food animals, the incidence of anthelmintic resistance and the escalating costs of the development of new anthelmintics all suggest that alternative control strategies must be developed. The most likely alternative for anthelmintics would be a vaccine, but this requires an improved understanding of the immune response, together with a more detailed knowledge of the host-parasite interactions.
We used C. oncophora infections in calves as a model to investigate the mechanisms involved in the immune response against GI nematodes. Development of immunity to C. oncophora is relatively fast. Using experimental infections with 100,000 L3 in 3-month-old calves, animals can be differentiated in High, Intermediate and Low responders. Until now, this differentiation relied solely on parasitological variables. In the first part of this thesis, the systemic and local immune response following a primary infection with 100,000 L3 C. oncophora was characterized. The results show that calves classified into different responder types based on parasitological variables, also feature a different immune response. The detailed analysis of the immunological and parasitological events occurring in the gut of Low and Intermediate responder animals allowed us to identify putative mechanisms involved in worm expulsion.
Baseed on the data from primary infected animals, we knew that acquired immunity is developed in Intermediate but not in Low responders. Under natural conditions, animals are re-infected after turn-out during the second grazing season. Hence, it is important to know whether the immunity generated during primary infection is long-lasting and protective against re-infection. To address this question, animals primed with 30,000 or 100,000 L3 were re-infected with a single dose of 100,000 L3. A detailed analysis of the parasitological features was performed to investigate two different issues: the effect of the infection dose on the development of immunity against re-infection and, whether the more resistant phenotype of Intermediate responders was sustained after challenge. Finally, the role of the B-cell and T-cell compartment and respective effectors was compared among primary and secondary infected animals.
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