Experimental infections of ruminants with Neospora caninum and the host immune response

Neospora caninum is a recently recognised protozoan parasite which is emerg ing as an important pathogen of cattle causing infertility problems and abo rtion. Diagnosis can be problematic as evidence of infection does not neces sarily imply cause of disease. It is not clear why some, but not all, anima ls infected with Neospora abort and whether this abortion mainly occurs in animals undergoing a primary infection or is as a result of a recrudescence of a previous infection. Controlled experimental infections of animals hav e allowed a more detailed analysis of the host parasite interaction. Experi mental infection of pregnant cattle with N. caninum tachyzoites successfull y infected and caused disease in the foetus thus demonstrating that N. cani num was a primary pathogen. Similar experiments using pregnant sheep showed that it was possible to achieve foetal infection and lesions in the foetal central nervous system and placenta which showed close similarities to tho se seen in both naturally and experimentally infected pregnant cattle demon strating that pregnant sheep may be a useful model for the disease in cattl e. Clearly there are many variables to consider when developing an experime ntal model of Neosporosis including: animal species, dose, route and parasi te life-cyde stage and strain, all of which will have a profound effect on the outcome of the infection. Work on the host immune response to N. caninu m in experimentally infected non-pregnant cattle has emphasised the importa nce of cell-mediated immunity and interferon gamma However the disease occu rs in pregnant cattle and the immunomodulation which occurs in the mother t o facilitate the pregnancy which includes a "damping down" of the interfero n gamma response may as a consequence favour multiplication of the parasite . The study of host/parasite relationship in the pregnant host is a highly complex area and is our main challenge is unravelling the factors involved in resistance and vulnerability to this opportunistic infection.