Sheep immunized with Toxoplasma gondii (Toxovax) prior to pregnancy were tested for their ability to withstand a challenge at 90 days gestation with 107 Neospora caninum (NC1) tachyzoites. The antibody responses in sheep following immunization with T. gondii were specific for T. gondii whereas peripheral blood mononuclear cells responded to both T. gondii and N. caninum antigen in vitro. This suggested that there was induction of crossreactive immune recognition in the sheep, at least at the cellular level. Following challenge of sheep at mid-gestation with N. caninum, no febrile responses were recorded in the group of sheep which had previously received Toxovax while significant febrile responses were recorded in the group of sheep which received N. caninum challenge alone. Antibody responses to N. caninum developed in all sheep following N. caninum challenge and antibody responses to T. gondii were boosted in the group of sheep which had previously been immunized with Toxovax. No antibodies to T. gondii were observed in the sheep which received the N. caninum challenge alone. Peripheral blood mononuclear cells from both groups of sheep responded to T. gondii and N. caninum antigen in vitro and interferon gamma was present in the cell-free supernatant from activated cells. However despite evidence of the induction of crossreactive immunity between T. gondii and N. caninum, this was not sufficient to prevent foetal death. The group of sheep which had received Toxovax prior to pregnancy and the group of sheep which only received the N. caninum challenge experienced 100% foetal death compared with 0% in the unchallenged control group. Vaccination prior to pregnancy with Toxovax did protect against foetal death following oral challenge at 90 days with 2000 T. gondii oocysts which caused 100% foetal death in a control challenge group.

Neospora caninum is a recently recognized protozoan parasite which has been described as causing a neuromuscular paralysis in dogs and is emerging as a major cause of bovine infertility and abortion worldwide. The parasite is known to infect a range of warm blooded animals but the disease predominates in dogs and cattle. It is not yet known if N. caninum can infect and cause disease in people. The dog has recently been identified as the definitive host and the parasite may be transmitted through the ingestion of oocysts or congenitally from mother to fetus. N. caninum is known to infect red foxes (Vulpes vulpes) and coyotes (Canis latrans) and the role of wildlife species as reservoirs of infection requires further investigation. Little is known about the range of parasite genotypes within the environment or the variation in virulence between different strains. RAPD‐PCR analysis of geographically distinct bovine and canine isolates has revealed little genetic variation. Epidemiological studies from different areas of the world have investigated the importance of N. caninum as an abortifacient agent and longitudinal studies have shown the high rate (approximately 80%) of congenital transmission within infected herds. Information on the rates of repeat abortion due to neosporosis are less well defined however current estimates put this at 5% suggesting that cattle may develop some form of protective immunity against N. caninum‐induced abortion. Diagnosis of the disease is based upon detection of the parasite in the tissues, most commonly using immunohistochemistry with additional information provided by serology. However, although positive fetal serology is a strong indicator of exposure to the parasite, care should be taken in the interpretation of maternal serology. As we understand more about the epidemiology of neosporosis we are also better able to interpret the results of diagnostic tests. The mere presence of the parasite does not necessarily infer that this was the primary cause of abortion. CD4+ T‐cells, interferon gamma and macrophages have all been found to significantly inhibit multiplication of N. caninum tachyzoites. The nature of a protective immune response and its modulation in the pregnant animal is discussed.

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