Neospora caninum is a protozoal parasite that was first discovered as a cause of disease in dogs (Bjerkås et al. 1984) and has since been found to be associated with abortion in dairy (Thilsted and Dubey 1989) and beef (Dubey et al. 1990c) cattle. Neospora caninum is now the most commonly diagnosed cause of abortion in cattle in many countries including New Zealand (Thornton et al. 1991). The life cycle has been reviewed by several authors (Dubey 1999; Reichel 2000; Antony and Williamson 2001). Once cows within a herd become infected, a high incidence of vertical transmission, at rates of up to 95%, maintains a reservoir of infected young animals entering the herd as replacements (Paré et al. 1996), although modelling studies have demonstrated that this is insufficient to maintain infection indefinitely within a herd (French et al. 1999).
Abortion due to neosporosis may occur as a result of a lethal infection of the fetus following a maternal parasitaemia, or damage to the placenta resulting in fetal compromise (Innes et al. 2002). This may occur when a pregnant cow is first exposed to N. caninum or when recrudescence of infection occurs. Fetal death may occur at any time during gestation and stillbirth of calves has been attributed to N. caninum infection (Dubey and Lindsay 1996). The main risk period for abortion is between the fifth and seventh months gestation (Anderson et al. 1991). It has been suggested that loss of pregnancy may occur
earlier than this and not be recognised as abortion but simply as a failure to conceive (Dubey and Lindsay 1996). A recent study in Spain found no association between
N. caninum infection and loss of pregnancy before 90 days of gestation (López-Gatius et al. 2004).
The timing of exposure of the fetus to N. caninum infection has a major effect on the outcome of pregnancy (Innes et al. 2002). Infection can result in abortion, birth of a congenitally-infected calf or birth of a normal calf. Cattle naturally or experimentally infected with N. caninum produce a specific antibody and predominantly T helper (Th) 1- type cell-mediated immune response, and the production of interferon gamma (IFN-γ) and other pro-inflammatory cytokines (Lundén et al. 1998). Normal pregnancy is characterised by a lack of strong maternal cell-mediated anti-fetal immunity and a dominant humoral and Th 2-predominant immune response (Wegmann et al. 1993).
Gestational age and immunocompetence of the fetus at the time of infection also affects pregnancy outcome (Williams et al. 2000); infection of the fetus later in gestation being less likely to result in abortion than early infection, presumably due to the relative maturity of the fetal immune system. Following experimental infection of pregnant cattle at 23 weeks gestation, it was found that all fetuses had specific antibodies to the parasite but that cell- proliferation and IFN-γ responses were highly variable between individuals (Andrianarivo et al. 2001).
Definitive diagnosis of abortion due to N. caninum infection remains difficult and relies on characteristic histopathological changes in fetal tissues (Barr et al. 1990) or demonstration of N. caninum DNA using a polymerase chain reaction assay (PCR; Lally et al. 1996b). Immunohistochemistry using antibodies to N. caninum can also be used to identify tissue cysts and tachyzoites within fetal tissue (Barr et al. 1991a). A high N. caninum
antibody titre in the dam is suggestive of the parasite being involved in the loss of pregnancy, and comparison of titres among aborting and non-aborting cattle can assist diagnosis of neosporosis as a cause of abortion when a number of cattle are affected (Paré et al. 1995a). However, diagnosis of N. caninum as the cause of abortion when there is no fetal tissue available for examination remains speculative.
Various serological tests that detect N. caninum-specific antibodies in cattle are commercially available (von Blumröder et al. 2004). Cattle that produce positive results to these tests have circulating antibody and this provides evidence for previous infection, but the significance of infection is difficult to establish and many pregnant, seropositive cattle go on to deliver full-term, healthy calves (Dubey and Lindsay 1996). A PCR test using whole blood has recently been developed (Okeoma et al. 2004a) but as the parasite may no longer be circulating by the time abortion is detected, its use may be limited to experimental studies. Diagnostic tests that detect N. caninum antibodies have been calibrated for different purposes; the detection of infected animals and detecting among infected animals those cattle aborting due to N. caninum infection (von Blumröder et al. 2004). For this reason, different cut-off points are used to determine positivity and the significance of test results, and there is not always agreement between tests.
It has been shown that the N. caninum IFAT titre can rise sharply around the time of abortion and then decrease markedly over the next 2 months (Cox et al. 1998). However, other studies have shown fluctuations in antibody titre among pregnant cattle that do not abort, suggesting that recrudescence of infection does not always lead to fetal death (Stenlund et al. 1999). Stressors such as parturition and negative energy balance in early lactation can suppress immunity (Lloyd 1983) and may allow recrudescence to occur. Changes occurring as a result of abortion from any cause may alter the balance of cell- mediated immunity (CMI) and result in recrudescence of a latent N. caninum infection, as has been recently reported to occur during pregnancy in infected animals (Okeoma et al. 2005).
Previous infection with N. caninum or the presence of antibodies to it do not confer protective immunity and abortion may still occur (Dubey and Lindsay 1996). Repeat abortion in the same animals has been reported (Thornton et al. 1994) but the prevalence of this can be difficult to quantify as many cows that abort are culled.
The current study is the first prospective cohort study of a group of naturally infected first-calving dairy heifers, managed in an extensive grazing environment. The aim was to compare the rates and timing of abortion between heifers that were seropositive vs
IFAT titres in the month before and 2 months after abortion, to determine their usefulness as a diagnostic tool on a dairy farm with a confirmed history of abortion due to N. caninum
over the preceding 7 years.