Potential involvement of Neospora caninum in naturally occurring ovine abortions in New Zealand.

Neospora caninum is an obligate intracellular parasite and is recognised as the leading cause of bovine abortion worldwide. Natural infection with N. caninum has been described in sheep but it has generally not been regarded as a significant cause of abortion. Recently, there have been several New Zealand cases of foetal abortions where N. caninum was detected which strongly suggested the involvement of Neospora in these abortions. However, there is minimal information about the prevalence of N. caninum infection naturally occurring in New Zealand sheep flocks and particularly its impact on reproduction success. Thus, this present study provides preliminary data on the role that Neospora is playing in ovine reproductive failure by establishing the prevalence of N. caninum antibodies and DNA in ewe blood and foetal material present in 21 New Zealand sheep farms with ongoing unexplained abortion problems and 10 farms with consistently high fertility levels. The results of this study demonstrated an overall seroprevalence of 1.4% which varied between Aborting/non-pregnant (1.8%), age-matched pregnant controls (0.6%) and high fertility (2.1%) ewes. However, despite the variation observed, there was no statistical difference between the three groups. In addition, Neospora DNA was detected by PCR in 13% of submitted foetal brains and in ewe blood from aborting/non-pregnant (6.9%), age-matched pregnant controls (3.6%) and high fertility pregnant (2.1%) ewes. When the PCR results were considered with the IFAT and IDEXX ELISA results, there was no correlation between serology positive and PCR positive blood samples. Taken together, these results reveal that reliance on ELISA-based serology or PCR alone may underestimate the involvement of Neospora. Furthermore, determining the involvement of Neospora appears to require a multi-facetted approach where diagnostic methods and serological cut-off values may need to be adjusted as further information about the effect of natural infections with N. caninum in the ovine host is elucidated.

Dose-titration challenge of young pregnant sheep with Neospora caninum tachyzoites.

Pregnant ewe lambs (7-8 months old at breeding) were inoculated intravenously at 90 days after joining with the ram with 50, 5 x 10(3), 10(6), or 10(8)Neospora caninum tachyzoites and outcomes were compared to a control group. Seroconversion was measured by indirect fluorescent antibody test (IFAT) at fortnightly intervals and by enzyme-linked immunosorbent assay (ELISA) at 28 days post-challenge. Seroconversion (by IFAT) occurred in all animals except for 2 sheep in the lowest dose group, including the 9 sheep in the control group. IFAT antibody titres rose and fell rapidly, peaking at 1:800 and in 8 animals was negative within 7 days of abortion. ELISA results more closely reflected infection dose and outcome of pregnancy than IFAT serology did. All 10 ewes in each of the 2 highest dose groups aborted, 5 of 10 ewes in the mid-dose group aborted and no ewes in the lowest dose or control groups aborted. Histological lesions consistent with N. caninum infection were seen in the brains of all 25 aborted lambs, as well as in 2 live-born premature lambs from the group receiving 5 x 10(3) tachyzoites and 2 clinically normal lambs (one from the control group). Polymerase chain reaction (PCR) detected N. caninum DNA in a variety of tissues from lambs, aborted foetuses and dams including 3 of 9 ewe-lamb pairs from the control group. The results from this study showed a strong relationship between challenge dose of N. caninum tachyzoites, the sample to positive (S/P) percentage in an ELISA test 28 days after challenge and pregnancy outcome. IFAT results did not correlate well with the level of challenge or the outcome of pregnancy and their relevance in studies of this kind should be questioned.