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Abstract
Herd history. A cluster of 19 abortions occurred in April and May 1994 on a seasonal dairy farm of 277 cows in Taranaki. Lesions consistent with neosporosis were seen in several foetuses. The age of the aborting cows ranged from 2 to 12 years. Pregnant rising 2-year-old heifers had been grazed off the main farm on a “run-off” until returning in May, when they were replaced on the “run-off” by the rising 1-year-old calves. None of these pregnant heifers aborted.
Methods. Sera were obtained from all calves (n = 32), 32 of 33 pregnant heifers, 59 of 60 3–4 year-old cows and 101 of 150 older cows, plus all cows that had aborted. Sera were tested using an indirect fluorescent antibody assay. Non-aborting cattle sera were screened at 1:400, and further dilutions were tested from 15 aborted cows.
Results. The percentages of seropositive cattle were: 32% of the calves, 3% of the heifers, 31% of the younger cows and 27% of the older cows. The proportion of seropositive heifers was significantly lower (p < 0.01) than in the other age classes. There was no significant difference in the proportion of seropositive animals between these other age classes (p > 0.05). Of the 15 cows that aborted, and for which further dilutions were tested, nine had titres of 1:12 800, four had titres of 1:6400, one had a titre of 1:1600 and one had a titre of 1:400. Twenty cows were the dams of pregnant heifers. Six of these cows were seropositive but all their offspring were seronegative. Fifteen aborting cows were held over to the following year when 13 became pregnant and calved normally. There were only two to three abortions in the whole herd in this following year.
Conclusion. These results suggest that either a point-infection occurred sometime in early 1994 with an infective period short enough so that the heifers did not get infected when they returned to the main farm, or some factor precipitated a recrudescence of latent infection with a subsequent rise in titres in about 29% of the animals on the main farm only. Although neither hypothesis can be proven or disproven in this observational study, the former hypothesis seems more likely.