Incidence of abortion and association with putative causes in dairy herds in New Zealand.
OBJECTIVE
To estimate the distribution and causes of abortion in dairy herds across New Zealand during the 2001/2002 breeding season.


METHODS
A questionnaire survey was sent to all veterinary practices that employed members of the New Zealand Veterinary Association. Veterinarians were requested to record the numbers of dairy herds serviced by their practice in which the rate of aborting cows was zero (0%), low (1-5%) and high (6-10% and >10%), and all laboratory submissions, specimens, tests and results related to their investigation of abortions in these herds, for each category.


RESULTS
Forty-two large animal practices serving 1,431 dairy clients from all except one region in New Zealand responded. The within-herd incidence of abortion was zero in 497 herds (34.7%), low in 876 herds (61.2%), and high in 58 herds (4.1%) in the season 2001/2002. Within the high category, 0.6% herds reported abortions at a frequency as high as may be expected in epidemic abortions (>10%). Compared to other regions, the abortion incidences tended to be relatively high in Northland, Auckland and the Bay of Plenty, and low in Hawke Bay, Waikato and Nelson (p<0.001). Cases were submitted for laboratory investigation from 51/934 (5.5%) herds. Even though submission rates increased with the level of abortion, most submissions originated from herds with a low abortion rate. The main agents identified among submissions as the cause of abortion were Neospora caninum (Nc) (35%) and bovine viral diarrhoea virus (BVDV) (16%). Among 39 submissions from low abortion herds, 33% were suspected due to Nc and 15% to BVDV, and this was not statistically different (p>0.10) from the distribution in 13 submissions from high abortion herds (46% Nc, 23% BVDV). While cases associated with Nc were submitted in April, May and June, submissions for suspected BVDV spread evenly over the months of March to July.


CONCLUSIONS
In the 2001/2002 season, approximately 4% of dairy herds in New Zealand recorded a relatively high incidence of abortion (>5%) and 0.6% herds experienced an incidence of >10%. BVDV and Nc were the most frequently diagnosed agents associated with abortion, regardless of whether the seasonal incidence of abortion within a herd was low or high.
Thobokwe
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Epicentre, Institute of Veterinary Animal and Biomedical Sciences, Massey University, Private Bag 11222, Palmerston North, New Zealand.
Heuer
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