Epidemiology of Bovine Viral Diarhhoea Virus Infection in New Zealand Dairy Herds

This dissertation summarises the findings of two separate studies relating to the epidemiology of bovine viral diarrheoa virus in New Zealand dairy herds. The objective of the first study was to estimate the distribution and causes of abortion in dairy herds across New Zealand during the 2001/2002 breeding season. A questionnaire survey was sent to all veterinary practices that were listed as members of the New Zealand Veterinary Association. Veterinarians were requested to indicate the numbers of dairy herds where the rate of aborting cows was None (0%), Low (15%) and High (>5%). The main causes of abortion among a total of 52 herd submissions were Neospora caninum (36.5%) and bovine viral diarrhoea virus (BVDV) (17.3%). This frequency distribution was similar in 35 herds submissions from the Low (1 to 5%) abortion category in that 34% were due to Neospora caninum and 15% were due to BVDV. We concluded that BVDV and Neospora caninum are the most frequently diagnosed causes of abortion in dairy herds in New Zealand with a problem of abortion, as well as in herds with a low level of abortion. The aim of the second study was to assess the relationship between results of a bulk tank milk antibody ELISA for bovine viral diarrhoea virus (BVDV) and the prevalence of BVDV sero-positive young stock as an indication of active infection of the herd. Bulk tank milk samples from 724 dairy herds in the Waikato, Bay of Plenty and Northland regions of New Zealand were tested for BVDV antibodies. From this random population subset, 20 herds were again randomly selected from each of the quartiles of the percentage sero-positive (%SP) ELISA result and contacted for blood sampling. ELISA antibody test results on blood of 15 calves aged 6 – 17 months from each of 50 herds were available for final analysis.