Bovine viral diarrhoea virus (BVD) primarily causes infertility and abortion in New Zealand dairy herds. The disease is notoriously difficult to eradicate and difficult to control. It spreads readily in all bodily fluids and via ‘fomites’ (e.g. fenceposts, gumboots). Bought-in cows, grazing calves away from home, and neighbouring cows and bulls all pose a threat to the BVD biosecurity of your herd.
Animals can be infected with BVD in one of two ways. Animals that are infected before they are born (in utero) will become Persistently Infected (PI). PI animals shed large amounts of virus into the environment and act as a major source of infection for others in the herd. Animals that are infected later in life will become Transiently Infected (TI). TI animals become infected with BVD for a short period (1-2 weeks) before developing immunity to the virus. They will shed virus into the environment during their infected period, but nowhere near as much as PI animals. However, if pregnant heifers or cows become transiently infected during their first trimester of pregnancy, the foetus will likely become a PI animal.
BVD testing has advanced considerably. It is now possible to detect antibody levels to the virus in bulk milk samples, as well as the virus in milk. High antibody levels indicate that the herd has been exposed to the virus and has developed immunity. Recently, BVD antigen (virus) testing in the milk has been refined to allow the detection of even one PI animal in a very large herd using bulk milk antigen testing. Dairy company bulk milk sampling is sent to LIC for testing and the results are reported to your vet. Your Anexa vet can request BVD testing for you. If you organise the testing through your LIC rep it is best to check that the results are also sent to your vet for interpretation to ensure you get the best advice for your herd.
Collection of BVD bulk milk samples is best when all animals, including the new heifers, are in supply. Unless you have a known problem with BVD on your farm, it is possible to run annual BVD antibody tests alone with no need to test for BVD virus. This allows you to monitor the immune status of your herd. If BVD virus is detected in the bulk milk sample, it is normal to blood sample the bottom 10-15% producing animals in the herd (using herd test information). If a herd was BVD virus negative last year and is now BVD virus positive but does not herd test, it may be possible to track down the offending animals by testing all new animals to the herd. All PI animals should be identified and culled promptly, preferably before the planned start of mating date, to minimise the devastating reproductive failure that the virus may cause.
Antibody levels are reported in the test results as an ‘SP ratio’. BVD antibody levels are separated into five bands for ease of interpretation:
1. Very High Exposure (BVD-Infected herds)
SP ratio greater than 1.0
The herd is likely to have active BDV infection (although antibody levels may remain high for some time after PI removal). About 40% of herds in this band have a milking PI and will be POSITIVE on the bulk milk BVD test. Many more of these herds will have non-milking PI animals (e.g. bulls, calves, carry overs, beefies or other dry stock).
2. High Exposure
SP ratio 0.75 to 1.0
Most of the cows in the herd have been exposed to BVD virus. Up to 10% of these herds will have a milking PI while the remaining will have been exposed via a non-milking PI animal. The milking herd may be exposed intermittently or had recent contact with a PI. There may be PIs amongst the replacement heifers. Once PIs in a milking herd are culled or have died, antibodies will fall slowly (depending on if heifers or the herd continue to be exposed from another source).
3. Moderate Exposure
SP ratio 0.5 to 0.75
Roughly half the herd has had contact with BVD. The BVD or PI exposure is likely to be historical or the herd may have occasional contact with BVD virus (e.g. over the fence with the neighbour’s herd). The heifers may have had contact with PIs at grazing. Recent introduction of infection or vaccination will result in a rising antibody titre, but active BVD will push levels to high or very high.
4. Low Exposure
SP ratio 0.25 to 0.5
Historical exposure to BVD or PI animals, with up to a third of the cows in the herd having some immunity. Generally older cows or specific mobs (e.g. purchased cows, returned heifers) may be immune. The milking herd is unlikely to have active BVD infection and should be protected against introduction of BVD virus. It is recommended that you assess the risk and implement a BVD control plan with your vet.
5. No Exposure (BVD-Free)
SP ratio less than 0.25
The herd shows little sign of previous exposure to BVD and was essentially free of BVD at the time of testing. This is a great result as the herd has not been affected by BVD infection. However, all or most cows are susceptible to BVD so on-going vigilance and biosecurity is paramount. Test all bulls or new stock. Consider vaccination to protect pregnant cows. Consult your veterinarian to develop a complete program to protect your herd against BVD incursions.
Remember these are general guidelines only. The trends over time and repeat bulk milk tests are important to better understand your herd’s BVD status. Test results may fluctuate due to animal movements without the introduction of new BVD infection. Milk changes in late lactation (or post colostrum antibodies) can occasionally raise antibody levels for about 2 months. Interpret results with care and in context of your herd’s management and biosecurity risks.
Contact your vet to arrange monitoring of your bulk milk results.
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