Course: Bovine viral diarrhoea | Last Updates: 23rd October 2015
Establishing herd BVD status and putting steps in place to control disease or maintain BVD-free status is crucial in preventing the potentially catastrophic effects of the disease on cattle health and reproduction.
Once exposure to disease is identified, cattle farmers should carry out a risk assessment with their vet and review biosecurity and vaccination policy. A strategy should then be put in place to identify and cull persistently infected (PI) animals, which are the main cause of BVD spread.
A PI is produced when a pregnant animal is exposed to the BVD virus in the first 120 days of pregnancy. The unborn calf is infected and is born shedding the virus. Preventing this infection and culling any PIs should form the focus of any control strategy. An ongoing monitoring policy is then crucial, whether you have a BVD vaccinated or non-vaccinated herd.
Testing strategies are based on milk, blood or tissue sampling and are designed to establish herd exposure and then identify specific PI animals for culling.
Antibody tests are used to establish exposure to disease. The existence of BVD antibodies shows an animal or herd has been exposed to the BVD virus, but is now immune. BVD antibodies will also be present in vaccinated herds.
Different tests are used to identify BVD virus. Virus will be present in an animal that has recently been exposed to BVD and is in the process of mounting an immune response. This animal is transiently infected and will test positive for antibodies after two to three weeks. Virus will also be present in PI animals, that will always test antibody negative, but virus positive.
Establishing BVD status
Bulk milk antibody samples are the first port of call when establishing status in a non-BVD-vaccinated dairy herd. These tests are useful for this purpose, but are not useful for identifying where BVD is coming from. They also have little use in BVD-vaccinated herds as you can’t tell the difference between the vaccine and natural antibodies.
In non-vaccinated dairy herds, if the sample is antibody negative, it is highly unlikely there is active BVD in your herd. Because the herd is naive to disease, any infection could be catastrophic, so do a risk assessment of possible routes of infection and continue to monitor bulk milk status every three to four months.
If the bulk milk sample is positive for BVD antibodies, there is evidence of exposure to BVD in your herd and it is possible you have a PI animal. Work with your vet to design an appropriate control strategy. The most effective route will be to vaccinate all breeding stock and identify and cull PI animals.
I would also review biosecurity policy, as bought-in animals and contact with neighbouring cattle pose an infection risk. If you are not a closed herd, consider becoming one. This will reduce risk from bought-in animals, but will have to be weighed up with the practicalities of rearing your own youngstock. Biosecurity that prevents contact with neighbouring animals will still be important.
In beef herds, whether vaccinated or not, blood testing five to 10 youngstock of between nine to 15 months of age for BVD antibodies is the best way to establish status. The presence of antibodies shows there is evidence of active infection. If negative, continue to monitor in this way every six months. If positive, vaccinate and identify PIs.
If BVD is identified as a risk, all breeding animals older than nine months should be vaccinated. The key is to ensure animals are vaccinated before service to prevent reproductive failure or the formation of a PI calf. The primary course should be completed a minimum of one to four weeks before service – check the particular product you are using.
Once exposure to BVD has been established, the next step is to find the PIs. In a dairy herd, carry out a bulk milk test for virus with a PCR test. This can tell you if there is even one PI within your milking herd, up to a certain size of herd. If the test is negative, there are no PIs in the milking herd on that day, however, you have only checked animals that went into the tank, so not cows that are dry or under treatment with milk withheld. Ideally, sample these animals individually, or retest in three months. If positive, you will then need to test individual blood samples for virus to find the PIs.
For youngstock, either ear tissue tag and test can be performed for all newborns or blood testing can be carried out. When individuals are virus positive, but antibody negative, they are either a PI or recently infected. To be sure they are a PI, they will need to be tested for a second time three to four weeks later for antibodies. If they are antibody negative on the second test, they are a PI and should be culled.
Blood samples can be used in a similar way across all ages of stock to identify PIs in beef herds. Blood tests can be expensive, so it is possible to pool bloods from groups of 10 animals (beef or dairy). Then, if one batch comes back positive, you can test the individuals in this group to find the PI. An alternative way in suckler herds is to sample all calves between birth and twelve months old and yearlings with no younger siblings for virus and then only sample dams of the identified PIs.
Once a beef or dairy herd has started to vaccinate and eliminate PIs, it’s important to track BVD, as everything born for the next nine to 10 months could be a PI. Tag and test or blood sampling should also be used on all newborn calves as a way of identifying PIs early.
Tag and test is a relatively new test that uses a special ear tag to automatically take an ear tissue sample when a calf is tagged. The sample is then sent away for analysis for virus. Any virus positive test should be followed up with a second test three to four weeks later to confirm it is a PI.
I would suggest continuing tag and testing calves as standard to make sure your BVD control strategy is working. You need to tag anyway and the value of keeping BVD out outweighs the expense of using these tags. Using these tags also helps when selling stock as it shows an animal has been tested.
In vaccinated herds it is imperative to monitor BVD by blood sampling a batch of non-vaccinated animals from nine months of age for antibody at least annually. If tests are positive, your vaccination and or biosecurity policy has failed – perhaps due to poor storage of vaccine or poor timing of vaccination. You will then need to identify PIs using bloods or milk testing for virus, and continue to tag and test or sample newborn calves. Testing nine- to 15-month-old stock for antibodies is also advisable for vaccinated herds that do test for virus in calves.
Biosecurity is crucial in maintaining BVD-free status or controlling BVD. Buying in virus-positive animals should be avoided at all cost. Ideally all bought-in animals should be tested for BVD virus before they arrive on farm. All purchased animals should also be quarantined on arrival and blood tested again for virus. In an ideal world, to reduce risk of introducing any infectious disease, stock should be quarantined for 28 days, although this is often not practical. In terms of BVD, bought-in stock should be kept separate from other animals – and particularly away from pregnant animals or animals due to be served – until test results come back. Animals should then be vaccinated. When buying in-calf animals, ensure the calf is also tag and tested/blood sampled for virus as soon as it’s born.
Bulls should also be blood tested and quarantined on arrival. Semen should be tested for BVD virus, as a bull could himself test virus negative, but his semen could be virus positive.
Preventing animals from coming into contact with neighbouring cattle is also crucial in preventing infection. It is well worth putting an extra field between grazing ground and a neighbour’s ground if possible, as well as double fencing.
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