Course: Bovine viral diarrhoea | Last Updates: 23rd October 2015
Bovine viral diarrhoea (BVD) is widespread in beef and dairy herds and represents a significant drain on the UK cattle industry, with Reading University putting the total cost at £61m/year.Its effect is far-reaching, causing reduced fertility, abortion and pneumonia. It also causes immunosuppression, which can make other common disease more severe, and mucosal disease, which is always fatal.
Estimates suggest 65% of UK dairy herds have been exposed to BVD, with the disease also widespread in beef herds. Part of the reason for its prevalence is linked to how easily the virus is spread between animals and the numerous routes of transmission. Any control strategy must take this into account and should be based on preventing the infection of pregnant animals.
What is bovine viral diarrhoea (BVD)?
BVD is a virus that belongs to the pestivirus family and is closely related to classical swine fever and border disease in sheep. The nature of these types of viruses means they can cross the placenta and infect the unborn foetus in pregnant animals.
What are the symptoms?
Despite the name, diarrhoea is one of the least common symptoms of BVD. The disease actually causes a wide range of symptoms, varying in severity from subclinical disease to death. Virus exposure most commonly results in fertility problems in breeding and pregnant animals. This may include early embryonic death, which results in lowered conception rates and more animals returning to service, as well as abortion, stillbirths and calves born with abnormalities. BVD can cause varying numbers of abortions at any stage of pregnancy and it’s important to get any abortions investigated.
In non-pregnant cattle and particularly youngstock, BVD exposure is more likely to result in a mild temperature and reduced feed intake with possible respiratory disease (such as pneumonia) and /or mild diarrhoea. The virus also causes immunosuppression, which can make symptoms worse and also means animals are likely to be more susceptible to other diseases. In adult dairy cattle farmers may also witness a drop in milk yields or cows being “off colour”, while both beef and dairy herds may see a drop in youngstock growth rates.
What is the difference between type 1 and type 2 BVD?
There are two different strains of BVD: Type 1 and type 2. Most of the BVD seen in the UK is type 1, with only a handful cases of type 2 reported. Both strains can have the same effect on fertility and overall health, however type 2 can be more severe and can result in bleeding syndrome and death. Type 2 is well recognised in Europe and North America and is potentially of serious concern to the UK and could be introduced by cattle movements.
How is BVD spread?
BVD virus can be spread through nasal secretions, urine and faeces, through nose-to-nose contact and urination, and cattle licking each other can cause transmission.
When an individual is exposed to the disease, the BVD virus will typically remain present in the animal for about two weeks while the animal mounts an immune response. During this time, they will be infectious to others. After two weeks, the animal will produce BVD antibodies and will be immune to the disease for a period of time.
When a bull is exposed to the disease, he may recover after two weeks, but his semen can remain infected with virus for a further 10-12 weeks. This can result in poor fertility and pose a potential infection risk to the animals he serves.
Very rarely bulls can have infected semen throughout their lives without the virus being present in blood and for this reason it is essential that breeding bulls have their semen tested for BVD as well as having regular blood tests.
However, the main route of BVD spread is through persistently infected (PI) animals.
What are persistently infected animals (PIs)?
A PI calf is produced when a pregnant animal is exposed to the BVD virus in the first 120 days of pregnancy (the first third). The virus can cross the placenta and due to the fact the unborn calf does not have a competent immune system, it remains infected with BVD and is born shedding large amounts of virus. It will then remain persistently infected throughout its life. If infection occurs later in pregnancy, a PI calf will not be produced, but other problems such as abortion or calf deformities may occur.
Early infection of the unborn calf creates added challenges, as it is impossible to identify if a dam is carrying a PI calf. The dam may well have fought off the disease and test antibody positive and virus negative for BVD, but her unborn calf may be virus positive and thus persistently infected, resulting in the production of a “Trojan mother”.
Once born, a PI calf may show signs of poor thrift and some may die. However, a significant proportion will appear normal and live for many years and continue to be a major source of infection. PI calves are also more likely to develop mucosal disease. This results from a PI being “super-infected” with another strain of BVD known as a cytopathic BVD virus. Calves with mucosal disease typically have ulceration of the gut, muzzle and tongue and always die.
As a result, identifying and culling PIs is the main way of controlling BVD spread in any herd. The ultimate aim is stop the cycle of infection, and particularly the exposure of pregnant animals to the disease. Correct biosecurity and checking the status of purchased animals is crucial in reducing the risk of buying Trojan mothers and PI cattle. Avoiding contact with neighbouring cattle over fences is also important, along with vaccination where appropriate.
For more information on establishing BVD status and controlling the disease, see our BVD Control Academy.
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