Disease Information: Bovine Viral Diarrhoea
What is BVD?
Bovine viral diarrhoea (BVD) is caused by an RNA virus called a Pestivirus. Unlike the name suggests, diarrhoea is not always a major symptom of the disease.
In fact, farmers may not know that their herd is infected as there are often no obvious clinical signs.
According to a survey by the Royal Veterinarian College, BVD has been rated by farmers and vets as by far the most important endemic viral infection in cattle.
It affects immunity - cattle with BVD are more susceptible to respiratory and infectious diseases. Sick calves respond poorly to veterinary treatment. Calves are more prone to scour, pneumonia and mucosal disease, which is fatal.
Effects on Pregnancy
BVD affects fertility - herds infected with BVD see an increased barren cow rate and more unexplained abortions than normal . If the animal is infected in the first 0-1 months of pregnancy, it is likely to cause early embryonic death. In 1-6 months it can lead to foetal death, abortion and mummification.
How BVD is spread
The BVD virus can cross the placenta in a pregnant cow and cause foetal infection, so it is spread from infected dams to their unborn calves. The dam later becomes immune to the virus but if the foetus is infected in the first trimester and doesn’t abort then the virus establishes itself in the calf, no anti-bodies are formed and it becomes persistently infected (PI) with the virus.
PI animals are known as ‘super-shedders’ i.e. they excrete the virus through their skin, urine, milk and saliva so they are highly contagious to any other cattle without immunity that they come into contact with. These animals are the main reservoir for the virus. 1-2% of the national population have been found to be PI animals (Houe 1999).
A significant proportion (28%) of PI animals live to 2 years or more, i.e. into the productive stage of their lifecycle (Booth and Brownlie 2012) and constantly spread infection. A PI dam always produces PI calves.
Why should I make sure my farm is BVD free?
Over 90% of UK herds have or have had BVD. BVD costs the UK cattle industry almost £40 million per year (Bennett & IJpelaar 2003), more than Leptospirosis, Johnes disease or IBK. In a 100 cow suckler herd the estimated economic loss is about £37 per cow per year.
Respiratory and enteric infections can be much worse in animals which also have BVD. When PIs are culled from a herd the incidence of these infections decreases, young calves are generally healthier and farmers thus have to administer less medication.
How can I protect my herd from BVD?
Good vaccines exist but they must be used correctly and dams must be properly protected before they go into pregnancy. RNA viruses are also more likely to mutate, so variations in the virus continue to arise.
There are tests that establish the immune status of dams but not of the calves they are carrying so farmers buying in-calf cows into their herd could be at risk of infecting a herd.
The best way of controlling and eradicating BVD is by diagnostic testing for the disease and subsequent culling of PI animals from the herd. The virus does not survive well in the environment or other species, so PI cattle are the main reservoir for spreading the virus. It is estimated that on average only 0.3%-3% of a herd is PI so the cost of removing them is far out-weighed by the economic advantages gained by removing the source of contagion.
Bio-Genesys Diagnostic Tests
BVD Antigen Test kit / Serum plus
A test kit using skin tissue from ear notches so calves can be tagged and tested from birth. This is the most reliable test for detecting PI animals. If you think the calf may be acutely infected then the test should be repeated 3-4 weeks later to confirm it is not PI.
BVD Antibody Test kit
Used for screening milk and blood samples for BVD antibodies. It is best to test young stock (9-18 months old). Procedures using antibodies test only for exposure to the disease and are generally used to screen for the likelihood of BVD virus on farms. Mostly used for testing bulk milk, it has the advantage of being a quick and cheap screening method but remember, it will only test the milking herd’s exposure (i.e. animals contributing to the tank at the time), and if BVD antibodies are present then a number of longitudinal tests are needed to ensure the levels of antibodies are decreasing with time.