(Also known as bovine viral diarrhoea virus, or BVD or BVDV, or mucosal disease virus)
Please note: the advice below is for general information only. The issues in managing the risk of bovine pestivirus can be complex and you will need veterinary advice to develop the strategy that best suits your particular circumstances.
What is bovine pestivirus?
As its name indicates, bovine pestivirus affects cattle - both dairy and beef.
Pestivirus is a group (genus) of viruses. It includes not only bovine pestivirus, but also other viruses that cause border disease in sheep and classical swine fever in pigs. Bovine pestivirus infection of or by other species is very rare.
Is it present in Tasmania?
Yes. Test results from the NRE Tas Animal Health Laboratory indicate that pestivirus in cattle may be as widespread in Tasmania as it is on the mainland, where an estimated 70% of herds are actively infected.
While the production losses of pestivirus can be substantial, this will not always be the case. In most cases, the aim should simply be to manage the risk effectively and this can prevent the infection becoming a costly problem.
Why should the pestivirus risk be actively managed?
Pestivirus can cause several problems. By far the most common in Australia are reproductive losses. These can show up as aborted or stillborn calves, deformed calves or simply a failure to conceive.
The productivity losses arising from pestivirus can range from minor to substantial. The key factor is whether the infection causes a "PI" (see below) that can then cause a wider spread of disease across the herd. Where a herd of non-immune and pregnant animals comes into contact with "PIs", the reproductive problems can become major.
Transient and persistent infections - it's important to understand the difference
Where an otherwise healthy animal becomes infected after birth, the infection is usually short-lived and is cleared by the immune system. With such transient infections, the short period of virus shedding (typically around 10 days) and, with Type 1 viruses, the low concentration of virus means that the risk of spreading to other cattle is not high. With Type 2 viruses, the virus shedding period is the same but concentration of virus excreted is much higher so the risk of spread to other cattle is also higher. Outbreaks of clinically severe disease have been seen overseas.
Where a cow becomes infected during the early stages of pregnancy, the foetus does not recognize the virus as being a foreign invader and the foetus does not produce antibodies to the virus. This is because during the early stages of pregnancy, the foetal immune system is still being developed. At this early stage of pregnancy, a foetus that becomes infected with pestivirus does not develop immunity to the virus and therefore if the calf survives (as it does in about 70% of cases), that calf is likely to be a "PI" - that is, it is likely to be persistently infected and thereby a carrier of the virus for the rest of its life.
It is the persistently infected animal, or "PI", that is by far the greater risk in turning a relatively minor pestvirus problem into substantial reproductive losses and other production losses.
What are the signs of pestivirus?
The main problem in combating pestivirus is that, in most cases, there are no signs beyond a small number of animals with ill thrift and/or reproductive failure. As a consequence, the signs are sometimes ignored on the basis that the odd animal not doing very well or having an occasional reproductive problem is normal.
Other signs in cattle that are infected later in life, apart from reproductive losses, are relatively uncommon but include diarrhoea, excessive drooling and breathing difficulties. In even rarer cases, there can be ulcers around the mouth and muzzle and also the feet, so it can look similar to foot and mouth disease. There may even be an occasional death.
The classic sign that an animal is a "PI" is ill thrift - a beast that just doesn't grow as well as its peers - although it is important to stress that some infected animals can grow normally into adulthood. These "PIs" usually die by eighteen months of age.
Testing for pestivirus
How you manage the risk will depend on whether your herd is already affected. The first step is to have your vet investigate any reproductive failures or ill thrift to determine whether pestivirus is the cause. That involves a blood test on a number of young animals.
Managing pestivirus - if your herd is already infected
If the test results are positive, you have various options and you should consult your vet for the best one in your specific circumstances.
A strategy practised in some herds is exposing heifers to known "PIs" well before mating, as this will cause most of them to become infected and thereby develop a long-lasting natural immunity. There are some potential downsides to this strategy. For example, there is no guarantee that all animals exposed will become infected and consequently immune, particularly if a single 'PI" is relied upon to infect a large number of other animals. Typically, an infection and immunity rate of 80% by this method would be considered a good result. Also, the fencing and farm practices must be such that there is no chance of the "PI" inadvertently mixing with cattle in early pregnancy. Another issue to consider is that mixing a possible "PI" with young heifers may interfere with Bovine Johnes Disease control strategies you may already have in place. So you should discuss it with your vet before proceeding.
Culling the 'PIs" and vaccination of the herd is another option. Again there are some potential downsides to this. Vaccination is not 100% effective, so good biosecurity and ongoing monitoring will still need to be maintained. If you decide to vaccinate, the program will need to be ongoing for many years at least. If a vaccination program successfully eradicates pestivirus from the property and the vaccination stops, there will be no immunity and, as a result, the proportion of susceptible animals in the herd will increase over time and the herd will need ongoing management as a closed herd (see below). Again, you should discuss this option with your vet before making a decision.
Managing the pestivirus risk - if your herd is not already infected
If there are no signs of active pestivirus in the herd, you can minimise the risk of infection by keeping a closed herd or checking for signs of pestivirus frequently and having any suspicious cases checked promptly. Even if there are no signs of active pestivirus in the herd, vaccination may still be a good option. Because circumstances vary from property to property, there is no "one size fits all" solution. You should always discuss your particular situation with your vet.
If you use an artificial breeding program
The pestivirus status of both the donor and recipient animals should be identified before proceeding. It is recommended that both the donor and recipient animals should test antibody positive (ie immune) four weeks before the program.
The risk factors
By far the most likely way of infecting cattle is through a PI. This could be a PI animal or a pregnant female carrying a PI foetus. A PI can infect another animal with as little as 1 hour contact time, so events such as sales, shows, other temporary high stocking situations, fence breakthroughs or even over-the-fence contact with neighbouring cattle are risks.
There have been reported cases of infection by contaminated semen and embryos, cattle trucks and even husbandry equipment. But these risks are quite low, compared with the risk via a PI.
Infection via sheep, goats or alpacas is possible but highly unlikely.
Every producer should have a pestivirus risk management strategy that is specific to their particular circumstances. The best person to advise on that is your veterinarian.