BVD

• When cattle with no previous immunity against BVD (susceptible animals) become exposed to the virus, they develop short-lived infections that last for a period of 2 to 3 weeks. These animals are referred to as Transient Infected or TI’s for short.
• TI cattle will have a period of fever, inappetence, diarrhoea (hence the name of the virus!) reduced milk production, lowered growth rates, they may also have respiratory signs. While they are actively infected they are also more susceptible to other infections such as IBR, mastitis and Salmonella as their immune systems are weakened. Even after recovery from initial infection animals which continue to be exposed will suffer from slower growth rates and reduced milk production due to the energy costs of suppressing ongoing exposure/ infection with the virus.
• Animals will usually start producing immunity to BVD within a few weeks which clears the virus from their system. Once animals have recovered from natural exposure, they normally have a strong immunity to BVD which will last for a number of years.
• Calves that are born to cows that have either recovered from a natural BVD infection or have recently been vaccinated against BVD get protective immunity through colostrum (maternal antibodies), which can stay in the blood for anywhere from 3 to 10 months. After this time, the calves become susceptible to infections with the virus.

What is happens to a foetus that gets infected with the virus?

The biggest impacts of BVD occur when a susceptible cow is infected whilst she is pregnant. This is because the virus can cross the placenta to infect the developing foetus. Depending on what stage of pregnancy the cow is in, the virus will have different effects on the developing foetus:

• Early pregnancy (days 0-40): Infection at this stage may result in early embryonic death. It also reduces conception rates and therefore increases the empty rates you will see on your farm.
• Mid pregnancy (Days 40-120): Infections at this stage most often result in abortion. However, if the foetus survives, the calf’s immune system will develop to not recognise the virus and so does not produce immunity to clear it. These calves are born persistently infected (PI) with BVD.
• Late pregnancy (Days 120 to 280): Infections at this stage can result in calves being born with birth defects such as small eyes, cataracts and neurological disease. Abortions can still occur at this stage. Normal-looking calves may be PI or may be born with an active and effective immune response to the virus.

What is a persistently infected calf?

Persistently infected (PI) calves as mentioned above, result when unborn calves are infected between days 40-180 days gestation. These cattle cause the largest impact in herds as they are constantly shedding large amounts of BVD. They can easily infect hundreds of cattle in their lifespan, enabling the virus to stick around for a long time in a herd.

Due to viral damage to their immune system, they are much more likely to get sick from other diseases such as lameness, pneumonia and parasitism, and are often much smaller than their herd mates. However, some PI animals can look completely normal which means it can be tricky to identify them in a herd without testing individual animals.

These animals are often culled due to poor performance or die due to the development of Mucosal disease in their first two years of life. Mucosal disease occurs when these PI calves are infected with a different strain of BVD (sometimes self-infected following a mutation of the virus!), and they develop severe fever, lethargy, diarrhoea and ulcers that rapidly lead to death.

Any PI heifers that survive long enough to calve will always give birth to a PI calf, continuing BVD transmission within the herd.

How do we diagnose a BVD infection?

We have a full range of BVD testing options available, but they can be split into two main groups; either testing to see if your animals have been exposed to the BVD virus or tests which look for the presence of virus itself in the sample taken.

If your stock have never been tested for BVD the simplest test is to measure the BVD antibody levels (exposure testing). This is done either through blood testing a small group of animals or, if possible, taking a milk sample from the vat.

When a milk sample isn’t available (young stock, bulls and beef animals) 10-15 blood samples are taken from randomly selected animals in the herd (or mobs of animals if testing different stock classes). Animals should be over 10 months of age and unvaccinated to prevent a false positive.
In dairy herds, a bulk milk BVD test can be used to check the level of exposure to BVD. It can be done anytime during the milking season but is best to do this test after calving to ensure all animals will be present.

If these screening tests reveal that your cattle have been exposed to the BVD virus, it can indicate that animals in the herd are either currently infected, or have been in the recent past. The next recommended step in these cases is to perform a PI hunt.

A PI hunt involves testing individual animals within the herd in search for the virus (antigen) which would indicate a PI animal. Persistently infected animals (PI’s) will be negative for antibodies to BVD since they are unable to mount an immune response but will show positive for the virus on antigen testing.

How can BVD get into my herd?

For a herd to be actively infected with BVD, a PI must be present somewhere in the herd which may be amongst the calves, replacement heifers, mixed-age cows, bulls and/or fetuses.

There are three big risk factors for BVD outbreaks:

1. Bringing IN new cattle to your herd. The biggest risk is purchasing replacement heifers, cows, or bulls with unknown BVD status. Another major risk is purchasing pregnant non-PI cattle that could be carrying PI calves after becoming infected with BVD during early pregnancy (referred to as “Trojan Cows”).
2. Moving cattle ON and OFF farm. Replacement heifers are particularly vulnerable animals on your farm as they are the least likely to have immunity against BVD and are most likely to get exposed when being sent offsite for rearing.
3. Allowing nose-to-nose contact OVER fence lines. Fence boundaries that allow nose-to-nose contact with neighbouring stock or if cattle routinely escape from pasture, significantly increase the risk of being exposed to the BVD virus.

How do we control BVD?

BVD control hinges on two key points:

1. Identifying and eliminating PI animals –Test and Cull
2. Preventing new PI’s from being produced – Biosecurity and Vaccinate

A farm doesn’t necessarily have to be doing both targeted culling and vaccination to have effective BVD control, but without one or the other, any efforts are reduced to monitoring BVD as it infects your stock.

The most appropriate and cost-effective BVD control plan is as varied as our clients’ farming operations. There is no one ‘perfect’ BVD control strategy for all farms. The best plan is formed by consultation with your vet, this will allow you to develop a plan which fits your farming system, budget and desired outcomes.