FAQs

BVD is very effective at spreading and persisting in cattle populations because of the small number of persistently infected (PI) animals that are constantly shedding large amounts of virus and infecting most susceptible animals they come in contact with over their lifetime. Most cattle farms in New Zealand have breaks in biosecurity which makes it very easy for BVD to get into the herd. Once you have a PI in the herd, the virus will usually not go away unless you perform testing and/or vaccination to eliminate PI animals. A lot of farmers still aren’t testing their herd every year to keep track of their BVD status to make informed decision about managing their risks (negative herds) and eliminating BVD (positive herds) so the virus continues to have a major impact on animal health and production.

No. Animals can only become persistently infected before birth if the dam gets infected with BVD and the virus crosses the placenta to infect the fetus before it has developed a working immune system. If animal was not PI at birth, there is no possible way for it to become PI in the future. Animals that are infected with BVD after birth will only develop a transient infection that lasts about 2 to 3 weeks before they fully recover and stop shedding the virus. As long as the animal has a healthy immune system, it is unlikely that they will get infected with BVD again in the future.

Yes. The BVD virus can infect sheep, goats, deer, bison, llamas, alpacas, and pigs causing similar problems with reproductive performance and youngstock health. However, because the virus is not really well suited to living in these species, it doesn’t spread as effectively and the evidence suggests that there is a low risk that they will transmit BVD back to cattle populations. Keeping BVD out of your cattle herd is the best way to keep BVD out of animal populations on your farm.

No. There is no known health risk of BVD to people.

BVD is a relatively fragile virus that is not able to survive outside an animal for very long periods. It is likely that most virus particles in the farm environment will be inactive within 3 to 4 weeks although the virus may survive longer in damp, cold, and dark conditions. It important to clean and disinfect any equipment, vehicles, and clothing that have been in contact with cattle from other farms.

Not necessarily. Always ask to see a signed veterinary certificate confirming that each individual animal tested negative for BVD and/or were administered the vaccine series according to the manufacturer’s label. There have been many cases where farmers purchased animals that were supposedly BVD free only to find out later that they were PI after they starting developing illnesses or were told the cattle were fully vaccinated when they had only received one of the two required doses to give full immunity. It’s also possible for unvaccinated cattle to become transiently infected with BVD before arriving on farm, which is why it’s important to quarantine them from the herd for 14 days.

The risk of your herd getting infected with BVD depends on a number of factors including how well you manage biosecurity at your farm boundaries, whether or not you vaccinate animals that are at risk, and the BVD status of any cattle farms your herd may be in direct or indirect contact with through the movements of animals, personnel, and equipment.

Based on the bulk milk testing data from LIC, about 5% of dairy herds (1 out of every 20 herds) that are negative for the virus on bulk milk PCR during one season will have a positive bulk milk PCR the season. Most beef farmers are not screening their herds for BVD every year and so we don’t have good information on their risk of becoming infected, but based on data from the BVD Free field studies, it could be at least 10% (1 in every 10 herds).

Trojan dams are pregnant cattle that are carrying PI fetuses from have been infected with BVD during early-to-mid pregnancy. These animals will give birth to a PI calf, which will bring BVD into your herd. There are currently no reliable diagnostic tests that can be performed on pregnant cattle to tell if they are carrying a PI calf. It’s best not to purchase pregnant cattle, but if you can’t avoid it then you can minimise the risks by:

1. Only purchasing pregnant dams from BVD negative herds that have good control programmes in place.

2. Testing their calves for BVD as soon as possible after birth to make sure that none of them are PI.

The clinical signs of BVD are not always obvious and can look like a number of other diseases or management issues. Previous studies have shown that only about 1 in every 4 farmers with infected cattle are aware that BVD is causing problems in their herds. Some things you may notice in your calf crop are low birth weights, birth defects (cleft palate, misshapen eyes, swollen heads), poor growth rates leading to uneven slaughter weights, an increase in other infectious respiratory and diarrhoeal diseases, and sudden deaths. In your breeding herd, you may notice poor fertility, higher returns to service, abortions, higher empty rates, and fewer calves born.

The best thing to do if you are unsure about your herd’s current BVD status is to talk to your veterinarian about running a screening test to check your cattle for antibodies against BVD, which can tell us if they have recently been infected with the virus. Knowing your BVD status can help you make more informed decisions about managing risks to your cattle.

Most PI animals will die before they reach slaughter age and those that do survive usually don’t a good slaughter weight. While this animal is on your farm, it will also be spreading BVD to other animals. You will usually end up losing a lot more money by keeping the PI around. It’s always best to send it immediately to slaughter.

Calves can be tested for BVD virus almost immediately after birth. If you are submitting tissue samples for calves under 35 days of age, then you can use either PCR or antigen ELISA as the diagnostic test. If you are submitting blood samples for calves under 35 days of age, then you should only use PCR to prevent interference from antibodies they got through colostrum.

Both the PCR and antigen ELISA tests are reasonably good (but not perfect) at telling the difference between PI animals and TI animals based on the amount of virus present in the sample. If the test results come back high positive, then the animal is most likely PI especially if it also looks stunted and ill-thrifty. If the test results are low positive, there is a greater chance that it may be TI and so you may want to wait for the confirmation test before slaughtering. If you are going to wait for the confirmation test, it is important to keep the animal quarantined from others so it doesn’t keep spreading the virus.

No. Animals can only become persistently infected before birth if the dam gets infected with BVD and the virus crosses the placenta to infect the fetus before it has developed a working immune system. If animal was not PI at birth, there is no possible way for it to become PI in the future. However, there is still a chance that it can become transiently infected with BVD and test positive for the virus. Additionally, although the diagnostic tests for BVD are very accurate in detecting PI animals, there is a very small risk when thousands of animals across the country are being tested that a few may test falsely negative the first time around.

An animal should not be certified as being non-PI unless it was individually tested for the virus using a blood, tissue, or milk sample and the results came back negative. There are too many potential errors with correctly matching calves to their dams even when parentage testing is performed. Any cows from the breeding herd or milking herd that are sold to other herds as replacements should be individually tested prior to sale.

Not necessarily since both transiently and persistently infected dams can give birth to PI calves. Dams of PI calves should be tested individually to make sure that they are not PI.

An animal should not be certified as being non-PI unless it was individually tested for the virus using a blood, tissue, or milk sample and the results came back negative. There are too many potential errors with correctly identifying which cattle contributed to the tank on the test date. Any cows from a milking herd that are sold to other herds as replacements should be individually tested prior to sale.

Not necessarily. If the youngstock antibody screening test is negative, that means it’s very unlikely that there were PI animals within or in contact with the group of cattle you sampled. However, there could still be PI animals in other mobs.

The current diagnostic tests we have to measure BVD antibody levels in blood or milk cannot tell the difference between antibodies that were produced in response to a natural infection with BVD from antibodies that were produced in response to vaccination. For this reason, the test results from bulk milk antibody ELISA and pooled serum antibody ELISA are not reliable when used in herds that vaccinate the milking herd or young replacement heifers. Vaccination does not have any impact on the antigen ELISA or PCR tests used to screen animals for active infections with the virus.

Using PI animals as “natural vaccinators” (i.e. infecting cattle so that they develop a natural and lifelong immunity to BVD) is a bad idea for several reasons:

1. In many herds, especially extensive beef herds, there may not be enough contact between animals to ensure that all susceptible breeding animals are exposed to BVD prior to the breeding season. This puts animals at risk of getting infected with BVD during the mating period when they are at greatest risk of having reproductive complications.

2. Animals that become transiently infected with BVD still experience decreased milk yields, slower growth rates, and increased susceptibility to other costly infectious diseases. These costs alone may be enough to offset the expense of vaccination.

3. As long as your herd has PI animals, you are at risk of infecting other herds that purchase your cattle, share grazing pastures, have nose-to-nose contact over fences, or come in contact with contaminated vehicles, clothing, and equipment.

Your pregnant dams are at the greatest risk of having reproductive problems or generating PI calves from transient BVD infections in the earlier stages of pregnancy. Their immune system will have the strongest response against the virus within 3 to 4 weeks after the last BVD vaccine dose (for animals that are being vaccinated for the first time) or booster dose (for animals that have previously been vaccinated). By giving it 4 weeks prior to mating, we are ensuring that animals are maximally protected when they need it most.

There are a number of reasons why you might still have PI animals in your herd even though you have a BVD vaccination programme in place:

1. There are PI dams still present in your herd: These animals will always give birth to PI calves so unless they are removed, you will almost always get new PI calves.

2. You have a long mating period: If you finished the vaccine series 4 weeks prior to mating according to the label directions you may still have cattle that aren’t getting pregnant until 3 to 4 months later depending on the length of the mating period. This means that the protective immunity from the vaccine may have dropped by the time they reach the risk period for creating PI calves.

3. The vaccine wasn’t stored or administered according to the label directions: Not storing the vaccine in appropriate conditions, giving the wrong volume of vaccine, giving the vaccines at the wrong time, or not completing the booster series are common and preventable causes.

4. They were exposed to more virus than their immune system could handle: Sometimes when animals are exposed to large quantities of virus or they have a compromised immune system, they may not be able to fight off the infection.

5. Some animals weren’t vaccinated:
It’s always possible that some animals were missed when the vaccines were administered.

6. You purchased ‘Trojan dams’ as replacements**
If you purchased pregnant animals as replacements, they may have already been carrying a PI calf.

Although vaccination will help prevent the birth of new PI calves from dams with no previous immunity to BVD, it doesn’t deal with any existing PI animals in the herd. These PI animals will continue spreading BVD to other susceptible cattle and will always give birth to other PI calves, which can keep the BVD cycle going in your herd for many years.

With so many cattle herds in New Zealand currently infected with BVD, there is an ongoing risk that your cows or replacement heifers will be exposed to BVD during pregnancy and give birth to PI calves. If only a small number of cattle were exposed to BVD, the antibody levels may not have increased enough to show up in the bulk milk tank or in the older heifers that were screened through the pooled antibody ELISA. Testing your replacement heifer calves to make sure they are non-PI is the safest way to make sure you don’t accidentally end up with more PI animals in your herd.

Based on a recent survey, less than 5% of cattle herds in New Zealand are completely closed off from contacts with outside herds and so there is ongoing risk that your herd may be exposed to BVD. Monitoring trends in the BVD antibody levels provides extra reassurance that haven’t been any major breaches in your biosecurity.

Not necessarily. Approximately 90% of PI calves are born to mothers that are not themselves PI, but instead have undergone a transient infection in the early part of pregnancy. Testing of successive generations of calves provides ongoing assurance that the herd has not been exposed and further PI calves created.

Doing something is always a better option than doing nothing when it comes to BVD control. The more farmers that take responsibility to get rid of BVD from their herds, the less risk there is for everyone of BVD getting back in.

If your milking herd came up positive for BVD on bulk milk PCR and you got rid of the PI animal(s), there is still a chance that pregnant dams were exposed to the virus during the risk period for creating new PI calves. If they give birth to a female PI that is kept as a replacement heifer, it will take about 2 to 3 years for that animal to join the milking herd depending on what age you calve heifers.