BVD Tests
Unlike many other cattle diseases, the tests we have for bovine viral diarrhoea (BVD) are highly accurate for detecting infected animals and infected herds. It’s never been easier to know your herd status and take action to remove persistently infected (PI) cattle.
What are the basic types of BVD tests?
The diagnostic tests for BVD can be divided into two main categories based on what they are looking for:
1. Tests that look for antibodies against the virus (Antibody ELISA)
These tests can be done on blood samples or milk samples to look for the presence of antibodies against the BVD virus. We can’t screen animals less than 10 months of age or vaccinated animals because the test can’t tell the difference between antibodies produced by natural infections with BVD from antibodies the animal may have received through colostrum or from antibodies the animal may have produced in response to the vaccine.
A positive test result in an unvaccinated animal over 10 months of age means that the animal has previously been infected with BVD and developed an immune response against the virus. Persistently infected (PI) animals will generally be negative for antibodies to BVD since the reason that they are persistently infected with the virus is that they are unable to mount an immune response against the virus. However, some PI calves may test positive for BVD antibodies if they received high levels of antibodies from their dam through colostrum.
2. Tests that look for the virus itself (Antigen ELISA or PCR)
These tests can be conducted on blood samples, ear notch samples, or milk samples and look for the presence of the BVD virus itself. The antigen ELISA test uses special antibodies that bind with unique proteins on the surface of the BVD virus whereas the PCR test looks for the unique genetic material of the BVD virus.
A positive test means that the animal is currently infected with BVD and actively shedding the virus. Some tests can measure the amount of virus that the animal is shedding, which can help us tell the difference between animals that are transiently infected (low shedders) and persistently infected (high shedders). However, you can have some TI animals that are high shedders and the only way to confirm an animal’s status is to test them again 3 weeks later to see if the virus is still present. PI animals will still be shedding BVD whereas TI animals will have recovered and become negative after 3 weeks. If the animal tests negative for the virus, it can be certified as being non-PI for life.
How do I screen my herd for BVD?
If your herd has never been tested for BVD before and you have mobs that are not currently vaccinated against BVD, the easiest way to find out your herd’s BVD status is through a simple screening test that measures BVD antibody levels. If there has been a PI animal in the tested mob, we would expect a large number of cattle to have recovered from infection and have high antibody levels against the virus.
Pooled Serum Antibody ELISA
For both beef herds and dairy herds, you can take blood samples from 10 to 15 randomly selected animals in each separately managed mob (defined as groups of cattle that have direct nose-to-nose contact with each other) and submit them for a pooled serum antibody ELISA to measure antibody levels. The animals should be over 10 months of age and unvaccinated to prevent false positive results. This test can be run any time the animals are already being yarded for other routine management events.
The test results are reported as sample to positive (S/P) ratios with the following interpretation:
Negative (S/P < 0.17)
Animals in the group are unlikely to have been exposed to BVD and it is very unlikely that there is a PI animal in the group. No further investigation is required at this time.
Suspect (S/P 0.17 to 0.75)
Animals in the group have been exposed to BVD, but at lower contact rates. This could be from (1) contact with a PI animal over fenceline boundaries, (2) a PI animal that was previously present in the group but subsequently died or was culled, (3) a PI animal that was only recently introduced into the group and has not yet had time to infect many other animals, or (4) a PI animal present in a group that is grazed extensively with low contact rates between animals. Conducting a PI hunt can help to confirm that there is no PI animal in the group.
Positive (S/P > 0.75)
It is highly likely that a PI animal is currently present or has very recently been present in this group. If these animals are being kept for breeding, it is highly recommended to conduct a PI hunt to make sure that any PI animals are removed before they cause more damage to the herd.
Bulk Milk Antibody ELISA
For dairy herds, you can easily request a bulk milk BVD antibody ELISA test to check the level of immunity against BVD in your milking herd. This test is not reliable for herds that vaccinate against BVD since this can cause antibody levels to be high in the absence of active BVD infections in the herd. Although this test can be done any time during the milking season, it is best to do this after calving has finished in case there were any PI animals born into the herd. Some herds will also test late in the season to look for signs that cattle may have been exposed to BVD during pregnancy.
The test results are also reported as sample to positive (S/P) ratios with the following interpretation:
Negative (S/P < 0.25)
Low (0.25 < S/P ≤ 0.5)
Moderate (0.5 < S/P ≤ 0.75)
High (0.75 < S/P ≤ 1.0)
Very high (S/P > 1.0)
Dairy herds with bulk milk S/P ratios > 0.75 are considered highly likely to currently have at least one PI animal (or have recently had at least one PI animal) amongst the milking cows, replacement heifers, or calves. However, it can take several years for bulk milk BVD antibody levels to drop after the PI animals have been removed. If the milking herd is no longer being exposed to the BVD virus either through natural infection or vaccination, we would the expect S/P ratios to decrease linearly over time. Sudden increases in the S/P ratios could mean that BVD has gotten back into the herd.
The most important thing to remember about BVD screening tests is that your status can change over time.
Although your herd may not have any PI animals today, your herd status can easily change if BVD gets back in through the birth of PI calves or buying-in PI replacement animals. That’s why it is important to keep monitoring antibody levels in your youngstock and/or bulk milk every year to look for evidence that your cattle have been exposed to BVD. Given how easily BVD can spread between herds in New Zealand through animal movements and contact with neighbouring cattle, it is also recommended to test your replacement heifer calves each year to make sure that none of them are PI animals.
What is a PI Hunt?
If your cattle have high levels of antibodies against BVD, there is a very strong chance that they are either currently in contact with (or have recently been in contact with) at least one PI animal. The next question becomes whether that PI animal is still present in your herd and/or whether that exposure was from contact with PI animals from other herds. The process of testing individual animals in a herd to find PI animals is called a PI hunt. Since there are several different ways that a PI hunt can be done,it is important to talk with your veterinarian about the best approach for your herd. It is also important to remember that pregnant cattle may be carrying PI fetuses (Trojan Dams) and so the herd cannot be declared PI-free until these calves have been tested after birth.
Individual Animal Antigen ELISA or PCR
Compared with many other infectious diseases, the diagnostic tests to find animals that are actively shedding BVD virus (antigen ELISA or PCR) are highly accurate and will detect almost 100% of PI animals with a single test as long as the correct procedures are followed. However, it is possible for some errors to occur when testing thousands of cattle across New Zealand each year and there may be a very small number of PI animals that are missed. Calves can be tested for BVD almost immediately after birth and many farmers find it convenient to collect ear notch samples on their replacement heifers at the same time they’re placing the NAIT tag or performing disbudding. Once an animal has tested negative for BVD virus, it can be certified as non-PI for life and does not need to be tested again.
Bulk Milk PCR
For dairy herds, bulk milk PCR is a convenient and inexpensive way to screen your entire milking herd for PI animals with a single sample and a single test. The test is sensitive enough to detect a single PI animal in milking herds with more than 1000 cows and is not affected by vaccination against BVD. If you are already performing routine herd testing to monitor lactation performance of milking animals, a convenient time to run a bulk milk PCR for BVD is on one of your regularly scheduled test dates since this will give you a more accurate record of animals that were contributing to the bulk milk tank and you already have individual milk samples collected from each animal in case the results come back positive. Some diagnostic laboratories offer discounted prices for BVD tests that are bundled with routine herd tests.
A positive result on bulk milk PCR means that there was at least one PI animal contributing to the bulk milk tank on the day of testing and means that you need to follow up by testing individual animals to figure out which one(s) are PI animals. Since PI animals are often among the low producers in the herd, some farmers will start by screening the bottom 10 to 20% of the herd and then working upwards until the bulk milk PCR comes back negative. Other farmers will test the entire milking herd at once to more quickly identify the PI animals and get a verified non-PI status for the remaining animals in the milking herd.
It is important to keep track of which animals were not contributing to the bulk milk tank on the test date and test these animals individually to make sure that they are not PI animals. It’s also important to remember that your milking herd can be negative for BVD, but you may still have PI animals in your replacement heifer mobs that can cause a new outbreak when they join the milking herd.
What is the “Diagnostic Gap”?
The "diagnostic gap” refers to the potential problem with using antigen ELISA to test calves under 35 days of age for BVD virus. Calves that receive an adequate dose of high quality colostrum from dams that are immune to BVD can have high levels of maternal antibodies circulating in their blood for several days to several weeks after births. Even in PI animals that produce large quantities of virus, there may be enough antibodies to bind most virus particles and prevent the test components from working correctly. The risk of false negative results (missing potential PI animals) can be minimised by only using ear notch samples instead of blood to perform antigen ELISA on calves under 35 days of age. It is important to note that the PCR test can be performed on any sample (blood, ear notch, or milk) at any time after birth since it looks directly for the presence of viral genetic material.
Check your understanding
Answer the questions below to check how well you understand the basics of BVD Tests.
Check Answer
The correct answer is (a).
The BVD antibody ELISA tests look for evidence that an animal has mounted an immune response against BVD. Maternal antibodies from colostrum can also be picked up on the BVD antibody ELISA test and bind to virus particles in the calf’s blood stream interfering with the BVD antigen ELISA tests.
Check Answer
The correct answer is (c).
This is a high S/P ratio indicating that the antibody levels against BVD are very high. Assuming that the test was not perfomed on young animals or vaccinated animals, the results mean that the animals have more than likely had very close contact with a persistently infected animal.
Check Answer
The correct answer is (c).
An animal that tests positive for BVD may be either transiently infected or persistently infected and should be re-tested in 3 to 4 weeks. Animals that are transiently infected will no longer be shedding the virus at this time.
Check Answer
The correct answer is (a).
The bulk milk PCR test will only screen animals that were contributing to the bulk milk tank on test date. Dry stock (including replacement heifers and animals yet to calve) as well as animals that were separated from the milking herd for illness or medication withdrawal times may be persistently infected.
Check Answer
The correct answer is (d).
A lot of farmers forget that unborn fetuses are also an important stock class on farm. If the PI hunt was conducted at weaning when most dams were pregnant, it is possible that they are carrying PI calves which will re-introduce BVD to the herd through birth the following calving season.