Knowing your BVD status is important for making smart decisions around BVD prevention and control. Because the effects of BVD can look similar to many other diseases, you may not realize that your herd is infected unless you do diagnostic testing. However, you should be suspicious of a problem with BVD if you have a higher empty rate than expected or if you have many calves that are smaller or more ill-thrifty than their herd mates. These calves could be PIs or they could be calves that were born weak from exposure to BVD after 180 days into pregnancy.
Types of Tests
There are two main types of diagnostic testing for BVD:
- Antibody tests (Antibody ELISA): These tests can be conducted on blood samples or milk samples and look for the presence of antibodies to BVD. 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 maternal antibodies or antibodies produced in response to the vaccine. A positive test 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 they are persistently infected with the virus and unable to mount an immune response against it. However when testing young animals, PIs can often test positive for BVD antibodies due to circulating maternal antibodies obtained through colostrum.
- Virus tests (Antigen ELISA or PCR): These tests can be conducted on blood samples, skin samples, or milk samples and look for the presence of the BVD virus itself. A positive test means that the animal is currently infected with BVD and actively shedding the virus. However, we can’t accurately tell if the animal is PI or transiently infected (TI) without re-testing in 3 weeks to see if the virus is still present. PI animals will still be shedding BVD in 3 weeks whereas TI animals will become negative. A negative test means that the animal is definitively not PI.
Compared with other diseases, the diagnostic tests for BVD are highly accurate with the sensitivity (probability that an infected animal will test positive) and specificity (probability that an uninfected animal will test negative) ranging from 98% to 100%. However, they are not completely perfect and there is still a very small chance that animals will be misclassified.
Screening Herds for BVD
If your herd has never been tested for BVD before and you do not currently vaccinate heifers or cows for BVD, the best place to start is with a simple screening test to look for antibodies against the virus. If your herd is managed as separate mobs, you may need to conduct screening tests on each individual mob, especially if there is not a lot contact between the different groups.
- Bulk Milk Antibody Test: For dairy herds, you can order a bulk milk BVD antibody test through LIC or through your veterinarian. Although this test can be done at any time during the lactation period, it is best to do this after the calving season has ended in case there were any PI animals born into the herd.
- Testing Youngstock: For dairy herds and beef herds, you can take blood samples from 15 randomly selected heifers between 10 and 18 months of age and submit them for an antibody ELISA test either on the 15 individual samples or on the pooled sample. You can do this anytime when the heifers are already being yarded for other routine management events.
The results for both tests are reported back as an S/P ratio, which measures the level of antibodies in the sample.
- Negative: this means that it is unlikely that your herd is infected with BVD. Talk with your veterinarian to develop a plan for keeping BVD out of your herd.
- Exposed: this means that your herd has likely been exposed to BVD in the past, but it is unknown whether PI animals are currently present in the herd.
- Positive: this means that your herd is likely actively infected with BVD and there is a good chance that PI animals are present.
If your herd comes back as exposed or positive, you will need to perform follow-up testing to confirm whether BVD is actively circulating. Also remember that just because the herd comes back negative on a screening test now doesn’t mean that there aren’t Trojan dams carrying PI calves that could cause a serious outbreak in the herds during the next calving season. For that reason, it is important to conduct annual screening to monitor changes in your herd’s BVD status over time.
The only way to confirm an exposed or positive status is by testing blood, skin, or milk samples from every animal on the farm to check if they are actively shedding the BVD virus. For dairy herds, it is possible to conduct a bulk milk virus test to determine if any animals contributing to the tank are shedding the virus. However, you will still need to screen any youngstock on the farm as well as animals that are not currently lactating or animals that have their milk withheld from the tank during veterinary treatment. Also remember that any pregnant cattle may be carrying PI calves and the herd cannot be declared BVD Free until these calves have been tested after birth. Many farmers choose to screen calves at the same time as placing ear tags since there are devices that can collect an ear notch skin sample for testing at the same time as the tag is placed.