BVD (bovine viral diarrhea) virus is a tricky pathogen that can affect cattle in different ways. It can cause abortion, mummification of a fetus, birth defects, stillborn full-term calves, normal-looking calves with immune deficiencies, and acute or chronic illness. BVD is an indirect cause of many other types of disease because it has adverse effects on the immune system.
Cattle can become exposed to BVD in several ways, and some U.S. studies have estimated that 70 to 90 per cent of infections go undetected, without visible symptoms. The only clue you might have that BVD virus is in your herd may be a poor reproductive rate due to pregnancy losses, or a higher than normal rate of sickness in calves.
Acute infections, in which a naive animal becomes exposed, triggers an immune response. The body fights off the infection and recovers. This form of BVD is not as big a concern as persistent infections in which the animal never gets rid of the virus. PI (persistently infected) calves are the result of the dam coming into contact with a certain type of BVD virus during early pregnancy, before the fetus’ immune system is fully developed. After this fetal infection, the calf is born carrying BVD virus for the rest of its life because it cannot recognize it as foreign and does not mount an attack against it.
Acute infections can raise havoc in a herd, resulting in abortions, sick calves, poor performance such as a drop in milk production, poor weight gain, reproductive inefficiency, and lowered resistance to other diseases. But persistent infection is the silent, sneaky thief. A persistently infected (PI) animal continues to shed the virus and creates a constant source of infection for the rest of the herd.
There are tests a producer can utilize to see if cattle are harbouring BVD virus, which is particularly important for seedstock producers.
There are two kinds of tests for BVD. One looks for the virus itself. The other tests look for antibodies to the virus, to know if the animal has been exposed.
Antibody tests are useful in some circumstances but a positive antibody test just means the animal has been exposed to the virus or has been vaccinated at some point in its life.
This test proved useful as a screening tool with unvaccinated dairy calves in some studies but didn’t work as well in large-scale studies with beef calves.
“This is probably because we don’t have that many unvaccinated animals in beef herds,” says Dr. John Campbell, with the department of large animal clinical sciences at the Western College of Veterinary Medicine in Saskatoon.
“So we mainly just use this test in outbreak situations to try to figure out if the animals have been exposed to the virus. Even then it can be confusing if they were vaccinated. It can be hard to decipher whether the titer is due to vaccination or due to natural exposure.”
The ultimate test is to look for the virus, primarily to find PI animals. “Those are the ones that were infected before they were born, at a certain stage of gestation. Those calves, from the time they are born, are little virus factories, shedding more virus than acutely sick animals. In many outbreaks these PI calves may not be the only source of the virus but they can be the main source,” says Campbell.
“Usually the reason we are doing BVD testing is to try to find those PI animals and get them out of the herd. They excrete large quantities of virus, and the virus test works fairly well to find them. If you test them with the antibody test they may not have a titer, however, because their immune system doesn’t recognize the BVD virus and they may not create antibodies. They may have a slight titer to a different strain of the virus and that doesn’t help us much,” he says.
There are a couple of virus tests. The most common ones require a skin sample, which could be from the ear.
“An ear notch is easy to do, and I often use a brisket punch to get a sample from the ear,” says Campbell.
“The test used at the lab here in Saskatchewan is immunohistochemistry which is a staining test. We fix the tissue sample in formalin and then stain it with a special stain that has antibodies attached to it. The sample actually lights up under the microscope when it has the virus in it — and this makes it easy to see,” he says.
If the virus is in the skin sample, this is a good indication the animal is probably persistently infected and it’s not just a transient infection. “The transient infections don’t get high quantities of virus in the skin,” says Campbell.
Some labs use an antigen capture ELISA test. “For that one they use fresh tissue rather than fixed tissue, so you don’t put it in formalin. It’s a slightly different test, measuring the virus in a slightly different way. It doesn’t have to go under the microscope and it uses different technology,” he says.
The fresh samples can be collected (such as a skin sample from each new calf at birth), frozen and shipped in a single batch. Then the lab can do either test.
“At our lab there is some benefit in testing multiple samples at a time because it is cheaper. A number of samples can be pooled on one slide for the immunohistochemistry test. A slide costs approximately $50 so it is cheaper per sample to pool them. With the antigen capture ELISA test they put the skin sample in saline fluid and let it sit there awhile, and then analyze the fluid for the virus.” Some labs can put multiple skin samples in the same vial/fluid and test them all together. If there’s a positive, however, then they have to retest the individual samples to identify which one it is.
With the immunohistochemistry test, individual samples are identified before they are put on the slide so they don’t have to retest if one or two show up positive on that slide.
“Check with your local laboratory to see if there is a possibility of pooling the antigen capture ELISA test samples,” says Campbell.
The virus can be identified from a blood sample, but the disadvantage is that you should probably retest that animal in a week or two if it shows up positive to be sure it’s not a transient infection. “We can’t be sure that this animal is persistently infected, with just one test,” says Campbell.
With a transient infection the virus is in the blood for a short period, but if that’s when you took the sample you might think it was a PI animal.
When PI animals are found, they should be removed from the herd, before breeding season. “You don’t want animals shedding the virus when you’ve got cows in early gestation, since that’s the biggest risk. If the cows are well vaccinated, the risk is much less, but the vaccines — even though they are good — are not 100 per cent protective. That’s the main reason for testing and trying to get rid of any PI cattle,” says Campbell.
There may be a few cows in the herd that for some reason don’t mount a good immunity when vaccinated, or you miss some, or the vaccine isn’t given appropriately. If the cow jumps when you administer the vaccine and some of it doesn’t get into the cow, she may not have enough of it to be effective. And you want every cow protected.
“In a dairy herd there might be the odd cow that encounters the virus at the right stage of gestation to have the fetus affected, but there’s more risk in a beef herd where we are calving and breeding them all in a short period of time. There could be a lot of cows at that vulnerable stage of gestation, and this could create a major problem. So it’s very important to vaccinate and also have good biosecurity to make sure you don’t get BVD coming into your herd in the first place,” he says.
If you do suspect you have BVD, testing is recommended. Even if you never bring new cattle into your herd, your cows could be exposed through fenceline contact with neighbouring cattle.
“If your herd is well vaccinated the risk is less, but if you have an abortion problem or something suspicious (like a bunch of calves that are sick and not improving) or evidence of BVD on a post-mortem examination, then it’s wise to test those animals,” Campbell says.
Seedstock producers may want to test, simply to make sure that cattle they are offering for sale are free of BVD. “There is more tendency today to test and prove that yearling bulls are negative, and it’s also a good idea to test replacement heifers,” he says.
“The main thing I try to get people to understand is that we are trying to protect the fetus. If a bull or a cow gets BVD they might be sick for a few days but unless it’s a violent strain they usually recover. The main concern with BVD is to protect the fetus from infection at the wrong time, and to prevent PI calves. So we try to protect the fetus in two ways — by vaccinating the cow as close to early gestation as we can, so her immunity is high at the proper time, and trying to prevent exposure via biosecurity and maybe testing if there may have been some exposure.”
The recommendation is to test young stock, and if you get a positive, you should test the dam. “A person could spend a lot of money testing the whole herd looking for the odd older cow that is PI because that doesn’t happen very often. So we usually just suggest testing young stock, or a dead calf, and then test their dams if they turn up positive. This can save a lot of time and money,” he says.