Importance of BVD

BVDV (Bovine Viral Diarrhea Virus) is a nasty pathogen that can affect cattle in many ways, often mimicking other diseases. This makes it difficult to diagnose without tests. BVDV can cause abortion, fetal mummification, birth defects and malformations, stillborn full-term calves, calves with immune deficiencies (persistently infected), acute or chronic illness with a variety of signs. This virus is also an indirect cause of many other diseases because it impacts the immune system. You may see animals with pinkeye, scours, etc. and not realize that the underlying cause is BVDV, which lowered the animal’s defenses and allowed another infection to get started.

Dr. Chris Chase, South Dakota State University, says BVDV continues to be a chronic problem in cattle, partly because some calves are born with this disease and are persistently infected; if the fetus becomes infected in utero (via infection of the dam at a certain stage of pregnancy) this is a life-long infection. “Most persistently infected (PI) calves don’t live long enough to become adults, but some do. They continually shed the virus and are super-shedders,” he says. PI animals are a source of infection for other animals in the herd. Even if they die as calves or yearlings, they have probably spread the virus to many other cattle by then.

PI calves are the Trojan horse of the beef industry, sneaking BVD into unsuspecting cattle operations. “This virus can cause immune dysfunction; some strains can cause the immune system to overreact when cattle are stressed, such as at weaning. If calves are subjected to stress and have BVD at the same time, it makes them much more susceptible to any respiratory diseases, coccidiosis, or whatever else they might be exposed to,” says Chase.

During the past 20 years the beef industry has tried to educate producers about the importance of vaccinating and testing for BVD but it’s still not very well controlled. “Even if your cattle are vaccinated, it won’t always protect them. If some get stressed, their immune response to pathogens may not be adequate. If they have close contact with a PI animal on the day they are stressed, they may become infected.”

Dr. Cheryl Waldner, University of Saskatchewan, says that to control BVD, we need to prevent the birth of persistently= infected (PI) calves that result from fetal infection in early pregnancy. Any cow, calf or bull can get an acute BVD infection, similar to the way a human gets a cold or influenza and then transmit it to other individuals. Risk for transmission from an acutely infected animal is present for a relatively short time, however (until that animal starts to recover), though in some cases bulls can harbor the infection for longer periods in their testes.

“The virus can be transmitted from these acute cases and infect other cattle, but most transmission in a herd is associated with exposure to persistently infected animals. These PI animals are life-long virus shedders that produce large amounts of the virus, forever,” says Waldner.

Chase says it’s important to find out what’s actually going on in your herd, with testing. “Doing some blood tests in the cows and measuring antibodies can reveal their immune status. With many diseases, antibody levels are usually not a good indicator of whether or not a herd has been infected, but with BVD it is. Antibody levels in an infected animal are much higher than what we’d see with an animal that has been vaccinated,” he says. Taking a few blood samples now and then can be a way to monitor BVD status of a herd.

If your cattle come into contact with other cattle, such as in a community pasture, Chase suggests checking all your calves the next year, by sending ear notch samples to check for BVDV. “You might have had a clean herd, but when a cow is pregnant, if she is exposed to this virus at a certain stage during the first 120 days of gestation, even if she’s been vaccinated, there is still some risk. If she has contact with a PI animal in someone else’s herd (even fence-line contact), she may be vulnerable.”

You probably wouldn’t know her calf is PI unless you test it, says Waldner. “Sometimes you might suspect a calf could be PI if he’s a little smaller and a poor doer, but there can also be calves, young cows or bulls that look healthy and are persistently infected,” she says.

Chase advises producers to test, and then if they have a relatively closed herd (not much contact or co-mingling with other cattle) it might not be necessary to ear notch all the calves every year. “But the advantage of testing all calves each year is that the PI animals can be identified and eliminated,” he explains. A PI animal is a threat to other cattle, so it should be euthanized and not sold, unless strictly for slaughter.

Biosecurity is crucial, but difficult with community pastures or fence-line contact, especially if your neighbor brings in new animals. “When you buy new stock, test and quarantine before you add them to your herd. If it’s a reputable seller who tested the cattle just prior to when you get them (and they don’t go through a sale barn or any place they might be in contact with other animals) you may not need to quarantine them. But if you buy animals you are unsure about, quarantine them for 30 days,” says Chase.

“Pull a blood sample when the animal arrives, and quarantine it, and pull another sample 30 days later. If the animal has never been tested, I recommend doing the ear notch when it arrives, to make sure it is not PI. If it’s not PI, a blood sample upon arrival and another one 30 days later will tell if it was exposed and became infected before it got to your place. The virus runs its course and most animals will clear the virus within 30 days,” he says. They mount an immune response, clear the virus, and are not a threat to other cattle anymore and can come out of quarantine.

“If there are several unexplained deaths in the herd, or feedlot, it is important to take an ear notch. Animals found down or dead might not show anything at necropsy, but could be PI animals. Even if you do see something else wrong, it’s wise to do an ear notch test, to be on the safe side and know whether the animal was PI,” he explains.

“If you tested the rest of the herd, you might not think there’s a problem if all the animals tested were clean, but if you forget about the one that died (and didn’t have it tested), it could have been a PI, creating some exposure in the herd.”

When To Vaccinate

Producers often ask when they should vaccinate their cows and what to use. “Inactivated products are generally safe at any time,” says Waldner. MLV (modified live virus) products are safe if used in calves, heifers and cows that are not pregnant, such as before the start of breeding season,” she says.

“Some MLV products are approved and labeled for use in pregnant animals under specific conditions; they could be given at the time of pregnancy testing (or given to calves suckling pregnant cows), but only when we know for certain that the cows were previously vaccinated for BVD as specified on the label, and you are following directions carefully.”

Most veterinarians recommend vaccinating cows after calving and well before breeding. Don’t vaccinate at the start of breeding season because this doesn’t give enough time to respond to the vaccine and build immunity. This takes at least 2 to 3 weeks for most products.

As long producers consult with their veterinarian and follow label directions, vaccinating females as calves, again as replacement heifers at weaning, and again before breeding, those cattle should have enough immunity that they can be safely vaccinated during pregnancy, using vaccines labeled for this option. “You can’t vaccinate a naïve pregnant cow or heifer with MLV products without adverse results such as abortions. Both the IBR and BVD components of a vaccine could cause a problem.”

If a producer vaccinates naïve pregnant heifers with MLV vaccine, there could be a serious wreck. They may not all abort; some may have encountered the virus (natural infection) earlier in life and may have some immunity. “It’s a bit like influenza in people,” Waldner says.

“Vaccination errors are not something you want to take a chance on, however. If these are your own animals that you raised and have consistently vaccinated according to label directions (and vaccinated as baby calves, again at weaning, etc.) they should have good immunity. Heifers should have 3 vaccinations before they become pregnant. Even if they didn’t have an adequate dose on one of those, your chances are good for strong immunity before their first breeding,” she says.

By contrast, if you purchase new animals without known vaccination history, it can be harder to track the new animals and keep them separate from the rest of the herd. If you don’t know their vaccination history, you are safer using MLV vaccine only before breeding in cows and heifers that are not pregnant, and inactivated vaccine when they are pregnant. “Regarding decisions on when to vaccinate and what type of product to use, producers consult with your veterinarian, to see what might work best for your situation,” explains Waldner.

Chase says it’s very important to use MLV vaccines in replacement heifers. “The second dose can be at about 5 to 6 months (usually at weaning time), and then another dose at least 60 days before breeding. The MLV vaccines also contain IBR—which can cause short cycling and interfere with reproduction if given too close to breeding,” he explains. A recent study by Zoetis showed that if heifers had a couple doses of MLV vaccine previously, you can use a killed vaccine before breeding and not have to worry about that problem.

In the last few years, the number of PI animals in beef herds has increased. “Part of the reason is that we are not monitoring for BVD as much as we did earlier, and also there is a strain called BVD1b that may have become more prevalent. Most of our vaccines are pretty good against BVD V1a and 2a because those are the two original strains we knew about. Now BVDV1b has emerged and it’s a bit different. Viruses keep changing and always seem to figure out a way to evade control,” he says.

It pays to work with your herd health veterinarian to guide you through all this. “Many people think that if they just vaccinate, everything will be ok. But if you don’t eliminate PI animals, you can’t vaccinate your way out of a BVD problem,” Chase says.

Testing

There are several tests available to see if an animal is harboring BVD virus. Dr. Gregg Hanzlicek, Veterinary Diagnostic Laboratory, Kansas State University, director of Production Animal Field Investigations, assists veterinarians in investigating complex herd health problems, working with pathologists and microbiologists in the veterinary diagnostic laboratory, and says there are several ways of diagnosing BVD.

“We are looking for carriers of the virus, so the tests we use are looking for the organism itself. With one with results available in a few minutes,” he says “The test with the most recent
technology, and used frequently, is the PCR test. One advantage is that it can quantify the amount of virus in the sample,” he says By using the PCR and assessing the number reported on the test result, we have a good idea whether that animal is a PI (permanently infected) or a TI (transiently infected) animal. “But it’s still wise to retest, to see if the animal cleared up or is still harboring the virus,” says Hanzlicek.

“If you have an animal with a positive test result, you should probably quarantine and retest that animal in 3 to 4 weeks, because both the ELISA and the PCR will find not only the cattle that are persistently infected, but also the animals that are transiently, temporarily infected. Those transiently infected animals will fight off the infection and no longer be shedding any virus, usually within a few weeks,” he explains.

“People think it’s a hassle to re-test in 3 to 4 weeks, but it’s best to remove both PI and TI animals from the group. A TI on the retest will be negative, and it would then be safe to put those cattle back in the herd. By contrast, any PI cattle should be sold to slaughter,” he says.

It’s wise to remove the TI as well as the PI cattle from the group, because the TI animal is shedding the virus for a while. “Whether the virus exposure is coming from a PI or TI makes no difference to the calf or cow that is exposed,” he says.

Test samples can be ear notches or blood. “Most people submit ear notches because it’s easier to collect than a blood sample. Once you ear notch the animals you can also tell by looking at them that they’ve had the BVD test. There are some shenanigans, however, with unscrupulous people ear notching cattle that haven’t been tested, sending them through a sale barn so buyers would
think they’ve been tested. This passes the disease to someone else. Our lab tests many loads of calves as they come into the Midwest and some loads have been found to contain multiple PI calves. In some of these situations you know that someone put PI calves together and sold them as a group,” says Hanzlicek.

Pooling diagnostic samples has advantages and disadvantages when sending samples to be tested. “Typically people request pooling for PCR to reduce cost. Pooling PCR samples can reduce the cost down to the ELISA cost or even less,” he explains.

“Don’t pool samples unless you are sure there are zero or very few PI animals in the group, because if the lab finds a positive pool then all the animals in the pooled group must be re-tested to find the BVD positive animals. If you know you might have some PI animals, request the test be completed individually, or in small groups of 6,” he says.

The technology has improved, making sample handling easier. “If you can’t sample all your calves at once, you can throw labeled ear notches into the freezer as you collect them. Each notch
should be submitted in a separate container such as a Ziploc bag. Then you can send them as a group later, for testing,” says Hanzlicek.

If there’s suspicion there might be BVD in a cow-calf herd, start testing the youngest animals first. “Make sure you are testing the calves before bull turnout or AI. If there’s a PI calf
in the herd during breeding season, it will be shedding virus, potentially transmitting it to bred cows in early gestation, and you could end up with more PI calves born the next year. To
find PI calves or screen for BVD, take an ear notch from every calf at birth or shortly afterward.”

If you find a PI calf, test the dam to make sure she is not PI–even though 97% of PI calves are not born to PI mothers, but mothers that were TI animals during the first four months of gestation. “These dams were temporarily infected at the wrong time of pregnancy. People often think PI calves are coming from PI dams, but that’s generally not the case,” he explains.

“People bringing calves to feedlots or stocker facilities need to make sure none of them are PI. It would be best if they could be tested before they are put on the truck, but that’s usually not practical, especially if calves are coming from sale barns. They should be tested upon arrival, especially calves coming from areas where previous PI animals were diagnosed. With many
of the tests, especially the PCR, results can be obtained within less than 24 hours. You can know the BVD status of those calves before exposing a large number of calves,” says Hanzlicek.

It’s wise to test any incoming cattle on cow-calf operations–bulls, or any purchased heifers or cows. “Isolate new animals for at least 30 days, until you know the BVD status. Testing and isolation would prevent many of the BVD issues that we see, and issues with some of the other diseases, as well.