BVD still a threat to Canadian cattle herds

Even closed herds can be at risk for BVD, making vaccination key to preventing this disease

Bovine viral diarrhea (BVD) can affect cattle in many ways, causing abortion, birth defects, stillborn calves, immune deficiencies in persistently infected animals, and other acute or chronic illness. BVD is also an indirect cause of many other diseases because it has adverse effects on the immune system.

An estimate a few years ago suggested that up to 80 per cent of cattle in North America have been exposed to BVD and 70 to 90 per cent of infections go undetected. The only clues of BVD may be poor reproductive performance due to pregnancy losses, higher than normal sickness rate in calves and lower than expected weaning weights.

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First descriptions of this disease in North America were 70 years ago — outbreaks of highly fatal diarrhea (hence the name), digestive tract ulcers, nasal discharge and abortions. Many veterinarians feel this disease should be renamed, since diarrhea is due to just one aspect of the disease. In most herds today you won’t find infected animals with diarrhea, but you might still find the virus.

Dr. John Campbell, professor at the Western College of Veterinary Medicine in Saskatchewan, says BVD is a unique virus because it often attacks the fetus in the pregnant cow.

“For cow-calf herds the main reason we vaccinate is to protect the fetus. A number of BVD vaccines have ‘fetal protection’ claims, which is often stated as FP on the label. The manufacturer is basically saying that this is a cow vaccine and it’s been shown to provide fetal protection if the cow comes into contact with the BVD virus while she is pregnant,” he says.

To prevent persistently infected calves, this protection is necessary in early gestation. Depending on timing of the infection, BVD can cause abortion, congenital defects in the calves, or ulcers, says Campbell. There are good reasons to have high levels of antibodies all through gestation, but to prevent persistently infected calves, it’s especially important in the first three months. Persistently infected calves are the main source of virus spread in a herd, and cow-calf producers are the only ones who can prevent this problem through well-timed vaccinations.

Any cow, calf or bull can get an acute BVD infection, similar to the way a human can get a cold or influenza and transmit it to other individuals. Risk for transmission from an acutely infected animal will typically be present for a relatively short time, though in some cases bulls harbour the infection for longer periods in their testes.

“The virus can be transmitted from these acute cases and infect other cattle, but most transmission is associated with exposure to persistently infected animals,” says Dr. Cheryl Waldner, Western College of Veterinary Medicine. Persistently infected animals are life-long virus shedders, producing large amounts of the virus, she adds, and they can quietly spread BVD in the herd.

A calf that’s a bit smaller and a poor doer might be suspected of being persistently infected, Waldner says, “but there can also be cattle that look healthy and are persistently infected.”

Vaccinating for BVD

Despite vaccine availability, BVD is still a problem in Western Canada. “The regional diagnostic laboratory identifies cases of BVD every year,” Waldner says.

This disease will probably always be a threat, because no vaccine is 100 per cent effective. Some animals are unable to mount adequate immunity when vaccinated, for various reasons. As well, some producers don’t vaccinate. There is always the risk of BVD from herds without effective control programs, and the possibility of wildlife harbouring BVD and transmitting it to cattle.

Waldner says that there’s good published data showing BVD vaccines work if used according to label directions. Field data from Western Canada suggests that producers can reduce the average time it takes for cows to rebreed by using BVD vaccine, she adds. Research also shows vaccines can improve pregnancy rates and cut abortion risk, she adds.

Herds using community pastures or bringing in new cattle are most likely to come into contact with this disease, with subsequent negative impacts on reproduction, unless protected by vaccination. In high-risk cattle, “field data shows that unvaccinated cows are four times more likely to be open in the fall and three times as likely to abort,” she says.

In 2016, Waldner and her colleagues asked all the herds that were part of the Western Canadian Cow-Calf Surveillance Network (funded through the Beef Cattle Research Council) about their vaccine practices.

“Most of those herds were vaccinating for BVD, but there is still room for improvement,” she says.

Researchers found that 93 per cent of herds were vaccinating for BVD in their cows, with a few more herds vaccinating their heifers. Eighty per cent were using BVD vaccines preventatively because they’d never had a problem. The rest were vaccinating because they’d had a problem and didn’t want to repeat that experience, says Waldner.

The high rate of vaccination seen in this study may not be representative of the industry, however. “Producers who participated in this program most likely interact with their veterinarians a bit more than average and may be more likely to adopt disease control recommendations,” Waldner says.

As well, only 55 per cent of these producers reported vaccinating their bulls. Bulls can potentially introduce BVD to the herd at the time of purchase, says Waldner, or spread it among the females if they become infected during the breeding season. And bulls can be at risk for contracting BVD if they go through a fence.

The good news is that the number of producers who are vaccinating for BVD has almost doubled in the past 15 years. “More people are learning about this disease and are encouraged about the effectiveness of vaccine products,” says Waldner.

“One paper published in 2015 that reviewed the existing literature talked about an 85 per cent decrease in fetal infection, on average, a 45 per cent decrease in abortions and a five per cent increase in pregnancy rates with BVD vaccination. That’s convincing evidence for the benefits of using a relatively inexpensive product,” she says.

Taking a sample to test for BVD with a tag punch. While some cattle may show signs of infection, other persistently infected animals can appear perfectly healthy.
photo: Courtesy Dr. John Campbell

There are pros and cons to both modified live vaccine (MLV) and killed vaccine, says Waldner. The survey found that 84 per cent of producers used modified live and nine per cent used an inactivated (killed) product. A few more herds were vaccinating their heifers with modified-live vaccines as compared to inactivated products. Most herds were using modified live before breeding or summer pasture turnout, she says.

Most veterinarians recommend vaccinating cows after calving and before the cattle go out to pasture, but not right 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 two to three weeks for most products.

“Those vaccines also contain modified-live IBR virus,” says Campbell. “That portion of the vaccine can cause temporary ovarian inflammation, and could potentially interfere with the cow’s first cycle if she is vaccinated too close to breeding.

“If it doesn’t work for you to run the cows through after calving and vaccinate them pre-breeding, you could vaccinate them while pregnant.” Campbell recommends vaccinating in late gestation, before calving. The idea is to vaccinate as close as possible to the early gestation of the next pregnancy, to protect the next fetus, he explains.

But in those situations, producers must consider vaccination history and check that the vaccine is safe to use on pregnant animals.

“Inactivated products are generally safe at any time. MLV products are safe if used in calves, heifers and cows that are not pregnant, such as before the start of breeding season,” says Waldner.

Some modified live products are approved and labelled for use in pregnant animals under specific conditions, Waldner notes. “They could be given at the time of pregnancy testing, but only when we know for certain that the animals have been previously vaccinated for BVD as specified on the label, and you are following directions carefully.”

Heifers should have two or three vaccinations before they become pregnant. That way, even if they didn’t have an adequate dose on one vaccination, chances are good for strong immunity before their first breeding, Waldner says.

“Vaccination errors are not something you want to take a chance on. If you are going to vaccinate pregnant cattle with MLV, you need good animal identification and good records,” she says.

If one purchases new animals with unknown vaccination history, it can be harder to track the new animals and keep them separate from the rest of the herd. If there are any questions about vaccination history, it’s safer using modified live vaccination only in animals that aren’t pregnant, or an inactivated vaccine.

If a person has any questions about these decisions, it’s best to consult a veterinarian, says Waldner. “There isn’t a blanket recommendation on what is best, in terms of timing.”

Minimize Risks

Build immunity early, beginning with calves at spring processing, says Waldner. Vaccinate again at weaning, and again before breeding, she suggests, as this reduces risk from other animals.

“Exposure to other animals — whether it’s because you are expanding your herd and bringing animals in, taking animals to shows, leasing bulls or buying untested bulls, or even sharing equipment and personnel — creates risks for disease introduction, even in what might otherwise be considered a closed herd,” says Waldner.

Waldner also recommends vaccinating bulls and purchased cows or heifers, as well as mature cows.

“Once a vaccination program is started, it’s critical to maintain it. If vaccination stops, the herd will be at significant risk if later exposed to BVD,” she says.

Most persistently infected animals die relatively young, but Waldner has seen some infected yearling bulls.

“If you bring in yearling bulls, it is important to know whether the supplier has a BVD control program. If you’re not sure, test the bull before exposing him to other animals,” she says. Veterinarians can test for BVD as part of a breeding soundness evaluation, she adds.

The thing to remember about BVD is that when buying a pregnant cow, we don’t know the status of the calf she’s carrying, Waldner adds. “If producers want a good BVD control program, they need to think about where BVD can enter a herd, what their herd status is currently and how they can minimize risks for it entering and spreading.”

Buying cows to breed from a well-vaccinated herd is usually safer than buying bred cows. The incoming cow can be vaccinated and tested if there is reason for concern. But with a bred cow, a producer would have to test the calf after it’s born to be sure it’s not persistently infected.

BVD incidents tend to show up as clusters in herds, says Campbell. “In beef herds, if BVD comes into your herd at the wrong time and your cattle are not vaccinated, a lot of fetuses may get infected at the same time, because the cows were all bred within a similar time frame. This can create a major wreck; we’ve seen some disasters in herds that didn’t keep their vaccine program up to date.

“I checked one herd that moved from Alberta to Saskatchewan and missed one year of vaccinating because of the move. They ended up mixing those cows with some already on the farm they bought, and had a big wreck in their calves at weaning time. It can happen in healthy herds if you simply miss a year or get behind in the vaccinations.”

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