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“Persistently infected (PI) calves are little virus factories,” said Jennifer Roberts, DVM, Boehringer Ingelheim. “I like to think of them as a trojan horse because while the calf may look normal on the outside, it’s a huge threat to the operation due to the level of virus that calf is continuously shedding.”

PI calves are animals that become infected with the virus during their time in utero. If the calf is infected before its immune system is able to recognize the virus as foreign, it can become a PI calf. This enables the virus to live and grow within that calf indefinitely.

“There are many consequences that can stem from a bovine viral diarrhea virus (BVDV) PI calf,” continued Dr. Roberts. “The virus itself does a very good job of suppressing the immune system, and because of that, you may see an increase in the incidence of other diseases, like respiratory disease in calves or mastitis in a milking herd. While these concurrent diseases are not necessarily caused by BVDV, the immunosuppressive effects of the virus make animals more susceptible to other illnesses.”

“I am sometimes asked to troubleshoot problems that dairy producers are experiencing in their herds, such as pneumonia or diarrhea that doesn’t go away, and often find that the underlying cause is BVDV,” added Ángel Abuelo, DVM, assistant professor in cattle health and wellbeing, Michigan State University. “There are so many ways that this virus can be transmitted, so it’s very important to implement effective biosecurity, vaccination and testing protocols on your dairy.”

Establishing a biosecurity program

While maintaining a closed herd is the best way to prevent this disease from establishing itself on your operation, this is not always possible due to dairy consolidations and expansions, off-site heifer growing, or even bringing your cattle to shows that can expose them to BVDV. On arrival, infected animals can leave the virus around the feed bunk, in the water trough and through the chute during processing.

“Any time you have cattle leaving the farm and coming back, there’s an opportunity for them to be exposed to a PI animal and become acutely infected with BVDV,“ said Dr. Roberts. “Even those acutely infected animals can shed the virus for a couple of weeks, and if they come in contact with cows that are at the right point in gestation for the fetus to become infected, it can cause a PI calf to be born.”

Both doctors agree that testing and/or quarantining any animals entering the farm is a great way to prevent the virus from entering your herd.

“The acute infection period is very short, usually 10 to 14 days, so if it is possible to quarantine new herd additions, the recommended period of isolation is 2 weeks prior to comingling with the rest of the herd,” said Dr. Roberts. “If you’re sending animals to a heifer grower, especially one raising heifers for multiple operations, I always recommend that the calves go to a heifer grower that requires PI testing.”

The importance of vaccination

“The most common way that BVDV spreads is through PI calves, so it’s also important to develop a targeted vaccination program that prevents BVDV PI calves from being born into your herd,” said Dr. Abuelo.

Vaccinating cows with a modified-live BVDV vaccine helps protect their health and reproductive efficiency, and enables them to deliver healthier, PI-free calves. That same pre-breeding vaccine also helps them produce antibody-rich colostrum, to protect calves from BVDV for several weeks to a few months after birth.

Calves can be vaccinated as early as 30 days of age with a modified-live virus (MLV) vaccine that includes antigens for BVDV. Since some MLVs can be inactivated by maternal antibodies, it is important to choose an MLV vaccine that works well with your colostrum program to enhance immunity in these young calves.

Protecting against the leading cause of PI calves: BVDV Type 1b

Thirty years ago, the majority of BVDV cases were caused by Type 1a. Now Type 1b has emerged as the most prevalent subspecies of BVDV in the United States, accounting for roughly 70% of reported cases.1,2

Viruses often mutate to escape detection by the animal’s immune system. Over time, viral mutations resulting from environmental pressures can lead to changes in the prevalence of viral strains causing clinical disease.

“The most surprising thing to me about BVDV has been the divergence of the different subspecies over the past 20 to 30 years,” said Dr. Roberts. “We know there are differences in the breadth of BVDV protection offered in the commercially available vaccines and it’s important to reevaluate vaccination protocols periodically as patterns in clinical diseases shift.”

Due to the increasing risk of BVDV Type 1b, Dr. Roberts recommends working with a veterinarian to establish a sound vaccination protocol that includes adequate protection against this particular subspecies. “It’s important to get at least two — if not three — doses of a modified-live virus vaccine that’s labeled to protect against BVDV 1b administered by the time that calf reaches breeding age. We want to make sure that each heifer on the farm has optimal protection prior to breeding in order to reduce the likelihood that she gives birth to a PI calf.”

“BVDV is a challenging disease, but thankfully we have tools available, like vaccinations, testing and biosecurity, to control it,” concluded Dr. Abuelo. “We just need to be aware of the consequences that BVDV can leave in dairy herds and use these tools to prevent and minimize the impact of the virus as best we can.”

For more information and resources on controlling BVDV on your dairy, please visit