Stop PI Calves Before They Start: Two Key Tips
Preventing calves persistently invected with BVDV starts with strong biosecurity and smart vaccination.
July 2, 2025
by Wyatt Bechtel for Boehringer Ingelheim
At first glance, a persistently infected (PI) calf may look just like any other in the herd. But beneath the surface, these calves are silent spreaders, continuously shedding bovine viral diarrhea virus (BVDV) and exposing healthy herdmates to infection.
How PI calves risk herd health
PI calves are animals that become infected with BVDV during their time in utero. When the calf is infected before their immune system is able to recognize the virus as foreign, they 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 BVDV-PI calf,” says Jen Roberts, veterinarian for Boehringer Ingelheim. “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. There can also be significant reproductive repercussions like early embryonic losses, abortion and birth defects. While these concurrent diseases are not necessarily caused by BVDV, the immunosuppressive effects of the virus make animals more susceptible to other illnesses.”
PI cattle can shed BVDV through many avenues, including respiratory secretions that spread the virus through the air of confined spaces, and bodily fluids such as milk, saliva, mucus, urine and manure.
Biosecurity can stop PI cattle from entering the herd
While maintaining a closed herd is the best way to prevent BVDV from establishing itself on your operation, it’s not always realistic. Dairy consolidations, expansions, off-site heifer growing, and even taking animals to shows are common events for many farms.
“Any time you have cattle leaving the farm and coming back, there’s a chance for them to be exposed to a PI animal and become acutely infected with BVDV,” explains 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.”
Roberts shares that testing for PI animals, and quarantining any new or returning animals, are great ways to prevent BVDV from finding a long-term home within 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 two weeks prior to commingling with the rest of the herd,” continues 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.”
Fig. 1: When a calf is infected with BVDV in utero, it will be born persistently infected and will shed the virus for its lifetime.
Prevent PI calves through vaccination
“The most common way 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,” stresses Roberts.
Vaccinating cows with a modified-live virus vaccine labeled for BVDV, like Express FP®, helps protect their health and reproductive efficiency, and enables them to deliver healthy, PI-free calves. That same prebreeding vaccine will also help cows produce antibody-rich colostrum to protect calves from BVDV and other respiratory disease threats right after birth.
When building a herd with strong immunity, vaccination shouldn’t stop with the dam. While maternal antibodies offer initial defense against disease, that protection wanes over time, opening the door for a gap in calf immunity. Research has shown that when exposed to a PI calf, 70%-100% of nonvaccinated or immune-suppressed cattle become infected.1 Fortunately, calves as young as 30 days of age can still generate a strong immune response in the face of maternal antibodies.
In a study, 30-day-old calves with maternal antibodies present were vaccinated with a uniquely adjuvanted five-way plus Mannheimia haemolytica modified-live virus (MLV) injectable-vaccine protocol, or an intranasal- and injectable-vaccine protocol. Five months later, both groups were challenged with BVDV Type 1b and M. haemolytica.
Results show that the five-way plus M. haemolytica MLV vaccine protocol provided a stronger immune response against BVDV Type 1b. It also decreased the level of BVDV shedding and kept rectal temperatures lower for several days, compared to the intranasal- and injectable-vaccine protocol.2
Not all respiratory vaccines are the same. Roberts suggests working with your herd veterinarian to solidify a vaccine protocol that best fits your operation’s needs.
BVDV Type 1b is the leading cause of PI calves
Thirty years ago, the majority of BVDV cases were caused by Type 1a. In more recent years, Type 1b has emerged as the most prevalent subspecies of BVDV in the United States, accounting for roughly 70% of reported cases.3
Viruses often mutate to escape detection by an 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,” says Roberts. “We know there are differences in the breadth of BVD 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, 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,” she points out. “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.”
While BVDV presents serious challenges, producers have reliable tools like vaccination, testing and biosecurity to manage it. Staying proactive and aware of BVDV is the first step in keeping your herd healthy and productive for the long run.
Editor’s note: This article was provided by Boehringer Ingelheim. [Lead photo by Boehringer Ingelheim.]
References:
1Fulton RW, Briggs RE, Ridpath JF, et al. Transmission of bovine viral diarrhea virus 1b to susceptible and vaccinated calves by exposure to persistently infected calves. Can J Vet Res 2005;69(3):161–169.
2Perkins-Oines S, Dias N, Krafsur G, et al. The effect of neonatal vaccination for bovine respiratory disease in the face of a dual challenge with bovine viral diarrhea virus and Mannheimia haemolytica. Vaccine 2023;41(19):3080–3091.
3Fulton RW, Cook BJ, Payton ME, et al. Immune response to bovine viral diarrhea virus (BVDV) vaccines detecting antibodies to BVDV subtypes 1a, 1b, 2a, and 2c. Vaccine 2020;38(4)4032–4037.
Angus Beef Bulletin EXTRA, Vol.17 No. 7-A
Topics: Health , Management
Publication: Angus Beef Bulletin
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