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 herd mates 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,” said Jen Roberts, DVM, 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 all 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,” explained 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.”
Dr. 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,” continued 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.”
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,” stressed Dr. 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 pre-breeding 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% to 100% of non-vaccinated 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. Dr. 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,” said Dr. 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, 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,” she pointed 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 impact is the first step in keeping your dairy herd healthy and productive for the long run.
About Boehringer Ingelheim - Animal Health business
Boehringer Ingelheim provides innovation for preventing and treating diseases in animals. The company offers a wide range of vaccines, parasite-control products, and medicines for pets, horses, and livestock to veterinarians, animal owners, farmers, and governments. As a leader in animal health, Boehringer Ingelheim values that the health of humans and animals is deeply connected and strives to make a difference for people, animals, and society. Learn more at www.boehringer-ingelheim.com/us/animal-health.
