What Is BVD in Cattle? How It Spreads and Affects Herds

Bovine viral diarrhea, usually called BVD, is a widespread infectious disease of cattle caused by a virus in the pestivirus group. It was first described in 1946 as an acute illness marked by high fever, severe diarrhea, and gastrointestinal damage, but in the decades since, researchers have learned that BVD’s real threat goes far beyond gut symptoms.¹ The virus suppresses the immune system, causes reproductive failure, and can create permanently infected animals that silently shed virus for life. For cattle producers, BVD ranks among the most economically damaging diseases in the industry, and understanding it requires grasping a few biological quirks that make this virus uniquely difficult to control.

The Virus Behind BVD

BVD is caused by bovine viral diarrhea virus (BVDV), a member of the pestivirus genus within the Flaviviridae family, the same broad viral family that includes hepatitis C and dengue.² There are two main species: BVDV-1 (now formally called Pestivirus A) and BVDV-2 (Pestivirus B), with a third group known as HoBi-like pestivirus that shows significant genetic differences from both.³ ⁴ Within each species, there are numerous subtypes, which is one reason a single vaccine does not always protect against every strain a herd might encounter.

One of the most important things to understand about BVDV is that it exists in two biotypes: cytopathic and noncytopathic. These terms describe what the virus does to cells in a lab dish. The cytopathic form kills cells; the noncytopathic form does not. In living cattle, the noncytopathic form is far more common and far more dangerous, because it is the form that can establish the lifelong persistent infections that drive the disease through herds.⁵ When both forms show up in the same animal, the consequences can be fatal, a situation discussed further below.

Why Persistently Infected Animals Are the Core Problem

If you only learn one thing about BVD, it should be this: persistently infected, or PI, animals are the engine that keeps the virus circulating in cattle populations worldwide. A PI animal is created when a pregnant cow is exposed to noncytopathic BVDV during roughly the first 125 days of gestation, before the fetus has developed a functioning immune system. The fetal immune system encounters the virus so early that it never recognizes it as foreign. The calf is born alive, apparently normal in many cases, but sheds enormous quantities of virus every day of its life through saliva, nasal discharge, urine, feces, and other secretions.

PI animals are the primary reservoir. Because their immune system treats the virus as “self,” they do not mount an antibody response against it and never clear the infection. A single PI calf in a herd continuously exposes every animal around it. Some PI animals are obviously stunted or sickly, but others look perfectly healthy for months or even years, making them difficult to identify without testing. This stealthy quality is a major reason BVD became so widespread before its biology was well understood.⁶

Identifying and removing PI animals is the single most important step in any BVD control program. Pooled ear-notch tissue testing is one standard screening method: a small skin sample is taken from the ear of every calf and tested for viral antigen or RNA.⁷ Any calf that tests positive is retested several weeks later. An animal that tests positive twice is persistently infected, because a transiently infected animal would have cleared the virus and seroconverted in the interim.

How BVD Spreads

Transmission happens through direct contact with infected animals and through contaminated secretions. A PI animal is by far the most efficient source. In dairy herds, where cattle are housed in close quarters, the within-herd transmission rate from a PI animal has been estimated at about 0.50 new infections per day. In extensively grazed beef herds, where animals have more space, the rate drops to roughly 0.11 per PI animal per day. That slower rate creates its own problem: it means not all breeding females in a beef herd will be exposed and develop protective immunity before they become pregnant, which increases the chance of generating more PI calves in the next generation.⁸

The virus can also spread through semen. In one study, twelve seronegative heifers were inseminated with semen from a persistently infected bull. All twelve became infected, seroconverting within two weeks.⁹ This is a significant concern for artificial insemination programs and underscores why semen used in breeding should come from tested, virus-free bulls. Fomites (contaminated equipment, clothing, and trailers), shared water sources, and nose-to-nose contact over fence lines are additional routes, though direct animal-to-animal contact with a PI animal remains the main driver.

What BVD Looks Like in a Herd

BVD is frustrating to diagnose on clinical signs alone because it looks different depending on the situation. In its acute form, cattle develop fever, diarrhea, oral ulcers, and reduced white blood cell counts. But most transient infections in otherwise healthy adult cattle are mild or entirely subclinical, meaning the animal fights off the virus and recovers without anyone noticing. The real damage from those subclinical infections often shows up indirectly: reduced milk production, poor weight gain, and a weakened immune system that opens the door to other infections.

Reproductive losses are frequently the most visible sign that BVD has entered a herd. Depending on when during pregnancy a cow is exposed, the virus can cause early embryonic death, abortion, stillbirth, birth defects (including brain malformations like cerebellar hypoplasia), and of course the birth of new PI calves. A sudden spike in abortions, returns to heat, or weak calves at birth should put BVD on the list of suspects.

Mucosal Disease

The most dramatic clinical form of BVD is mucosal disease, which is almost always fatal. It occurs only in PI animals and is triggered when the animal becomes superinfected with a cytopathic strain of BVDV in addition to the noncytopathic strain it has carried since birth. The cytopathic strain can arise by mutation within the animal’s own persistent virus or, as research has demonstrated, can come from an antigenically different cytopathic strain introduced from outside.¹⁰ The result is severe erosions and ulcers throughout the gastrointestinal tract, profuse bloody diarrhea, rapid wasting, and death typically within days to weeks. There is no effective treatment.

Immune Suppression and Respiratory Disease

Beyond its own direct damage, BVDV is a potent suppressor of the immune system. It targets white blood cells, impairing both the innate defenses (the body’s first-responder system) and the acquired immune response (the targeted antibody and cell-mediated defenses that develop after exposure to a pathogen).¹¹ This makes BVDV-infected cattle highly vulnerable to secondary bacterial and viral infections.

The connection between BVD and bovine respiratory disease (BRD) is especially significant in feedlot settings. BRD, sometimes called “shipping fever,” is already the leading cause of sickness and death in feedlot cattle. When BVDV is part of the mix, the outcomes get worse. Transcriptomic analysis of naturally occurring BRD in Australian feedlot cattle found that BVDV was associated with a distinct pattern of immune suppression, with roughly four out of five affected immune genes being downregulated, while secondary bacterial agents drove the actual lung inflammation.¹² In practical terms, BVDV opens the door and the bacteria walk through it.

The Economic Toll

BVD costs the cattle industry money through a combination of reduced milk yield, lower pregnancy rates, increased calf mortality, higher rates of secondary disease, and the cost of testing and control measures. Economic modeling of pastoral dairy and beef herds in New Zealand estimated that a BVD outbreak in a naïve herd costs roughly NZ$22 per cow per year in a dairy operation and about NZ$41 per cow per year in a beef operation over a five-year period. For dairy herds, the biggest hit came when PI replacement heifers joined the lactating group and caused transient infections that dropped milk production. For beef herds, the main loss was through reduced pregnancy rates, meaning fewer calves to sell.¹³

Those figures are conservative and apply to a specific production system. The actual costs vary widely depending on herd size, management intensity, vaccination status, and how long the virus circulates before being detected. Globally, BVDV is considered one of the most economically important infectious agents in cattle because it affects both productivity and reproduction simultaneously.¹⁴ Losses from immune suppression and secondary disease are particularly hard to quantify because they are often attributed to the secondary pathogen rather than to the underlying BVDV infection that set the stage.

Testing Strategies

Effective BVD control hinges on finding and removing PI animals, and several testing approaches exist depending on herd type and management system.

For beef herds, ear-notch testing of every newborn calf is the most common approach. A small tissue sample is collected, often at the same time as ear tagging, and submitted for antigen capture ELISA or PCR testing. One important wrinkle: calves tested very young may carry maternal antibodies from their dam’s colostrum, and those antibodies can mask the virus and cause a false-negative result.¹⁵ For that reason, some testing protocols recommend waiting until maternal antibodies have waned or retesting calves that initially tested negative if herd history suggests BVD exposure.

For dairy herds, bulk tank milk testing offers a practical alternative. RT-PCR testing of bulk milk can detect the presence of a single PI cow among several hundred lactating animals.¹⁶ If the bulk tank tests positive, follow-up testing of individual animals or smaller groups can identify the PI. Combining virus detection in milk tanker samples with antibody testing in bulk tank milk has proven to be a feasible and economical screening method for dairy herds, capable of confirming PI-free status across groups.¹⁷ In one regional survey using this approach, virus-positive samples were traced back to three PI cows and one PI calf on three farms out of 79 sampled routes.

Regardless of method, the logic is the same: screen broadly, follow up positives, confirm PI status with a second test, and remove confirmed PI animals from the herd promptly.

Vaccination and Biosecurity

Vaccination is a central tool for BVD control, though it works best as part of a broader program rather than as a standalone solution. Both modified-live and killed vaccines are available. A meta-analysis examining the effectiveness of BVDV vaccination for preventing reproductive disease found that vaccinated cattle experienced about 45% fewer abortions and roughly 85% fewer fetal infections compared to unvaccinated animals. Pregnancy rates also improved by about 5% in field trials.¹⁸ Those are meaningful reductions, especially the drop in fetal infection, because preventing fetal infection is what prevents new PI calves from being born.

Vaccination alone cannot eliminate BVD from a herd, though, because no vaccine provides perfect fetal protection, and PI animals that already exist are not helped by vaccination (their immune system does not respond to the virus). That is why biosecurity practices are essential. Producers who achieve BVDV-free status need to maintain it by annually testing all calves born to confirm no new PIs have been created, isolating and testing purchased animals before they join the herd, and preventing fence-line contact with neighboring cattle of unknown status.¹⁹ The risks of reintroduction are real: a single purchased animal that is PI or transiently infected can restart the cycle in a clean herd.

National and Regional Eradication Programs

Several European countries have pursued organized BVD eradication, with the Scandinavian countries leading the way. Sweden, Norway, Finland, and Denmark implemented systematic programs based on identifying and removing PI animals, combined with movement restrictions and surveillance, and have largely succeeded in eliminating the disease. Other countries, including Germany, Switzerland, Ireland, Scotland, and England, have programs at various stages of implementation, each adapted to local livestock industry structure and political will.²⁰

Eradication is biologically feasible because the virus depends heavily on PI animals as its reservoir. Remove all PI animals from a population, prevent new ones from being created through vaccination and biosecurity, and the virus runs out of places to hide. In practice, though, the journey from control to eradication is slow and expensive, requiring sustained buy-in from farmers, veterinarians, and governments. Countries without coordinated programs tend to have widespread circulation, and individual producers in those regions face the constant threat of reintroduction from neighboring herds.

BVD Beyond Cattle

Although BVDV is primarily a cattle disease, it is not strictly limited to cattle. The virus has been reported in over 40 domestic and free-ranging species. Persistent infection, the same lifelong virus-shedding state seen in cattle, has been documented in at least eight other species: white-tailed deer, mule deer, eland, mousedeer, mountain goats, alpacas, sheep, and domestic swine.²¹

For cattle producers, the practical significance depends on geography and management. A beef herd that shares pasture with wild deer or is in close proximity to sheep or alpacas has a potential exposure route that fencing and cattle-only biosecurity will not fully address. Wildlife reservoirs are a particular concern in regions pursuing eradication, because even after all PI cattle are removed, a PI deer or mountain goat in the area could theoretically reintroduce the virus. The frequency of cross-species transmission in real-world conditions is not fully characterized, but the possibility is well enough established that eradication programs must account for it in their risk assessments.

The HoBi-like Pestivirus Wrinkle

A relatively recent development in BVD virology is the emergence of HoBi-like pestivirus, sometimes called BVDV-3 in older literature, though its formal classification remains a work in progress. First identified in contaminated fetal bovine serum from Brazil, this virus is genetically distinct from both BVDV-1 and BVDV-2.²² It causes similar clinical disease in cattle, including respiratory and reproductive signs, and can produce PI animals.

The concern with HoBi-like pestivirus is that standard diagnostic tests and vaccines developed for BVDV-1 and BVDV-2 may not reliably detect or protect against it. Countries where HoBi-like strains circulate need diagnostic tools capable of identifying all three virus groups, and vaccine manufacturers face pressure to broaden their products’ coverage. For now, HoBi-like pestivirus is primarily reported in South America, Southeast Asia, and parts of Europe, but global cattle trade means it could appear anywhere. Its emergence is a reminder that the BVD story is still evolving, and surveillance programs need to keep up with viral diversity rather than assuming the pathogen stays the same.