Bovine viral diarrhea (BVD) is caused by bovine viral diarrhea virus (BVDV), a single-stranded RNA virus in the Pestivirus genus. The virus circulates in cattle herds worldwide and produces a surprisingly wide range of problems, from mild digestive illness to severe reproductive failure and a fatal condition called mucosal disease. What makes BVDV especially damaging is its ability to create “persistently infected” animals that silently shed massive amounts of virus for their entire lives.
The Virus Behind the Disease
BVDV belongs to the family Flaviviridae, the same family that includes hepatitis C in humans. Scientists classify it into three genotypes: BVDV-1, BVDV-2, and BVDV-3, with at least 30 subgenotypes identified so far. BVDV-1 is the most common worldwide, while BVDV-2 strains tend to cause more severe acute disease. This genetic diversity is one reason the virus is difficult to control with a single vaccine.
Beyond genotypes, the virus comes in two biological forms, or biotypes: cytopathic and non-cytopathic. The non-cytopathic form is far more common in nature and is the one responsible for establishing persistent infections. The cytopathic form destroys cells directly and plays a critical role in triggering mucosal disease when it appears in an already persistently infected animal. Understanding these two biotypes is key to understanding why BVDV behaves the way it does in a herd.
How the Virus Spreads
BVDV moves through a herd by two main routes: horizontal transmission (animal to animal) and vertical transmission (from a pregnant cow to her fetus). Horizontal spread happens primarily through direct nose-to-nose contact. Infected animals shed virus in nasal secretions, saliva, urine, feces, milk, and semen. The virus can also travel indirectly on contaminated equipment like needles, boots, clothing, and shared tools, and it can even spread through the air over short distances.
Persistently infected (PI) animals are the most significant source of virus in any herd. These animals shed enormous quantities of BVDV continuously, far more than a transiently infected animal ever would. A single PI calf introduced into a clean herd can expose every animal it contacts. The virus can also enter a herd through contaminated semen used in artificial insemination, making bulls and semen donors important to screen.
What Creates a Persistently Infected Animal
Persistent infection is the most consequential feature of BVDV biology. It happens when a pregnant cow is exposed to the non-cytopathic form of the virus during roughly the first 125 days of gestation. At that stage, the fetal immune system hasn’t developed enough to recognize the virus as foreign. The fetus becomes infected, and its immune system essentially learns to treat the virus as “self.” The calf is born without any antibodies against BVDV and will shed virus in large amounts from every bodily secretion for its entire life.
There’s a critical window between days 125 and 150 of gestation where the outcome is uncertain. Some fetuses infected during this period become persistently infected, while others mount a transient immune response and clear the virus. After 150 days, the fetal immune system is mature enough to fight back, and infection typically results in only a temporary illness in the calf.
Calves born to PI dams are always PI themselves. Many PI animals appear healthy at birth, which is part of what makes them so dangerous to a herd. They can look normal for months or even years while continuously spreading virus to every animal around them.
Three Forms of Disease in Cattle
BVDV produces three distinct disease patterns, each with different causes and severity.
Acute BVD is what happens when a non-persistently infected animal encounters the virus for the first time after birth. Either biotype can cause it. Symptoms range from mild fever, nasal discharge, and diarrhea to more severe illness with high fever, oral ulcers, and immune suppression. Many acute infections are so mild they go unnoticed. The real danger of acute infection in adult cows is often not the diarrhea itself but the reproductive consequences if the cow is pregnant.
Congenital persistent infection results from fetal exposure during early gestation, as described above. PI animals may appear healthy initially, but they tend to have shorter lifespans, poor growth rates, and higher susceptibility to other infections because their immune systems never fully function normally.
Mucosal disease is the most severe and always fatal form. It occurs only in PI animals, triggered when a cytopathic strain of BVDV emerges or is introduced alongside the non-cytopathic strain the animal already carries. This “mixed infection” causes devastating erosions and ulcers throughout the digestive tract. Affected cattle develop profuse, often bloody diarrhea, stop eating, and typically die within days to weeks. There is no treatment.
Reproductive Damage
The reproductive toll of BVDV is where herds take the heaviest losses. The virus affects both cows and bulls and can cause problems at nearly every stage of reproduction. In cows, infection can lead to low fertility, embryonic death, abortion, mummified fetuses, and stillbirths. The specific outcome depends largely on when during gestation the cow is exposed.
When infection happens during the period of fetal organ formation, the virus disrupts normal development. This can produce calves with a range of birth defects: underdeveloped cerebellums, fluid on the brain, eye degeneration, underdeveloped lungs, jaw deformities, joint contractures, and general growth retardation. These congenital defects lead to significant losses through fetal death, difficult births, and cows culled for ongoing reproductive problems. In dairy operations, this translates directly into fewer replacement heifers and lower calves born per insemination.
Economic Cost to Herds
A global review of direct monetary losses from BVDV found costs ranging from $0.50 to $687.80 per animal, reflecting the enormous variation in how severely different herds are affected. Dairy operations tend to bear higher costs: direct losses per dairy cow average about $24.85 more than per beef cow, driven by the impact on milk production and the tighter reproductive demands of dairy management. These figures capture only direct losses like reduced fertility, deaths, and treatment costs. They don’t account for indirect costs such as reduced growth rates across the herd, increased susceptibility to respiratory and digestive diseases, or the cost of testing and control programs.
Identifying Infected Animals
Finding PI animals is the cornerstone of any BVDV control program because removing them eliminates the primary source of virus. The main testing methods are PCR (which detects viral genetic material) and virus isolation (which grows the virus in a lab culture). PCR results come back in about two business days, while virus isolation takes 10 to 14 days.
For herd-level screening, pooled PCR testing can check blood samples from up to 10 animals at once, making it cost-effective to screen large groups. If a pool comes back positive, individual animals are then tested to identify which one is infected. For dairy herds, bulk tank milk testing using both PCR and virus isolation together increases the chance of catching a positive. A positive bulk tank result can mean there are PI animals in the milking herd, acutely infected cattle, or both.
Calves younger than 61 days must be tested using whole blood rather than serum, because maternal antibodies from colostrum can mask the virus in serum-based tests. Any animal that tests positive should be retested several weeks later. A transiently infected animal will clear the virus and test negative on the second round, while a PI animal will remain positive.
Vaccination and Prevention
Two types of vaccines are available: modified-live virus (MLV) and killed virus. Killed vaccines are the safest option, particularly for pregnant cattle, but they produce a weaker immune response and require multiple doses plus boosters to maintain protection. Modified-live vaccines stimulate a broader, longer-lasting immune response with fewer doses, but they carry some risk if used improperly, especially in pregnant animals or those with compromised immune systems. Strategic timing of vaccination relative to breeding is important to minimize these risks.
Vaccination alone doesn’t eliminate BVDV from a herd. The most effective control programs combine vaccination with rigorous testing to identify and remove PI animals, strict biosecurity when introducing new cattle, and screening of bulls and semen used for artificial insemination. Because PI animals are the engine that keeps the virus circulating, removing them is the single most impactful step a producer can take.