Bovine Viral Diarrhea (BVD) is a widespread viral disease affecting cattle herds globally. This infection, caused by the Bovine Viral Diarrhea Virus (BVDV), manifests in diverse ways, ranging from mild, transient illness to severe reproductive failure and fatal chronic disease. The virus is a member of the Pestivirus genus. Its ability to suppress the host’s immune system makes it a serious threat to herd health and productivity, requiring early and accurate detection.
Recognizing the Acute Clinical Signs
The most common presentation of BVD in non-pregnant cattle is an acute, transient infection that often goes unnoticed or appears as a mild sickness. Affected animals may exhibit a short-term fever, lethargy, and a temporary reduction in appetite or milk production. Diarrhea is not always present, and if it occurs, it is often mild and resolves quickly.
The virus’s immunosuppressive nature makes cattle more susceptible to secondary bacterial and viral infections, such as bovine respiratory disease or mastitis. This immune suppression can persist for up to two months, complicating recovery from other common diseases. Reproductive failure is also common if a breeding female is acutely infected, resulting in embryonic death, a return to heat, abortion, or stillbirths.
Identifying Persistently Infected (PI) Animals
A unique outcome of BVDV infection is the creation of a Persistently Infected (PI) animal, which is the primary reservoir for the virus in a herd. This occurs when a pregnant cow is infected with the non-cytopathic BVDV strain between approximately 30 and 125 days of gestation. During this period, the developing fetus recognizes the virus as “self” because it has not yet established a fully functional immune system.
The resulting calf is born immunotolerant and continuously infected, meaning it never produces antibodies against that specific BVDV strain. PI animals shed massive amounts of the virus throughout their lives, making them highly effective transmitters. While some PI calves may appear weak or stunted, many others look completely healthy. This outwardly normal appearance requires definitive laboratory testing to identify these constant sources of infection.
Laboratory Diagnostic Procedures
Definitive diagnosis of BVDV relies on specific laboratory tests, categorized as virus detection and antibody detection. Virus detection methods identify the presence of the virus itself, confirming an acute infection or identifying a PI animal. Primary tests include the Antigen Capture ELISA (AC-ELISA) and Polymerase Chain Reaction (PCR).
The AC-ELISA detects the viral antigen in samples like serum, whole blood, or an ear notch (a small skin sample). Ear notches are preferred for young calves as they avoid interference from maternal antibodies. PCR testing detects the BVDV’s genetic material (RNA), offering a sensitive and rapid method for individual and bulk-milk screening.
Antibody tests, such as the Antibody ELISA, measure BVDV-specific antibodies in the animal’s blood or milk. A positive antibody result indicates the animal has been exposed to the virus or successfully vaccinated. These tests assess the overall exposure status of a herd rather than diagnosing active infection in an individual animal.
Interpreting Test Results and Management
A positive result from an antigen-detection test (AC-ELISA or PCR) signifies that the animal is actively shedding the BVD virus. If the animal is not a calf, it is likely a PI animal, but a re-test is necessary to distinguish a permanent PI from a transiently infected animal. A definitive PI diagnosis is confirmed if an animal tests positive for the virus on two separate occasions spaced three to four weeks apart.
Once a PI animal is confirmed, the most effective management step is its permanent removal from the herd. PI animals are the largest risk factor for spreading the virus, and their culling is necessary to break the cycle of transmission. Herd management also involves implementing strict biosecurity measures and establishing a strategic vaccination program to protect susceptible breeding stock and reduce the risk of future PI births.