Bovine Viral Diarrhea (BVD) is a costly infectious disease affecting cattle worldwide. It is caused by the Bovine Viral Diarrhea Virus (BVDV), a member of the Pestivirus genus. The disease imposes a significant financial burden on the beef and dairy industries globally. Economic losses stem from reproductive failures, reduced productivity (such as decreased milk yield and poor growth rates), and the costs of managing secondary infections. The clinical presentation of BVD is wide-ranging, depending on the animal’s age, immune status, and the specific strain of the virus involved.
The Virus and How It Spreads
The BVDV pathogen is a single-stranded RNA virus classified as a pestivirus, a group that also includes the viruses responsible for classical swine fever and ovine border disease. This virus is highly dynamic and is categorized into two main species, BVDV-1 and BVDV-2, with multiple genetic subgroups existing within each.
The virus is predominantly spread through direct contact with infected animals, which shed the virus through various bodily excretions and secretions. Infected cattle release the virus in nasal discharges, saliva, urine, feces, milk, and semen. Indirect transmission is also possible through contaminated equipment, feed, water, or biting insects, although direct contact remains the most common route. The virus can survive in the environment for a short period.
Infection in an immunocompetent animal is typically transient, meaning the animal clears the virus within a few weeks and develops protective antibodies. However, infection of a pregnant cow can lead to vertical transmission across the placenta to the fetus. Infection during the first few months of pregnancy (specifically between approximately 40 and 120 days) can result in the birth of a persistently infected calf.
Recognizing the Signs of Infection
The signs of BVD infection vary widely, ranging from subclinical cases that go unnoticed to severe, fatal disease. In non-pregnant adult cattle, transient infection often causes mild and non-specific signs, such as a brief fever, lethargy, and a temporary drop in milk production. Due to the virus’s immunosuppressive effects, acutely infected animals are more susceptible to secondary infections, frequently leading to pneumonia or scours.
The most significant impact of BVD is seen in reproductive performance and fetal development. Infection in susceptible pregnant females can cause early embryonic death, leading to a return to estrus, or later-term issues such as abortion and stillbirths. Calves that survive in utero infection may be born weak, premature, or with congenital defects, particularly affecting the eyes and central nervous system.
The most devastating clinical manifestation is Mucosal Disease (MD), a severe, often fatal form that occurs only in persistently infected (PI) animals. MD is characterized by acute signs like high fever, severe, bloody diarrhea, and erosive lesions or ulcers throughout the mouth and digestive tract. Affected animals rapidly lose weight and typically die within days of the onset of these severe symptoms.
The Critical Role of Persistently Infected Animals
The Persistently Infected (PI) animal is the most important factor in the spread and maintenance of BVD within cattle populations. A PI calf is created when a susceptible dam is infected with the non-cytopathic BVDV biotype during the first trimester of gestation, before the fetus’s immune system can recognize the virus as foreign. Because the virus is encountered before the immune system is competent, the fetus recognizes the virus as “self” and becomes immunotolerant, failing to mount an antibody response.
PI animals are infected for their entire lives, continuously shedding large quantities of the virus in their secretions and excretions. They act as a constant reservoir, effectively “super spreaders” that can infect the entire herd and overwhelm vaccination programs. While some PI calves may appear small, unthrifty, or fail to thrive, others can look outwardly healthy and normal.
Mucosal Disease develops when a PI animal is superinfected with a cytopathic strain of BVDV, which can arise from a spontaneous mutation of the resident non-cytopathic virus. This superinfection triggers the lethal disease in the immunotolerant host, which the animal cannot survive.
Testing and Herd Management
Controlling BVD centers on identifying and removing PI animals, protecting susceptible breeding stock, and maintaining biosecurity. Diagnostic testing is paramount, with methods available to detect both the virus and the animal’s antibody response. The most common approach for PI identification involves testing tissue samples, often taken as an ear notch, using antigen-capture ELISA or PCR (Polymerase Chain Reaction).
Ear notch testing is a simple, highly effective method for mass screening of young stock and is the preferred test for determining PI status. Herd surveillance can also be performed by testing bulk tank milk using PCR, which can detect the presence of one PI animal in a herd of up to 200 lactating cows. The prompt and permanent removal of any PI animal from the herd is the most critical control measure, as they are the source of continuous infection.
Biosecurity protocols are necessary to prevent the introduction of new infection sources. These protocols include isolating and testing all new cattle entering the herd, particularly replacement heifers and bulls. Vaccination programs are another component of a comprehensive control strategy, helping reduce transient infection and protecting the fetus from developing into a PI animal. Both killed and modified live vaccines are available, and their use should be determined in consultation with a veterinarian to establish immunity in breeding stock before gestation.