Bovine Viral Diarrhea (BVD) is a complex viral disease caused by the Bovine Viral Diarrhea Virus (BVDV), a member of the Pestivirus genus, which affects cattle worldwide. BVDV infection causes reproductive losses, immune suppression, and general poor health in affected animals. Clinical outcomes range from mild illness to severe, fatal disease, making its presence a serious economic concern for livestock operations.
The Nature of Transient BVD Infection
The “come and go” aspect of BVD is explained by the acute, or transient, form of the infection. When a susceptible animal is exposed to BVDV, the virus replicates, causing a short-term infection that typically lasts one to two weeks. Clinical signs are often subtle or nonexistent, but may include mild fever, depression, decreased milk production, or transient diarrhea. During this time, the animal sheds the virus for about 10 to 15 days before its immune system eliminates the pathogen.
Once the virus is cleared, the animal develops an antibody response and becomes immune to that specific strain of BVDV. However, the transient infection targets lymphoid tissue, leading to a temporary state of immune suppression. This weakened immune status makes the animal highly susceptible to secondary infections, such as respiratory disease or scours. The infection can trigger costly complications that overshadow the original mild viral symptoms.
Understanding Persistently Infected Animals
BVD persists in a herd due to the Persistently Infected (PI) animal. A PI animal results when a pregnant cow is exposed to BVDV during early gestation, generally between 40 and 120 days. During this period, the developing fetus lacks a functional immune system and recognizes the virus as “self.”
The PI animal is born infected and remains infected for life, continuously shedding massive amounts of BVDV into the environment. These animals are the primary reservoir for maintaining the disease within a herd and spreading it. Many PI calves appear outwardly healthy, making them difficult to identify without specific testing.
PI animals often exhibit poor performance and are typically smaller than their contemporaries. A severe, fatal condition known as Mucosal Disease (MD) can occur if the PI animal is subsequently infected with a different biotype of BVDV or if the virus strain mutates. MD is characterized by severe diarrhea and erosive lesions, resulting in death.
Herd-Level Transmission and Risk Factors
Transmission of BVDV is directly linked to PI animals, who excrete the virus in virtually all bodily fluids, including saliva, nasal discharge, urine, and feces. Direct contact with a PI animal is the most efficient method of spread, leading to transient infections in susceptible cattle. The virus can also be transmitted through contaminated semen from an infected bull.
The introduction of the virus into a clean herd is a significant risk, most commonly occurring through the purchase of new animals. Buying untested replacement stock, especially pregnant cattle carrying a PI fetus, is a major pathway for BVDV entry. Other risk factors include shared equipment, movement of personnel or vehicles between farms, and nose-to-nose contact with neighboring cattle.
Transiently infected animals contribute to the spread during their brief shedding period, but the PI animal is responsible for the long-term perpetuation of the disease. Once introduced, the virus circulates rapidly, infecting pregnant cows and creating new PI calves, ensuring the cycle continues. Movement of cattle to shows or markets also presents an opportunity for exposure and reintroduction.
Strategies for Eradication and Prevention
Eradicating BVDV relies on a three-pronged strategy: testing, biosecurity, and vaccination. The most immediate action is identifying and removing all Persistently Infected (PI) animals, as they are the constant source of infection. A common and accurate method for PI detection is ear notch testing, which uses tissue samples to confirm the presence of the virus.
Once a PI animal is identified, it must be isolated and culled immediately to break the chain of transmission. The dam of a PI calf should also be tested, though most are not PI themselves. All newborn calves should be tested as soon as possible after birth to prevent a new PI from integrating into the herd.
Biosecurity measures are paramount for preventing reintroduction of the virus. This includes rigorous quarantine for all incoming animals, testing them for PI status before they join the main herd, and double-fencing shared boundaries. Attention to indirect transmission via fomites, such as contaminated equipment or clothing, is also necessary.
Vaccination is a complementary tool used to enhance immunity and protect susceptible animals, especially pregnant females, from negative reproductive consequences. The goal is to prevent the formation of new PI calves, requiring the breeding herd to be protected before and during early gestation. However, vaccination alone is insufficient, as the volume of virus shed by a PI animal can overwhelm vaccine-induced immunity.