Bovine Viral Diarrhea Virus Type 1 and Type 2 (BVD)

BVD, BVDV

The majority of BVDV infections are subclinical, with severity of disease determined by strain virulence and host susceptibility. However, BVDV infections almost always produce some degree of immunosuppression determined by virulence of the infecting strain. If the animal is exposed to other pathogens while it is immunosuppressed, morbidity and death -loss can be greatly increased. Initial infection is in the respiratory or vaginal mucosa, followed by systemic viral replication and widespread infection of the immune, respiratory, reproductive, and enteric systems. Infected cattle shed BVDV in various body fluids and tissues including saliva, nasal discharge, blood, feces, urine, and aborted fetal tissues. Persistently infected animals can shed more viruses and for longer periods than acutely infected cattle.

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Etiology

BVDV has two biotypes, cytopathic (CP) and noncytopathic (NCP), a distinction based on the characteristics of the virus when grown in a laboratory in tissue culture. The NCP strain reproduces without damage to infected cells and represents about 95% of field isolates, while the CP type results in death of target cells. Antigenic variations of the viral surface proteins result in two BVDV types, type 1 and type 2. Both type 1 and type 2 BVDV can be CP or NCP. Neither the type 1 or type 2 designations or the CP/NCP biotype relates to the virulence, or disease-causing ability of the virus.

Symptoms
Respiratory infection with BVDV is characterized by signs typical of viral respiratory disease, including fever, depression, inappetance, and ocular and nasal discharge, followed by diarrhea several days after onset. Sores or ulceration in the mouth and gums may be present, along with reduced milk production in cows. Bovine viral diarrhea includes several distinct disease syndromes, all caused by BVDV:
Respiratory infections, with signs similar to IBR, plus oral and tracheal ulceration
Thrombocytopenic (bleeder) syndrome, where type 2 BVDV infects blood cells and bone marrow, causing destruction of red blood cells, reduced clotting function, bleeding from wounds, lesions, and internal organs.
Reproductive disease resulting in embryonic loss and abortions.
Persistent infection (PI), which results when a calf is infected in utero with NCP BVDV during the first trimester of gestation and survives, and results in a continuing reservoir of BVDV infection. Animals with PI are often outwardly normal.
Mucosal disease occurs when an animal with PI is exposed to a CP strain of BVDV, resulting in explosive diarrhea and ulceration throughout the digestive tract and in most cases, death.

Diagnosis

Diagnosis may be made on clinical signs typical of viral respiratory disease, including fever, depression, inappetance, and ocular and nasal discharge, followed by diarrhea several days after onset. Sores or ulceration in the mouth and gums may be present, along with reduced milk production in cows. Virus isolation from blood, milk, and tissues is useful in diagnosing BVDV.

Treatment

There is no specific anti-BVDV therapy. Treatment is limited to antibacterial therapy for secondary bacterial infections and supportive treatment.

Prevention

To limit entry of a PI animal into the herd, new additions should be quarantined until their PI status can be determined. Persistently infected cattle should be culled. Direct contact with infected animals is the primary method of transmission, so isolation for two weeks of new arrivals or clinically affected cattle can minimize disease spread.
Vaccination is routinely practiced for BVD due to the economic impact of the disease in beef and dairy production. Modified live virus (MLV) vaccines can provide reproductive duration of immunity. The bovine fetus is very sensitive to fetal BVDV infection, so that even a few virus particles crossing the placental barrier can result in PI or abortion. Zoetis MLV-BVDV vaccines cannot cause PI because the vaccine virus is a CP strain, and only NCP strains are associated with PI.
Zoetis development of fetal protection (FP) vaccine strains for its MLV-BVDV vaccine introduced a new dimension to vaccination of calves for BVD. The FP strains of BVDV used in Bovi-Shield GOLD^® FP^® and PregGuard GOLD^® FP^® vaccines have been shown to reliably avoid infecting fetuses in previously immunized dams, thereby allowing pregnant cows to be vaccinated.
*LABEL INDICATIONS: Do not use in pregnant cattle (abortions can result) unless they were vaccinated, according to label directions, with any BOVI-SHIELD GOLD FP or PREGGUARD GOLD FP vaccine prebreeding initially and within 12 months thereafter. Do not use in calves nursing pregnant cows unless their dams were vaccinated within the past 12 months as described above. To help ensure safety in pregnant cattle, heifers must receive at least 2 doses of any BOVI-SHIELD GOLD FP or PREGGUARD GOLD FP vaccine with the second dose administered approximately 30 days prebreeding.