Bovine Viral Diarrhea Virus, or BVDV
Did you know that “Bovine Viral Diarrhea” is actually an all-inclusive term for a clinical disease manifestation that potentially involves one or two distinct viruses? Two distinct biotypes of the virus, two viral states of infection and five distinct clinical forms of acute disease are seen with the Bovine Viral Diarrhea Virus (BVDV). This complex is also known as “Bovine Virus Diarrhea”, “Mucosal Disease”, or “Bovine Pestivirus Disease Complex”.
The BVDV viruses are Pestiviruses that are members of the Flavivirus family. The two species of viruses involved in BVDV are designated as BVDV1, for Bovine Viral Diarrhea Virus 1, and BVDV2, for Bovine Viral Diarrhea Virus 2. There are two distinct biotypes of the virus which are classified based on how the virus grows in tissue culture. These biotypes are known as cytopathic and noncytopathic. There are two states of infection: chronic and acute. A chronic infection occurs when a cow is persistently infected with the virus, showing only mild or no clinical signs of infection. These cattle are still able to transmit the virus to susceptible individuals. An acute infection occurs when a cow is suddenly infected with full-blown clinical signs of disease. In addition there are five distinct clinical forms of acute disease: severe acute BVDV infection, hemorrhagic BVDV infection, acute BVDV infection/bovine respiratory tract disease and acute BVDV infection/immunosuppression, and mucosal disease.
Acute BVD infections may be seen clinically as an intestinal tract infection (enteric form), respiratory tract infection, or an infection of the reproductive tract. The disease may run anywhere from a subclinical (no outward signs of disease) to disease so severe that it results in the death of the infected individual. Factors involved in the severity of the disease include: the strain of the virus (some strains of the virus are more severe or pathogenic), the immune status of the host (whether the cow was vaccinated or previously exposed to the virus), the reproductive status of the cattle (whether a cow is pregnant or not), and the presence and severity of secondary infections.
Most acute infections involve animals having no immunity to the virus and are seen clinically as diarrhea, oral erosions or pneumonia which is typically mild. High mortality rates are seen most commonly with the hemorrhagic form of acute disease. Most acute infections do not exhibit long term-effects. Acute infection in pregnant cattle causes them to abort, have stillbirths, teratogenic (birth defects) effects may occur to the fetus or a calf may be born with a persistent infection. Calves infected with the virus prior to birth are often weak and unthrifty. Calves will sometimes appear healthy although infected and will develop clinical disease at a later date which may develop into full blown gastrointestinal disease and even death. One study found that calves exposed to the BVDV in utero after 125 days of gestation had a twofold higher rate of severe clinical illness during their first 10 months of life compared with cattle not exposed in utero.
All acute infections cause suppression of the immune system. This immunosuppression causes the infected individual to be more susceptible to secondary infections with bacteria, additional viral infections, etc.
The noncytopathic BVDV is the predominate biotype found in nature. Noncytopathic BVDV can also establish a lifelong infection in its host. These persistent infections are a result of the animal being exposed in utero (as a fetus) before the immune system of the calf is developed.
Persistently or chronically infected cattle are believed to be the major source of infection for other animals. Currently there is not a consistently accurate test for the detection of persistently infected BVDV carriers. False positive and false negative tests may be achieved. There is currently a need for a cost-effective, highly accurate test for the detection of these carriers that can be easily conducted chute-side. Until such time, persistent carriers will not be fully eliminated.
The virus is present in large numbers in the secretions and excretions of infected animals. Transmission is by direct contact or through the use of contaminated feed or equipment. The semen of inapparent carriers has also been found capable of transmitting the virus. The typical incubation period is from 5 to 10 days.
There is no specific treatment for any form of BVDV other than supportive care. Severely affected cattle are often slaughtered for salvage or euthanasia should be considered. Persistent carriers, when identified, should also be culled and destroyed.
Vaccination programs currently exist with the efficacy dependent upon the type of virus used for vaccination, the type of vaccine killed or modified-live, and the subgenotype of the virus incorporated in the vaccine. The more prevalent subgenotypes seen in the United States are BVDV1a, BVDV1b, and BVDV2a. Most commercially available vaccines and currently available diagnostic test are currently based on the BVDV1a and BVDV2a strains only. In other parts of the world different variations exist and would have serious consequences if introduced into a population of cattle never having been exposed to that particular variant.
Although BVDV is known as a pathogen of cattle, the virus also infects many other species of wild and domestic ruminants and pigs. Some of the wild ruminants susceptible to the virus include the white-tailed deer, the mule deer, the fallow deer, elk, the red deer, the roe deer, the eland, and the mousedeer. White-tailed deer display similar clinical signs of infection as cattle. Deer can also be persistently infected. There has been transmission of the virus demonstrated between cattle and deer.
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Ridpath, Julia and Robert Fulton. “Knowledge gaps impacting the development of Bovine Viral Diarrhea Virus control programs in the United States.” JAVMA, Vol 235, No. 10. November 15, 2009. Pp. 1171-1179.