Bovine Virus Diarrhea/Mucosal Disease (BVD)

Causative agent: BVD virus is an RNA virus from the family Togaviridae, genus pestivirus
BVD is a very complex disease. The behavior of this virus in cattle populations has not been completely established (9). Little is known about the behavior of the virus in bison herds, other than that it is present in bison herds (26) and that it causes disease (19). There are many different strains of the BVD virus identified in cattle populations (9). The strains of the virus found in bison have not been reported (19). BVD virus is found in all countries of the world where cattle are produced (9). Sixty to 80% of cattle tested have antibodies to the virus (9). The Yellowstone National Park survey of 1991-92 demonstrated antibody titers in 31% of bison tested (26).
Transmission of the disease in cattle is thought to be by direct contact between animals (horizontal) and across the placenta to the fetus (vertical)(9). In cattle, BVD virus can be shed in nasal discharge, saliva, semen, feces, urine, tears, milk and discharges following abortion of a fetus (9). Some cattle can become persistently infected. These animals shed large quantities of the virus for the rest of their lives and are probably the main source of infection in cattle herds (9).

Clinical signs:
There are several different manifestations of BVD in cattle (9).

  • Acute BVD

Acute BVD has not been described in bison. In cattle, acute BVD occurs in animals that have not been previously exposed to the BVD virus. This form of BVD can be an inapparent (symptomless) infection or it can cause acute explosive diarrhea associated with elevated body temperature and anorexia. These cattle are sick for a few days to a week and then recover. Occasionally affected cattle may die directly from the virus or from secondary infections. BVD infection in young calves may cause severe diarrhea (scours). Acute BVD is seen in bison and can cause mortalities in bison at any age (19).

  • Fetal infections

Fetal BVD infections have not been reported in bison. In cattle, they are seen following an acute BVD outbreak. Fetal infections can cause abortion, stillbirths , weak calves, and fetal abnormalities. If the fetus is infected with the virus in the first 125 to 150 days of gestation, the fetus may become persistently infected. When these calves are born they are infected and remain so for the rest of their lives (9).

  • Mucosal disease

Mucosal disease has not been reported in bison. In cattle, this disease only occurs in animals that are persistently infected. It is thought that all persistently infected calves die of mucosal disease at some time in their life, but how long they may live is unknown. Some persistently infected cattle remain relatively normal and shed the virus to other animals in the herd for years. They will pass the virus on to their own calves producing more persistently infected calves.
Persistently infected calves may be unthrifty, poor doing calves that have low weaning weights. Persistently infected calves may have a rough hair coat or they may have skin lesions. They may develop lameness with associated foot lesions. These calves may become suddenly ill at any time in their life with diarrhea. They may have an elevated temperature, anorexia, nasal and ocular discharge, and lesions in the mouth and feet. They may die quickly or become chronically ill, taking months to die.
If there many persistently infected animals in the herd, mucosal disease can occur in outbreak form. Outbreaks are thought to be triggered by the introduction of a different strain of BVD virus into the herd.

  • Thrombocytopenic form of BVD

This form of BVD has not been reported in bison. In cattle, this form of BVD is similar to the acute form of BVD in that animals are exposed to the virus and subsequently develop disease. This form of BVD produces hemorrhage into all mucous membranes, bloody nasal discharges, bloody diarrhea and bleeding from injection sites. Mortality associated with this form of BVD is very high.

Postmortem findings:
The pathological changes associated with BVD in bison have not been documented (19). In cattle, postmortem findings include shallow ulcers in the mouth, pharynx, esophagus, rumen, and omasum. There may be erythema and hemorrhages in the abomasal mucosa. There may also be lesions in the feet (19).

In cattle, blood samples for virus isolation or polymerase chain reaction (PCR) testing give reliable results (9), especially in cattle that are persistently infected. Most commonly the diagnosis is made on postmortem examination and submission of samples to a diagnostic laboratory. Skin samples from dead, persistently infected animals can be used to isolate the virus even if the carcass is decomposed or partly eaten by scavengers (19).

There are no specific treatment protocols established for treating BVD in cattle (9) or bison (19). Cattle and bison with BVD have not responded to any treatment (9,19).

There are no protocols established for controlling BVD in bison. In cattle, control of BVD has proven to be difficult for some forms of the disease. Vaccination may not provide adequate protection against the occurrence of mucosal disease and the formation of persistently infected cattle. Vaccination may provide adequate protection against acute BVD infection and the thrombocytopenic form of the disease in cattle.
There are many commercially available BVD vaccines. They are all designed for use in cattle and none of them have been registered for use in bison. The effectiveness of these vaccines in bison has not been established. The use of these vaccines in bison should be approached with caution. There is considerable variation among these vaccines. There are variations in the constituents of both modified live and killed virus vaccines. The adjuvants and immune stimulants found in killed BVD vaccines may have unfavorable affects on bison calves (19). Modified live BVD vaccines have been observed to cause diarrhea in recently weaned bison calves on bison ranches (19). Yearling, two year old ,and adult bison on bison ranches have not been observed to be as susceptible to the harmful effects of some of these vaccines (19). In the past, modified live virus BVD vaccines have been associated with abortion when administered to pregnant beef cattle. These vaccines therefore should probably not be administered to pregnant bison cows.
Vaccination programs for bison should not be a carbon copy of vaccination programs that have been designed for use in beef cattle. Vaccines should be chosen with careful consideration. The time of administration and the method by which vaccines are administered should be discussed thoroughly with bison ranchers. Programs should be developed that are sensitive to the reproductive and productive cycles of bison as well as to their behavior.

Handling Note!
The behavioral characteristics of bison have forced bison ranchers to develop new and creative methods for handling them. Handling of bison, however, is still a very common cause of injury and death. At some times of the year certain groups of bison should not be handled at all. Handling of bison cows from the onset of calving season until weaning time should be done with extreme caution. Cows with calves at foot will often challenge, rather than move away from people trying to chase them. Moving bison cows with calves at foot through narrow runways or confining them in small corrals will often cause cows to trample and seriously injure their calves.


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