Seventy to ninety percent of all bovine viral diarrhea virus (BVDV) infections occur without the appearance of clinical signs. In fact most cases of BVD remain undetected in the herd. The outbreaks of hemorrhagic Type 2 BVDV in Canada and Pennsylvania in the early 1990's has redirected public attention to BVD's potential threat. Unfortunately, many people have lost sight of the fact that the overwhelming majority of Type 2 infections, like Type 1, show no clinical signs and result in mild infections.

Background
BVD is caused by a virus which infects livestock and other ruminants such as llamas and deer. Related viruses include hog cholera virus in swine and border disease in sheep. The BVD virus appears in two genetically distinct groups, designated type one and type two. This numerical classification does not relate to the ability to cause disease. Type one and Type 2 BVDV each include hundreds of different strains. Common problems associated with the virus include diarrhea, general illness, abortion, oral ulcers, and immunosuppression. Type one or two can result in high mortality depending on the strain involved and the level of immunity in the cattle. However, cattle with an acute type 2 infection usually experience a more severe disease syndrome than cattle infected with type 1.

BVDV typically enters an animal via the oral/nasal route. Acute or persistently infected (PI) cattle shed the virus in mucus, saliva, feces, and urine. BVDV can also be transmitted, to a much lesser extent, by contaminated equipment, semen, fetal calf serum, and biting insects.

PI animals are the primary source of new BVD infections within the herd. Fetuses infected at less than 150 days gestation may die, have birth defects, or become persistently infected. After 150 days gestation, most calves develop an antibody response to the BVD virus and eliminate it. All calves born to PI dams will become PI BVD animals.

Severe Type II BVDV
Also known as bleeder syndrome, Type 2 BVDV is associated with hemorrhagic syndrome with high mortality. During a Type 2 BVDV infection, the virus adheres to and destroys platelets, or blood clotting cells. Nearly 300,000 platelets circulate within the bloodstream of a normal cow, compared to the fewer than 10,000 platelets in a cow suffering from Type 2 BVDV with hemorrhagic syndrome. As a result of decreased clotting ability, an animal with hemorrhagic syndrome may bleed for several hours from an injection site.

Infected cattle may initially experience decreased feed intake, a low grade fever, and bloody diarrhea. However, the first sign related to hemorrhagic syndrome is bleeding in the eyes. At necropsy, visible signs of infection include bleeding into the large muscles and organs. It should be noted than BVD-induced hemorrhagic syndrome is an uncommon finding in California.

Prevention and Control
Colostrum is the first line of defense against BVDV Types 1 and 2. To adequately protect all calves, each calf should receive adequate high quality colostrum containing BVDV antibodies shortly after birth. A dam with BVDV antibodies will transfer temporary passive immunity to the calf through her colostrum. This protection usually lasts 4-8 months. Protected by maternal antibodies, the calf is termed seropositive for BVDV.

To induce active immunity, calves should be vaccinated no earlier than one week of age. Since it is not possible to distinguish seropositive and seronegative calves without a blood test, all calves should be vaccinated. Vaccination will have little effect on calves with high maternal antibody from the colostrum; however, it will protect seronegative calves from infection and boost the immunity of calves with low maternal antibody.

Over 140 BVDV vaccines are licensed in the United States, most of which are produced from a Type 1 strain. Currently, four to five vaccines contain a Type 2 strain. Type 1 vaccines are effective in protecting cattle against Type 1 BVDV and provide partial protection against some Type 2 viruses. To provide a broader range of protection, some producers are using a rotational vaccination schedule - alternating use of vaccines produced from different strains of BVDV types one and two. Vaccines are available as modified live or killed. Modified live vaccines generally provide better cross protection against strains of BVD than killed vaccines. All vaccines should be used according to the manufacturer's instructions.

The groups most susceptible to BVDV are seronegative animals, calves that did not ingest sufficient colostrum, and cattle under environmental or disease-related stress. PI cattle are the primary source of infection as they continuously shed large amounts of virus in their feces, saliva, and nasal discharge. Producers can identify PI animals with a blood test and cull them to decrease the amount of BVD virus circulating on the dairy.

Ideally, newly purchased cattle should be vaccinated before entering the herd and isolated on the farm for two to three weeks. Unvaccinated new arrivals can develop BVDV when exposed to PI cows in the herd. Likewise, unvaccinated home herds can break with BVD if a newly purchased animal is shedding the virus. Sick animals should also be quarantined to prevent the spread of infection.

Another source of BVD exposure is contact with infected cattle in a neighboring pasture. Show cows interacting with animals outside the herd are at higher risk of exposure. Producers should be aware that sheep, goats, and deer also carry BVDV and can cross-infect cattle. These cross infections seldom cause visible disease in cattle.

Semen from an infected bull can transmit the disease. Most artificial insemination companies screen for BVDV so the risk of infection is minimal. However, on a farm with a history of BVDV, the bulls used for natural service should be tested for the virus.

BVDV can also survive in fetal calf serum. Before using the serum during an embryo transfer procedure, it should be tested for the virus. For export purposes, some companies treat BVDV contaminated fetal calf serum with chemicals or irradiation to kill the virus. This is not a common practice on commercial operations.

Prevention of BVDV may seem an overwhelming task considering the fact that types one and two contain hundreds of different strains. The informed dairyman can successfully meet the BVD challenge by following strict biosecurity guidelines, eliminating PI animals, vaccinating the entire herd, and providing calves with colostrum.

To complete a BVDV control or prevention plan, the herd should be vaccinated against BVDV according to the manufacturer's instructions. Your local veterinarian can help you design a vaccine program to suit your herd.