When the BVD virus mutated into a new form that killed more than 10,000 dairy calves and cows in Canada and Pennsylvania in a little over a year's time in 1993 and 1994, a lot of producers who hadn't given due respect to this mysterious killer gained a sudden new appreciation.
It's a respect that hasn't been lost on veterinary researchers, though. Many of them believe Bovine Viral Diarrhea Virus to be the country's most economically important virus in both beef and dairy cattle. In fact, USDA veterinary researcher Steven Bolin has speculated that if not for current vaccination programs, fully half of all U.S. cattle would be clinically sick with it.
In the last two decades, advances in molecular genetics have led to an increased understanding of the wide range of clinical diseases associated with this mysterious virus.
BVDV is a pestiviruses, a small, enveloped virus that compensates for its relative susceptibility to environmental conditions by having a high potential to mutate in response to immune pressure. That "antigenic drift" leaves BVDV able to persist in the cattle population. It also creates a lineup of differing BVD viruses that are extremely variable in their degree of virulence. Some strains cause serious health problems, while many others are apparently quite avirulent, producing little to no signs.
Despite that complexity, one fact remains clear, BVD can and should be controlled. Some estimates say that up to 95 percent of U.S. cattle herds have animals that would test positive for BVDV virus. If you delay control measures until clinical signs show up, you're inviting a wreck. Control is usually managed by a combination of three tools:
1. Appropriate vaccination
Disasters in the past caused when carrier animals, called "persistently infected," were vaccinated with modified-live BVDV—which then caused the disease—soured some producers against vaccination.
BVDV control via vaccination is complicated by the virus' diversity, which can compromise cross-protective immunity from vaccination. Traditionally, modified live vaccines have been used for their ability to produce quick cell immunity, to have a longer duration of activity, and to better cross-protect against both biotypes of BVDV. Killed vaccines were generally regarded as less risky because of their inability to cause the disease or to create calves born persistently infected due to vaccination.
Generally, MLV vaccination is appropriate for building initial immunity in young breeding stock before they enter the herd. Then, a killed vaccine can be used as a safe booster for adult and young cattle. Although only one product on the market, Vira Shield from Grand Labs, contains antigens for Type 2 BVDV, research suggests there is some cross protection between Type 1 and 2 vaccines.
2. Identify and cull carriers.
Acute or persistently infected cattle are the primary source of re-infection in herds. Blood testing and culling infected carriers can help eliminate the disease from your herd. However, although new tests are under development, that fact that no test can differentiate PI animals makes testing a long and costly commitment. Work with your veterinarian to suggest a practical start.
3. Keep out infected animals.
The best way to prevent BVD is to keep it out through effective biosecurity measures. (See "Ranch biosecurity: Still a contradiction in terms?" in May 1998 Practical Health, pp. 8-10.)
The different sides of BVD
BVD viruses are the cause of these disease syndromes:
Peracute BVDV disease
Fetal BVDV disease
BVD History
1946—BVD first distinctly termed a cause of gastroenteritis with severe diarrhea in New York dairies.
1953—Mucosal disease first described in Iowa cattle. Though first thought to be a separate disease from viral diarrhea, researchers came to realize the same virus caused both.
Late 1960s-70s—Researchers identify BVD Virus as a close relative of hog cholera virus and the bovine border disease virus.
1993—Severe, acute BVD infection strikes Ontario and Pennsylvania dairy herds. The new mutation comes to be referred to as Type 2 BVD.
Source Alves, D., CEPTOR, Dec. 1994.
At risk?
Are you putting yourself at risk of a BVD outbreak by your vaccination practices? Try this checklist, based on actual BVD outbreak investigations.
qYou don't vaccinate for BVD.
qYou don't know whether your vaccine contains BVD.
qYou vaccinate breeding stock once—even twice—per year with killed vaccine, but you never initially give a primary series, a vaccine plus a booster spaced three to four weeks apart.
qYou vaccinate calves at six months of age but never revaccinate before they enter the breeding herd.
qYou vaccinate calves under six months of age but never fully re-vaccinate after six months of age.
qYou don't ensure that purchased cattle or boarders have been given a primary series before including them in the herd's annual killed vaccination program.
qYou refuse to vaccinate without laboratory confirmation.
qYou don't vaccinate bulls.
qYou leave a few dry cows, cows you plan to cull, or heifers unvaccinated.
qYou don't vaccinate cows before breeding to prevent abortions or P.I. calves, or to protect newborn calves through passive immunity.
qYou don't ensure that calves receive initial "natural vaccination" by getting a full gallon of colostrum from immune cows within 12 hours of birth.
Reprinted with permission from Practical Health, Farmland, August 1998.