What can 40 years of vaccinations and 160 currently licensed vaccines do to eradicate a disease? When you are talking about bovine viral diarrhea virus, apparently not much—it is still thriving. Whereas cattle diseases such as foot-and-mouth have made headline news and are the subject of strict regulations, BVDV has remained, comparatively, in the shadows, unfettered by governmental regulations.
Scientists first identified BVDV in New York state dairy herds in 1946. Now, more than 50 years later, BVDV infections are seen in all ages of cattle throughout the world and have a major economic impact on the cattle industry.
"As far as we can tell, our serology indicates that we have not affected the level of BVDV at all," says Julia Ridpath, PhD, a microbiologist at the National Animal Disease Center in Ames, Iowa. "Our serology says that around 80 percent [of cattle in the United States] has been exposed, which is about what it was in 1960, so we are not making a big dent."
In response to the problem, the National Animal Disease Center held a meeting April 4-5 to discuss the ins and outs of the disease. Various researchers, diagnosticians, practitioners, producers, regulators, and biologic producers presented their research and ideas for controlling the disease.
|Severe acute BVDV may affect any age animal.|
A reproductive problem
Bovine viral diarrhea virus is a single-stranded, enveloped RNA virus that can cause acute diarrhea, fetal infections, and mucosal disease in cattle. Mucosal disease can result in high fever, epithelial lesions, anorexia, and even death. Animals that are acutely or persistently infected with BVDV infect other animals by shedding the virus through nasal and oral secretions, feces, and urine. No treatment is available.
Two species of the virus exist: BVDV-1 and BVDV-2. Although severe acute infection has been reported only with BVDV-2 strains, most of these strains are no more virulent than BVDV-1. Both strains cause reproductive problems. "The biggest problems with BVDV are the reproductive problems," Dr. Ridpath said. When a fetus is infected early in its intrauterine development, it may spontaneously be aborted or be born with congenital abnormalities.
For a small virus, 12.3-kilobase, it packs a wallop of a punch, causing the beef and dairy industries substantial economic losses. Infections reduce milk production and reproductive performance, retard growth, increase the susceptibility to other diseases, cause early culling, and increase mortality, especially among young stock.
In Denmark, estimated losses have varied from a few thousand to up to $100,000 per herd, said Dr. Hans Houe, a professor of veterinary epidemiology at the Royal Veterinary and Agricultural University in Denmark. Estimation of losses at the national level, he said, range between $10 and $40 million per million calvings. Losses in the United States are estimated to be at the high end of this range.
Close relative to hog cholera, similarly contagious
According to speakers at the conference, a variety of factors have let the disease thrive, one of which is the way the virus spreads. BVDV, a close relative to the pestivirus hog cholera, is highly contagious and lingers in the environment. It can be transmitted via contact with infected animals as well as infected housing, equipment, transport vehicles, clothing, and boots.
According to Dr. Hana Van Campen, who works in the Veterinary Diagnostics Laboratory at Colorado State University, BVDV also exists in wildlife such as free-ranging populations of mule deer, elk, and bison, and these animals are reservoirs for the disease.
There may also be problems with the vaccines, including the fact that some don't induce a high antibody titer. "We don't know whether the vaccines are being used improperly or the vaccines themselves don't work in the field setting," Dr. Ridpath said.
According to Dr. Kenny Brock, a professor in the Department of Pathobiology at Auburn University in Alabama, BVDV is a pathogen that is uniquely adapted to cattle by its ability to cause persistent fetal infections following in utero infections within the first 150 days of gestation. Its unique pathogenesis gives it a selective advantage allowing continual mutation and antigenic variation within the cattle population.
Because of the variation in the virus itself and the fact that some vaccines are made from virus strains that are 40 or 50 years old, efficacy is questionable. "There is cross-protection between isolates, but we just don't know at what level that cross-protection is effective," Dr. Ridpath said.
Roping in the virus
According to Dr. Brock, use of a modified-live or killed vaccine can provide a certain level of protection from acute disease and the development of persistent infection in fetuses of vaccinated cows, but it is clear that vaccination programs alone cannot control or eliminate the disease. In naturally exposed and vaccinated herds, BVDV infections are not self-limiting and may persist in herds over time, underscoring the ability of the virus' genome to remain fluid under selective pressures. Good biosecurity measures are key.
The emergence of strains that cause more severe disease in young calves has generated interest in protecting calves from acute BVDV infection at an earlier age than previously considered necessary, according to Dr. Janice Endsley, a veterinary pathobiologist at the University of Missouri-Columbia. Trials, she says, indicate that vaccinating young calves for BVDV while maternal antibody is present generates T cell- mediated immunity to BVDV.
Currently, however, the United States does not regulate for BVDV. Farmers are not required to vaccinate or test for the virus. Even divulging an animal's status is left to personal choice. "There is no requirement for them to do anything," Dr. Ridpath said. "You can sell a persistently infected animal."
But several European countries have taken action and implemented eradication or control programs that first identify and eliminate persistently infected animals. Further steps are based on the density and seroprevalence of the regional cattle populations.
Sweden, for example, implemented a voluntary program that relied on the certification of BVDV-free herds and continual monitoring at the expense of the farmers. The program did not include vaccination. Irene Greiser-Wilke, PhD, a researcher at the Institute of Virology in Hannover, Germany, says the program is helping. In 1993, roughly 35 percent of the herds were BVDV-free compared with 87 percent in 2001.
According to a position statement on bovine viral diarrhea virus from the Academy of Veterinary Consultants, the beef and dairy industries need to adopt measures to control and target eventual eradication of BVDV from North America. Dr. Ridpath agrees. "Until the National Cattlemen's Beef Association or the dairy association says we won't put up with this anymore, it won't change."
Without a BVDV-specific monitoring system, however, taking advantage of the National Animal Health Monitoring System already in place is one option. For example, BVDV surveillance could be piggybacked with Johne's disease surveillance. "As long as you are getting [Johne's] samples, it would be just as easy to get the BVDV samples," Dr. Ridpath said.
Whatever the solution, one thing was clear from the April meeting: controlling BVDV is going to take collaboration among diagnosticians, researchers, producers, and field veterinarians. According to Dr. Ridpath, "Everybody is realizing that we all have to get in the same boat and row."