 |
||||
|
||||
|
||||
Bovine Viral Diarrhea virus continues to have a significant impact on US cattle producers, despite the fact that there are more than 140 federally licensed vaccines to prevent the disease, according to the USDA. In 1992, two BVD genotypes, or strains, of the virus were discovered. With this discovery, many vaccine manufactures have developed a BVD vaccine for both Type I and Type II. However, with the incidence of BVD on the rise, researchers are devoting more resources to produce still better vaccines and develop additional diagnostic tools to identify infected animals. Dr. Chris Chase of South Dakota State University's Veterinary Science department says BVD is not just a problem in cattle. "I think it is the problem right now," he says. "We're certainly seeing more of the disease at our diagnostic lab." Acute vs. Persistent Infections Cattle infected with BVD fall into two categories: acute infections and persistent infections. Acute infections result from direct exposure with an infected animal. Once infected, the clinical signs of acute illness are typically mild, according to Kenny Brock of Auburn University College of Veterinary Medicine, Pathobiology department. Mild fever and diarrhea may be observed following an acute infection with the virus in calves or cows. Cattle become persistently infected (PI) when they are exposed as a fetus between the first and fourth month of gestation. During this time, the fetus' immune system is developing. If the gestating cow is exposed to BVD, the fetus is also exposed. The immune system of the fetus does not recognize the virus as a pathogen, but rather as part of its immune system. Therefore the fetus does not develop an immune response to attack the virus. The increase in BVD cases is in part due to the difficulty of identifying persistently infected animals. By the time it is identified, the calf has had the opportunity to spread the disease to the entire herd. The virus is spread through nasal and oral secretions, feces and urine. "PI animals who survive are constantly producing and shedding the virus into the environment," says Brock. "Additionally, calves born of persistently infected cows will also be persistently infected with BVD." "PI cattle are a source of infection for other cattle as long as the PI calves are alive," Chase says. "It doesn't matter how good your vaccine is. You'll always have a problem because of the continuous exposure." Once a calf is infected, the virus weakens the immune system, setting the stage for a secondary bacterial infection to take over. "BVD is immuno-suppressive by nature, making it a major contributor to respiratory disease," says Chase. "If there's a few PI calves shedding the virus to the others, the herd's immune system is weakened. Add some stress, and you could have a full-blown wreck on your hands." Prevention Vaccinating against the disease is a move in the right direction, but using vaccination as the sole means of control is very difficult, says Chase. He recommends a combination of three management practices to prevent herd infection.
Type of Vaccine Researchers have now identified two strains of Type I, "A" and "B." There is a lot of diversity in the virus which makes designing a vaccine very difficult, says Chase. "The virus can change relatively fast." There are advantages and disadvantages to both killed and modified live vaccines, says Brock. Modified-live vaccines (MLV) establish an immune response by replicating in the calf and allowing his immune system to develop antibodies to fight off the disease. "In an open herd with cattle moving in and out, it is important to introduce the vaccine and establish immunity as quick as possible," says Brock. "The MLV establishes immunity faster and provides a more natural exposure to the virus. If biosecurity measures are not in place, a MLV gives more immunity, faster than a killed vaccine." Although modified-live vaccines provide more long-term immunity and are administered with only one injection, slight suppression of the immune system may result. Killed vaccines also establish an immune response by introducing the virus in the calf's system, but the virus does not replicate. According to Chase, a killed vaccine puts less stress on the animal. "If the cattle are highly stressed, a killed vaccine may be easier on the cattle." However, two doses are required to establish immunity. Additionally, the level of immunity may be lower. Consider the amount of stress and the environment when deciding between a killed or modified live vaccine, Chase says. "Just as important as deciding which type of vaccine to use is simply choosing to vaccinate," adds Brock. Calves nursing pregnant cows must receive the killed vaccine. With the modified-live vaccine, there is a risk of the cow's fetus being exposed to the live virus from the suckling calf. If that occurred, the fetus -- if it survived -- would be persistently infected. BVD was once considered a disease in dairy cattle. However, its presence in beef cattle is growing, says Chase. "It's definitely here, and it's big." In the next issue of Feed*Lot, we will examine the effects of BVD in the cow herd and replacement heifers. |
||||
|
|
||||
|
||||
|
All information is copywrited by Feed Lot magazine and cannot be printed or re-printed
without the publishers express consent. Please contact
Feed Lot Magazine for reprint and copy authorization.
|
||||