BVDv (bovine viral diarrhea virus) often infects cows without signs that the virus is spreading in their body. The virus has the ability to pass through the placenta of the cow to the fetus and can create a calf that is persistently infected (PI) with BVDv.
The ability of cattle to protect themselves from the many disease agents and complexes to which they're exposed is amazing. And it all boils down to a balance between the immunity cattle have to a disease and the level of challenge from potential disease organisms.
Immunity, or resistance of the herd to disease, is affected by many factors: natural exposure, vaccination, nutrition, age, neo-natal colostrum and physical stress. Challenge is the amount of infective organisms an animal is faced with.
“We stress that the ability of an organism to cause disease is not a fixed characteristic,” says David Thain, University of Nevada Extension veterinarian. “Immunity depends on many factors affecting the ability of an organism to invade tissues and produce chemical toxins, which do the real damage to an animal's system.”
Cattle can acquire resistance to a disease either through “passive” immunity or an “active” triggering of the immune system.
Passive immunity gives temporary protection though the transfer of immune substances from resistant individuals, says Thain.
An example of passive immunity and an important factor in cattle herd immunity is the passing of antibodies from dam to calf via the colostrum.
When a calf is born, it has no passive or active immunity to disease, explains Mel Pence, DVM, University of Georgia College of Veterinary Medicine. “This situation is unique. In humans, immunity flows across the placenta from the mother to the baby; but the only immunity a calf gets is from the colostrum provided after birth.”
Colostrum-induced passive immunity works very well except when diseases enter the herd that cows have not been exposed to or vaccinated for. This is especially critical during the calving season.
“To further complicate the situation, maternal antibody levels vary between calves and may last one day or several months,” he says.
Maternal antibodies have a half-life of approximately 21 days; that is, in 21 days a calf will have only one-half the amount of maternal antibodies it absorbed from the colostrum. In another 21 days, the calf will have only one-fourth the original amount (½ of ½) of maternal antibodies, and so on.
“Because of the rapid reduction in maternal antibodies in the calf,” Pence says, “it's important that we stimulate the dam by vaccination to stimulate a humoral immune response to produce as high a level of colostral antibodies as possible to prolong passive immunity in the newborn.”
But, by their very nature, infectious agents have built-in mechanisms for evolving very rapidly to evade animal defense mechanisms. Many disease agents very effectively take advantage of new opportunities inadvertently presented by changes in livestock management and environmental stress.
Providing active immunity
Producers are usually most concerned about providing active immunity through vaccination with products containing a virus or bacteria. The use of vaccines to elicit active immunity is based on the principal of the antigen-antibody reaction, says Clell V. Bagley, Extension veterinarian, Utah State University.
“The vaccine serves as the antigen and stimulates the immune system to produce antibodies against a specific agent,” he says. “Later, when the actual infectious agent penetrates the body defenses, the antibody is present and able to inactivate the invader.”
Immunization to a disease may occur when cattle are exposed to the naturally-occurring bacteria or virus. But getting cattle enough exposure to only get them a little sick but recover, and then develop a good immune response, is difficult.
“Natural exposure to disease generally produces a better, longer lasting immunity than vaccination, if the cattle live through the disease,” Bagley adds. But most scientists and veterinarians don't recommend depending on natural exposure to achieve effective immunization.
“Immunity to a specific disease can be affected by a vaccination only if the total program is effective,” Pence explains. “In order for immunity to protect cattle, we must understand what viruses or bacteria we need to vaccinate against.”
Thain says even though cattle have been vaccinated, don't expect they're going to be 100% protected. And he says he cannot overstate the value of studying and understanding product labels.
“But, be aware that you can do everything exactly according to label directions, and follow recommendations to the letter on nutrition and calf health, and the calves may still be vulnerable to a disease,” says John Paterson, Extension beef cattle specialist, Montana State University.
“Building immunity requires adequate protein and proper levels of various trace minerals that will help the animal effectively respond to the active immunity intended from the vaccine,” Paterson says.
Vaccine types and timing
The decision to use modified-live virus (MLV) vaccines or killed virus vaccines is an issue of concern for the beef cattle producer. This decision should not be made based on general recommendations, but rather on the advice of the veterinarian who understands the details of the operation and the needs of the individual producer.
It's the responsibility of the cattle producer to administer vaccines at the appropriate time, Thain says. Timing of vaccination protocols should also be done with the advice of an attending veterinarian.
“The vaccine must be given before exposure to the challenge and with enough time to allow immune response to develop to a protective level,” he says. “In general, immune response requires 10-14 days for a detectable response to develop following initial exposure to the antigen.”
Some producers are concerned about causing abortions in the cows if the nursing calves are vaccinated with a MLV, but research vets now say that the risk for such an event is miniscule.
“There's no reason why a MLV labeled for such use should not be used on nursing calves,” says Bethany Lovaas, DVM, University of Minnesota. “MLVs will provide better protection against the viral respiratory pathogens because they will stimulate two different populations of immune cells, whereas a killed vaccine will only stimulate one of these populations.”
BVD fetal protection
Many of the new generation of BVDv MLVs have a label claim for fetal protection and have been approved for use in pregnant cows and calves nursing pregnant cows. Fetal protection is defined as “the ability to prevent transplacental transmission of BVDv and subsequent fetal infection in pregnant cattle.”
With a nationwide initiative by veterinary organizations and through organized efforts in many states to eliminate BVDv (see “Ranch BVD-PI Biosecurity,” page 18), the ability of vaccination to prevent fetal infection is important in reducing the potential for BVDv transmission.
The USDA has published requirements for vaccine manufacturers to follow in determining fetal protection. Labeling guidelines of BVDv vaccines — Center for Veterinary Biologics Notice 02-19 — may vary depending on the infecting BVD virus. A vaccine may provide different levels of protection to the fetus when different strains of BVD viruses are used for virus challenge.
However, if these products are used after exposure of the fetus to BVDv, some PI calves may have already been created in the pregnant cows.
The label claims for fetal protection don't cover the gamut of protection needed to protect against all the various strains of BVDv in the environment, warns Thain.
“It's important to remember that vaccination of pregnant animals will not provide complete protection of the fetus,” he says. “Therefore, it is important to ensure that pregnant animals are not exposed to BVDv, especially during the first trimester of gestation.”
He says a producer should begin a BVD biosecurity program by vaccinating all replacement heifers with an MLV. This treatment should be after the heifers are six months old, but at least one month before breeding.
“If possible, cows should be vaccinated 30 days before the beginning of the breeding season,” Thain adds. “This provides the best chance for immunity and the best chance the fetus will not become infected with the virus.”
Thain also warns that vaccination is not always 100% effective in a herd.
“No matter how well protected individual animals are, the BVD virus can still challenge the animal enough to overwhelm the immunity we provide,” Thain says. “But properly timed and properly administered vaccination is very important in keeping animals healthy and productive.”
Ranch BVD-PI biosecurity
Prevention of BVDv should center on the identification and elimination of PI cattle before they're introduced to a ranch's breeding herd.
The general principles of biosecurity as applied to BVD virus include increasing the resistance of the host individuals to acute infection through a sound vaccination program.
“But vaccination alone is not a herd-health program and should not be considered as the sole safeguard to control infection by the BVD virus,” says Montana State Veterinarian Marty Zaluski. “The tremendous amount of virus secreted by a PI calf can overwhelm a level of immunity that is protective under less severe exposure.”
He recommends that all ranchers and cattle feeders work with their attending or consulting veterinarians to incorporate appropriate components of a biosecurity program as part of overall herd health management.
“The single largest risk factor for BVDv infection is the purchase of animals carrying the infectious agent,” Zaluski says. “The most important piece of information that a purchaser can have is documentation about the health status of the herd of origin.”
BVD at the ranch
Management and control of BVDv infection in cattle herds must consider the two ways that BVD virus passes from one animal to another within a herd.
The first is horizontal transmission, which occurs when a transiently (temporarily) or PI animal releases the virus and the virus enters a susceptible animal. The second is vertical transmission of BVDv from an infected dam's bloodstream to her fetus during pregnancy.
“PI calves can develop in the uterus of the dam if the heifer or cow is exposed to BVDv,” Zaluski adds. “This is the only way a PI animal is created.” Most literature reports the susceptible period is the first part of gestation — 30-150 days after conception.
Fetal infection with BVDv can lead to fetal death, birth of deformed calves, underdeveloped calves or a visually normal calf at birth.
Once a calf is BVD-PI it will always be a PI animal. If an animal is not PI at birth, it can never become PI. Further, PI cows will always produce a PI calf.
“The primary source of BVDv infection is PI cattle, which usually have a very high and virulent amount of virus circulating in their bodily fluids,” Zaluski explains. “And they shed the BVD virus continually.”
To reduce the risk of exposing fetuses to the virus, PI carriers should be identified and eliminated from the herd before cows are exposed to breeding.
The Montana BVD-PI Herd Biosecurity Project uses a screening process approach based on analysis of fresh “ear notch” samples from individual animals submitted by the producer or attending vet.
The focus of the Montana project is on “whole herd” screening of cattle herds for PI status. This generally involves screening, which begins with breeding bulls, open replacement heifers and new crop calves — with all samples taken well in advance of the breeding season.
Perpetual testing for PIs from a cow-calf management standpoint is not necessarily recommended. Once a rancher screens his/her herd (whole herd screening) there can be reasonable assurances of PI-free cattle with the following management in place:
A vaccination protocol based on modified-live vaccines.
A sound biosecurity program.
Testing of any new herd additions.
Surveillance of suspect animals. Other key points
Cows don't need to be tested for PI status unless they have a positive PI calf.
The main negative effect of BVDv is it can inhibit conception and/or cause abortion.
BVDv suppresses the immune system of infected cattle.
Individual animal ID is critical to match tissue samples with the calf and its dam.
If an animal tests negative for PI status, there's no need to ever retest that animal.
PIs that live to be breeding females will always produce a PI calf.
Bulls should be purchased as BVD PI free or tested prior to the breeding season.
A plan should be developed to eliminate PIs from the herd.
No PI animal should enter commerce.
Fact sheets on the Montana BVD PI Herd Biosecurity Project can be obtained through the Montana BQA website: www.mtbqa.org. Click on “Projects.”
— Clint Peck