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Cow Tales

Kenny Barrett Jr., DVM, MS

Beef cattle practice is slowing down across the upper Midwest and Great Plains. Most of the herd bulls have been tested for breeding soundness. Many have been turned out while some are awaiting their special day. Producers are busy putting up hay and fretting over rain. The mama cows are out to grass with babies at side. There is nothing like a great spring and early summer on a ranch. Fall, it seems, will be here soon and weaning is just around the corner. This installment of Cow Tales will focus on BVD virus, including its effects on cattle and what it means to the rancher.

The cattle virus most discussed today is BVD. BVD or BVDV stands for bovine viral diarrhea virus. Most often BVD has very little to do with diarrhea but rather leads to pneumonia and a variety of reproductive effects. There are two types of BVD, type 1 and type 2. Each type is further divided into cytopathic and noncytopathic strains. Cytopathic indicates a strain that destroys cells grown in a culture dish in a lab. Noncytopathic strains do not destroy the cells. As a virus, it requires a host animal for survival. Consequently, infected cattle serve as the primary source of the virus to infect other animals. Infected animals shed virus in every bodily secretion. Any beef or dairy animal with nose to nose contact will certainly be exposed. So what happens after exposure?

The primary effect of BVD is suppression of the immune system. This one statement is worth saying again: suppression of the immune system. In fact, modified live BVD vaccination is stressful to calves. BVD weakens the immune system making it difficult for the body to engage its biological weaponry crafting and easier route for bacteria and additional viruses to gain a foothold. This is the major reason BVD is a component in pneumonia in young stock. Most breeding animals have mature immune systems and, whether due to vaccination or natural exposure; can deflect the major thrust of a BVD onslaught. Breeding females, however, can develop a transient viremia endangering their unborn calves without showing any outward signs of disease themselves.

Viremia simply means virus circulating in the bloodstream. Because an unborn calf in the uterus is nourished by the dam’s blood they are at risk from blood-born insults. One of four outcomes is possible in this scenario. Early in pregnancy the infected fetus can abort or the virus can cause birth defects. The defects observed originates from which fetal cells are dividing and differentiating at the time of infection. Fetuses infected during the latter part of gestation generally mount an effective immune response and eliminate the virus. Sometimes these calves are born weak and unthrifty. If the fetus is infected during the middle part of gestation the breeding female can give birth to a calf that is persistently infected. During the middle part of gestation the fetal calf is developing their own immune system. The immune system is sampling everything it can to recognize all of its own cells as “self.” This is important to keep the body from attacking itself throughout life. However, a BVD infection at this crucial time will cause the immune system to mistakenly think of BVD as “self.” The immune system will never mount an offensive against that type of BVD virus. These calves are persistently infected and will remain so until they die.

Persistently infected (or PI) calves are born completely normal. There is one major difference: these calves expel huge amounts of BVD virus into their environment infecting all of the cattle they encounter. Because BVD suppresses the immune system, these calves usually succumb to disease as calves. However, it is important to note some PI animals survive and enter the breeding herd creating huge problems. A breeding PI female will have a persistent viremia throughout her gestation exposing the fetus to BVD the entire time. Any calf born from a PI heifer or cow will always be PI.

Vaccination and PI status are the primary concerns for beef cattle producers. It is difficult to find a bull producer, bull stud, or bull leasing operation that doesn’t test and advertise their animals as BVD tested. Again, the chances of finding a breeding age animal that is BVD-PI positive are slim, but the consequences of adding one to a cow herd would be devastating. Buying cattle from mixed or unknown sources increases the odds of encountering a PI animal. The greatest risk occurs in the feeding sector. Feedlots are well aware of BVD. Some test suspicious pens of cattle while other managers advertise all cattle as tested on arrival.

Producers should work with a veterinarian to ensure adequate sampling and tissue handling for accurate results.

There are many vaccines for BVD. As mentioned earlier there are two types of BVD virus. Vaccination against one type is not necessarily protective against the other. Therefore, most commercial vaccines include both types of BVD in their formulation. Producers need to be aware of BVD, the meaning of PI, and work with their veterinarian to formulate a complete herd health approach. The details of testing animals for BVD and BVD-PI status can be somewhat confusing. Consult with your veterinarian to determine your risks associated with BVD. Remember, the primary role of BVD in animal health is suppression of the immune system and a BVD-PI female will always have a PI calf while a non-PI female can give birth to a PI calf under the right circumstances.

Beef cattle practice is slowing down across the upper Midwest and Great Plains. Most of the herd bulls have been tested for breeding soundness. Many have been turned out while some are awaiting their special day. Producers are busy putting up hay and fretting over rain. The mama cows are out to grass with babies at side. There is nothing like a great spring and early summer on a ranch. Fall, it seems, will be here soon and weaning is just around the corner. This installment of Cow Tales will focus on BVD virus, including its effects on cattle and what it means to the rancher.

The cattle virus most discussed today is BVD. BVD or BVDV stands for bovine viral diarrhea virus. Most often BVD has very little to do with diarrhea but rather leads to pneumonia and a variety of reproductive effects. There are two types of BVD, type 1 and type 2. Each type is further divided into cytopathic and noncytopathic strains. Cytopathic indicates a strain that destroys cells grown in a culture dish in a lab. Noncytopathic strains do not destroy the cells. As a virus, it requires a host animal for survival. Consequently, infected cattle serve as the primary source of the virus to infect other animals. Infected animals shed virus in every bodily secretion. Any beef or dairy animal with nose to nose contact will certainly be exposed. So what happens after exposure?

The primary effect of BVD is suppression of the immune system. This one statement is worth saying again: suppression of the immune system. In fact, modified live BVD vaccination is stressful to calves. BVD weakens the immune system making it difficult for the body to engage its biological weaponry crafting and easier route for bacteria and additional viruses to gain a foothold. This is the major reason BVD is a component in pneumonia in young stock. Most breeding animals have mature immune systems and, whether due to vaccination or natural exposure; can deflect the major thrust of a BVD onslaught. Breeding females, however, can develop a transient viremia endangering their unborn calves without showing any outward signs of disease themselves.

Viremia simply means virus circulating in the bloodstream. Because an unborn calf in the uterus is nourished by the dam’s blood they are at risk from blood-born insults. One of four outcomes is possible in this scenario. Early in pregnancy the infected fetus can abort or the virus can cause birth defects. The defects observed originates from which fetal cells are dividing and differentiating at the time of infection. Fetuses infected during the latter part of gestation generally mount an effective immune response and eliminate the virus. Sometimes these calves are born weak and unthrifty. If the fetus is infected during the middle part of gestation the breeding female can give birth to a calf that is persistently infected. During the middle part of gestation the fetal calf is developing their own immune system. The immune system is sampling everything it can to recognize all of its own cells as “self.” This is important to keep the body from attacking itself throughout life. However, a BVD infection at this crucial time will cause the immune system to mistakenly think of BVD as “self.” The immune system will never mount an offensive against that type of BVD virus. These calves are persistently infected and will remain so until they die.

Persistently infected (or PI) calves are born completely normal. There is one major difference: these calves expel huge amounts of BVD virus into their environment infecting all of the cattle they encounter. Because BVD suppresses the immune system, these calves usually succumb to disease as calves. However, it is important to note some PI animals survive and enter the breeding herd creating huge problems. A breeding PI female will have a persistent viremia throughout her gestation exposing the fetus to BVD the entire time. Any calf born from a PI heifer or cow will always be PI.

Vaccination and PI status are the primary concerns for beef cattle producers. It is difficult to find a bull producer, bull stud, or bull leasing operation that doesn’t test and advertise their animals as BVD tested. Again, the chances of finding a breeding age animal that is BVD-PI positive are slim, but the consequences of adding one to a cow herd would be devastating. Buying cattle from mixed or unknown sources increases the odds of encountering a PI animal. The greatest risk occurs in the feeding sector. Feedlots are well aware of BVD. Some test suspicious pens of cattle while other managers advertise all cattle as tested on arrival.

Producers should work with a veterinarian to ensure adequate sampling and tissue handling for accurate results.

There are many vaccines for BVD. As mentioned earlier there are two types of BVD virus. Vaccination against one type is not necessarily protective against the other. Therefore, most commercial vaccines include both types of BVD in their formulation. Producers need to be aware of BVD, the meaning of PI, and work with their veterinarian to formulate a complete herd health approach. The details of testing animals for BVD and BVD-PI status can be somewhat confusing. Consult with your veterinarian to determine your risks associated with BVD. Remember, the primary role of BVD in animal health is suppression of the immune system and a BVD-PI female will always have a PI calf while a non-PI female can give birth to a PI calf under the right circumstances.


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