Bovine viral diarrhea virus has been described as one of the most serious pathogens in beef and dairy cattle.
It is responsible for causing early embryonic death and abortion, severe diarrhea, suppression of the immune system and a systemic condition known as mucosal disease, which usually results in death.
Persistently infected animals are the main source of the virus in cattle populations. Persistently infected calves are infected with the virus as fetuses between 40 and 120 days of gestation.
The fetus can become infected if an unvaccinated pregnant cow is exposed to the virus during this critical time period, and the calf will be born persistently infected with BVD virus.
These calves do not recognize BVD virus as a foreign agent, and their immune system will allow it to replicate freely in their bodies. They become walking virus factories.
Many of these animals eventually die of mucosal disease, but they are an important source of the virus while alive.
Estimates vary, but .3 to .5 percent of weaned calves are persistently infected with BVD virus.
Despite a relatively low prevalence, these persistently infected animals can spread the virus if mixed with other animals.
BVD virus is also known to severely suppress the immune system when cattle are exposed to the virus. This may result in making them more susceptible to other infectious diseases such as respiratory disease.
Scientific studies have attempted to measure the health and disease effects of being exposed to BVD virus in feedlot cattle.
Dr. Dan Grooms from Michigan State University and his coauthors recently published an article in the Journal of the American Veterinary Medical Association, which again tried to answer this question.
Grooms’ study involved three trials in which he compared groups of weaned calves that were exposed to persistently infected BVD animals with groups of weaned calves that had no exposure.
He and his colleagues then recorded health and productivity measures to determine if being exposed to a persistently infected animal made weaned calves more likely to get sick.
The study was also designed to allow researchers to examine the effects of vaccinating calves to see if it reduced the effects of being exposed to BVD virus.
The results were varied, but in general the experiments showed that persistently infected cattle adversely affect the health and performance of weaned calves in the feedlot.
Cattle that were exposed to BVD virus tended to be more likely to be treated for respiratory disease.
These results are similar to other larger studies that have examined the effect of BVD virus in feedlot cattle. Some studies have shown that being exposed to a persistently infected calf in the feedlot pen can make cattle more likely to get sick, although the effects have not been consistent across every study.
Some of Grooms’ experiments were able to show a protective effect in the calves that were vaccinated for BVD virus.
It would appear that BVD virus can have negative consequences for feedlot cattle, but the magnitude of the effect probably depends on the strain of the virus, the type and age of the cattle and management practices.
Feedlot producers typically vaccinate incoming cattle for BVD virus, but this management practice does not eliminate the persistently infected animal.
These animals were infected as a fetus, and the only way to prevent this from happening is to ensure the mother is properly vaccinated against BVD virus.
Vaccinating for BVD virus on arrival at the feedlot may have some benefits in terms of protecting other calves if a persistently infected calf is bought as part of a group of weaned calves.
However, significant exposure and transmission of the virus may occur before immunity from the vaccination has time to take effect.
BVD has important economic consequences for cow-calf and feedlot producers. The ultimate control of this disease rests in the hands of the cow-calf sector. Consult a veterinarian for advice about the most appropriate vaccination program to protect a herd from this virus.
It is important to remember that our vaccination program for BVD is designed to protect the fetus from becoming infected and therefore must focus on protecting the cow during early gestation.
John Campbell is head of Large Animal Clinical Sciences at the University of Saskatchewan’s Western College of Veterinary Medicine.