Persistently infected calves are important sources of bovine viral diarrhea virus in cattle, but they might also cause respiratory disease at the feedlot level.
Dr. Dan Grooms of the Michigan State University recently told the Academy of Veterinary Consultants in Denver, Colorado, that these calves occur at a relatively low prevalence of three to five of every 1,000 weaned calves arriving at the feedlot, but they can play an important role in how disease occurs within the feedlot.
The persistently infected calf sheds huge quantities of virus and will spread it to other exposed calves, usually those in the same pen.
BVD virus can temporarily suppress an animal’s immune system, which makes it more susceptible to other infections from other bacteria and viruses.
This means that a feedlot calf that shares its environment with a persistently infected animal might be more likely to get respiratory disease because of the immunosuppressive effects of the BVD virus infection.
Grooms summarized three studies that looked at the effects of exposure to BVD virus on feedlot cattle health.
One of the studies involved two truckloads of weaned calves (92 calves per truckload), which were transported to Michigan State’s Beef Cattle Research Center.
All of the calves were unvaccinated for BVD virus and had not previously been exposed to the virus.
Two persistently infected BVD calves were added to one of the two trucks just before transport, and half of the calves on each truckload were given a modified live BVD vaccine when they arrived at the university.
The calves exposed to the persistently infected animals were housed separately throughout the feeding period.
Calves that were continuously exposed to persistently infected BVD calves were twice as likely to be treated for respiratory disease than the calves that were not exposed to persistently infected calves.
Calves that were vaccinated for BVD virus were 57 percent less likely to require re-treatment than calves that were not vaccinated.
Another study created five groups of calves on a farm before transporting them to the university.
Half the calves in each group received a modified live BVD vaccine two weeks before weaning. A persistently infected BVD calf was introduced to two of the five groups, which created four treatment groups:
- Exposed to persistently infected calves preweaning and vaccinated for BVD preweaning.
- Exposed to persistently infected calves preweaning and not vaccinated for BVD preweaning.
- Not exposed to persistently infected calves preweaning and vaccinated for BVD preweaning.
- Not exposed to persistently infected calves preweaning and not vaccinated for BVD preweaning.
The calves were placed together in one pen once they arrived at the university’s feedlot.
The study did not find a link between exposure to cattle persistently infected with BVD and treatment rates, but it did demonstrate that cattle vaccinated for BVD virus pre-weaning were less likely to be re-treated.
The third study involved a group of 500 pound beef calves, half of which were given a modified live BVD vaccine three weeks before weaning.
The calves were moved to the university’s feedlot at weaning, and four BVD persistently infected calves were introduced. They stayed with the group throughout the feeding period.
The treatment, re-treatment and re-pull rates did not vary significantly between the treatment groups in this study.
The three studies attempted to replicate typical feedlot scenarios, in which weaned calves may be exposed to persistently infected BVD calves when they are placed together in the auction market or feedlot.
The results are variable, but these and other similar studies have demonstrated that calves that are constantly exposed to persistently infected BVD calves may have higher treatment rates for respiratory disease, probably as a result of the immunosuppressive effects of BVD virus.
However, the results are not consistent, and other factors probably affected the results, such as age and immune status, stocking density and perhaps the strain of BVD virus involved. Several studies, including some of Grooms’ research, were unable to show a significant impact from being exposed to a persistently infected calf.
Ideally, we would like to prevent exposure of weaned calves to persistently infected BVD calves. Controlling BVD virus largely depends on preventing these persistent infections from occurring.
It is important to understand that persistently infected calves are created only through fetal infection. As a result, our vaccine strategies must protect the fetus by vaccinating the cow.
This disease can significantly affect the feedlot, but the primary control strategy can be carried out only by the cow-calf producer.
The critical time period for fetal protection is the first four months of gestation, and the cow needs a high antibody level to provide that protection to the fetus.
John Campbell is head of Large Animal Clinical Sciences at the University of Saskatchewan’s Western College of Veterinary Medicine.