BVD in feeder cattle Dr. John Campbell | Back to Vet Advice
Bovine viral diarrhea virus (BVDV) may be one of the most important infectious diseases of feedlot cattle. Its ability to suppress the immune system of calves may allow other bacteria or viruses to cause disease. The unique way in which this virus persists in the cattle population also makes it a difficult disease for the feedlot operator to control.
The persistently infected calf
A pregnant cow with no immunity to BVDV when exposed to the virus will also infect her fetus. These infections can result in abortion or in congenital abnormalities. However, if the fetus is in the first 125 days of gestation, the resulting infection can become persistent. This calf will be born and will have a life-long persistent infection with BVD virus. They are sometimes said to be immuno-tolerant to BVD virus and will always carry the infection.
The persistently-infected (PI) calf is especially important in the spread of BVD virus in the cattle population. These animals excrete vast quantities of BVD virus which can be spread to other animals in the herd. Although many of these PI calves are poor doers and unthrifty, some may appear quite normal. If a PI calf is part of a group of feedlot calves, it has the ability to spread the virus to many of the other animals in the group.
It is important to remember that a PI calf was infected as a fetus while in the early stages of gestation. How many PI calves are out there? Although the evidence is far from complete, it would seem that there are relatively few PI calves in the population. Some U.S. studies have estimated that about four per cent of cow-calf herds have PI calves present in them, and that only about 0.3-0.5 per cent of the population are persistently infected. Despite the small numbers, these PI calves are very important in spreading BVD virus within the cattle population.
Immunosuppression from BVD infections
When a calf which with a competent immune system is exposed to a PI calf, it may become infected with BVD virus. In many cases, the BVD infection may result in very few visible signs. Occasionally, cattle may display symptoms of diarrhea but in many cases they seem to recover quickly. However, the BVD virus can cause significant damage to the calf's immune system making it susceptible to other infections. Diseases such as bovine respiratory disease (shipping fever), salmonella infections, and mycoplasma infections that cause pneumonia and arthritis can be the result of the immunosuppression. BVD virus may often be overlooked as an initiating cause of many infectious diseases that occur in feeder calves.
Control of BVDV at the feedlot and the cow-calf herd
The importance of the PI animal in spreading BVD virus makes it a difficult disease for the feedlot owner to control. Vaccination with a modified live BVD vaccine on arrival at the feedlot may help protect some of the animals from infections and has been shown to have a significant economic benefit in some recent studies. Pre-vaccinated calves prior to arrival at the feedlot would almost certainly provide better protection for these animals from BVD virus as well.
However, the ultimate control of this disease still rests with the cow-calf producer. Appropriate vaccination programs for the cow herd consisting of utilizing a modified live vaccine pre-breeding would help to protect the pregnant cow and fetus from infections and help to prevent the PI calf from occurring. As well, instituting biosecurity protocols that prevent the introduction of persistently infected animals into the cow-calf herd are a necessary step to eliminating this disease.
Cow-calf producers who effectively control this disease will perhaps one day have a marketing advantage if they can demonstrate that their calves have been appropriately protected against BVD virus.
Dr. John Campbell is a professor in the department of large animal clinical sciences at the Western College of Veterinary Medicine.
