PI calves may be weak, runted and 10-15 pounds lighter than normal. However, on a dairy especially, PI calves often look normal though they are immunologically frail and prone to disease.
The birth of a persistently infected calf only occurs via in utero exposure. At birth, a calf is either PI or it isn’t. Persistent infection can only occur two ways: if the dam is PI or if a non-PI dam is exposed to BVDV and passes it on to the fetus during that critical 60-100-day-of-gestation window. The virus crosses the placenta before the calf’s immune system is developed – before MHC-1s have developed for self-recognition – and the immune system recognizes that strain of BVDV as part of itself and not as foreign protein.
It’s rare for PI bulls to create PI calves at breeding through infected semen because it’s too early in the trimester at conception to cause PI calves. Infection at this time, however, can create early embryonic death. The danger of PI bulls is when they are exposed later to pregnant cows and the shedding of virus through nose-to-nose contact at that critical time can create PI calves.
Closing the PI loop
Closing the loop of persistent infection on the dairy involves identifying an infected calf and, when possible, her dam. “I like to test calves as early in life as possible to identify PIs and remove them to reduce the impact of BVDV in the herd, as well as the heifer-growing facility,” says Smith.
Many dairies or heifer growers are now ear-notch testing calves for persistent infection. The problem is when they find these calves and either euthanize or dispose of them, they often don’t go back to find that PI calf’s dam, which may or may not be PI itself. “Persistent infection is often along family lines,” says Cortese. “You must go back and test the dam to see if she’s a PI.”
Cortese notes that 7 percent of PI animals will be from PI dams. “If I just get rid of the PI calves, I still have infection through the cows on the dairy. If you’re going to spend money and time to find a PI calf, the dairy needs to know so they can test the dams.”
Smith says once a producer has an understanding of the dynamics of BVDV and how PI calves are produced, they are usually willing to test the dam. “Identification is rarely a problem when the calf is a heifer,” he says. “Bull calves are not identified on some dairies, and therefore, traceback is much tougher in that situation.”
Looking for BVDV in aborted fetuses can also identify problem dams. Any time Smith works up an abortion problem, he tests the fetus and dam for the presence of BVDV, and occasionally finds some positives.
Economics of PIs
In a 2002 study by Larson, et al, the economics of one PI animal in a beef herd were estimated. It was calculated that a reduced pregnancy rate of 5 percent, an increased pre-weaning mortality of 10 percent and a decreased weaning weight of 0.5 percent could have an economic effect of $14.85- $24.84 per year decreased return to fixed costs per beef cow exposed.