Suckling calves are commonly in contact with the breeding herd during early gestation, prior to the time the bovine fetus develops a competent immune system. As a result, PI suckling calves are considered to be the primary source of BVDV infection in breeding herds causing pregnancy loss, pre-weaning mortality and the induction of PI calves in the next generation.

Although PI suckling calves are considered to be the primary reservoir for BVDV in a herd, PI adults can also be present in a herd. Adult PI animals are not as common because mortality of PI calves prior to and after weaning has been reported to be very high due to fatal congenital defects and secondary infections that cause enteritis, pneumonia, and arthritis. However, 17% to 50% of PI calves may reach breeding age in some situations. PI breeding females not only are a source of horizontal transfer of BVDV, but will always produce a PI calf themselves. Witttum et al., showed that 7% of PI calves were born to dams that were PI. Meaning that while PI dams can be a direct cause of a small percentage of PI calves, a vast majority (93%) of PI calves are born to non-PI dams due to transient infection of the dam during gestation.

Male PI calves will occasionally be selected for use as breeding bulls. The amount of BVDV excreted in the semen of persistently infected bulls is very high (10⁴-10⁶ TCID₅₀/ml). BVDV-contaminated semen is an efficient horizontal transmitter of disease from bull to seronegative females. If PI bulls are used for natural service, PI calves are not common, but seronegative cows may not be able to conceive. Once immunity has developed, cows can conceive and give birth to normal (non-PI) calves. If PI bulls are used for AI, all or most seronegative females bred with the semen will become infected although most will not produce a PI calf.