The most important consequence occurs if the cow is infected with the BVD virus between 42-125 days of gestation. In this short window of time, the unborn calf contracts the virus and is never able to get rid of it for the rest of its life. When this calf is born, it is “persistently infected” or a “PI” calf.
The “PI” animals are the reason BVD disease continues. They are the primary source of virus transmission because they shed an extremely high number of virus particles throughout their lives. A PI animal is a virus factory, producing millions of virus particles in all body secretions including feces, urine, saliva, nasal discharge, milk, semen, uterine secretions, and aborted membranes. These contaminated body fluids are deposited on the grass, in ponds, watering troughs, feed troughs-virtually everywhere the animal goes. Although it is often assumed that PI calves are stunted, will grow poorly and usually die young, some are absolutely normal and will survive well into adulthood and have calves. Unfortunately, if a PI female gets pregnant, her offspring will be a PI calf 100% of the time. This is important because if one PI heifer survives and is out in the pasture constantly shedding virus, many (if not all) of the heifers in her group will be exposed during the highest risk time for creating more PI animals. This risk continues within the cow herd when the heifer freshens and enters the milking string.
Source: Michelle Arnold, DVM, Ruminant Extension Veterinarian