PI-BVD information:

The primary concern of spreading the BVD virus is through PI cattle.  PI animals are very efficient transmitters of the virus.  They usually have a very high amount of virus circulating in their blood and other fluids, and they shed the BVD virus continually.  PI-BVD calves are “created” during pregnancy when BVD virus from an infected dam’s bloodstream crosses the placental wall to her fetus during the first part of gestation.  This is the only way a PI animal is created.  Infection can lead to fetal death, the birth of a PI calf, or the birth of a normal calf.  It’s important to note that a calf born BVD-PI will always be a PI animal.  If a calf is not PI at birth, it can never become PI.  BVD-PI calves can grow to adult age without any outward signs of BVD virus infection.  The virus is perpetuated when these PI animals survive past yearling age and enter the breeding herd.  Infected heifers or cows (that conceive) will always produce a PI calf. 
A PI bull has the dangerous potential to spread the BVD virus to all cattle he comes in contact with.

---