A PI, or Persistently Infected calf is one that is born infected with BVD virus and is a life long carrier and shedder of the virus.
How do calves become PI?
The unborn fetus is targeted by BVD virus when its dam (cow or heifer) is exposed to the virus while pregnant.
The virus enters the bloodstream of a susceptible (not vaccinated or inadequately vaccinated) female and immediately gains entrance into the developing fetus. The risk period for the formation of PI calves is thought to be from approximately the 40th day of pregnancy through to the 150th day of pregnancy.
What happens to calves born PI?
The calf is often born normal but carries the virus and spreads millions of virus particles daily to the rest of the herd.
The PI calf will often serve as the source of infection the next year to the herd. PI calves are usually destined to die early. Most of them die between 7 and 14 months of age either from BVD virus itself or due to a secondary virus or bacteria entering the animal.
What does the virus do?
The virus is very immunosuppressive which allows other agents to enter the calf and kill it. PI calves have been referred to as "walking time bombs" due to the massive amounts of virus being shed to herd mates or pen mates. BVD virus has severe negative production effects on animals exposed to the virus, which result in poor gain, poor performance and increased susceptibility to both disease and dying.
What are the potential signs that a PI calf is in a herd?
If a PI calf is in the herd you may see signs indicative of BVD, including:
- Increased number of open cows
- Delayed or strung out calving periods
- Abortions
- Weak or dead calves at birth
- Birth defects
- Increased percentage of sick calves including scours, pneumonia and lameness
- Runty or poor doing calves
- Calves dying post-weaning
