A Persistently Infected or PI calf remains infected with BVD throughout its entire life. The persistent infection comes about as a result of infection of the calf in the womb during the first 120 days of pregnancy. This occurs if the cow herself is PI, or if a cow becomes infected with the BVD virus in early pregnancy. PI calves cannot produce an immune response to combat the virus. Consequently, they shed virus for their entire life.
