Although not entirely clear, the outcome of an acute infection is probably related to several factors including strain of virus, age of host, immune and physiologic status of the host, and the presence of other pathologic agents. The majority of acute BVDV infections are caused by noncytopathic viruses. Cattle acutely or persistently infected with BVDV are the primary source of virus.
Infected animals shed virus in nasal and oral secretions, feces and urine.
The primary virus entrance route is probably oral nasally. Other less important routes of entry may include infected semen, biting insects, and contaminated instruments. Following entry and contact with the mucosal lining of the mouth or nose, initial replication occurs in epithelial cells with a predilection for the palatine tonsils. From here, the virus is able to spread systemically through the blood stream. Spread can occur through both free virus in the serum and virus infected leucocytes, particularly lymphocytes and monocytes.