BVD infection results in a wide variety of clinical signs, due to its immunosuppressive effects, as well as having a direct effect on respiratory disease and fertility.

Following viral entry and contact with the mucosal lining of the mouth or nose, replication occurs in epithelial cells. BVDV replication has a predilection for the palatine tonsils, lymphoid tissues and epithelium of the oropharynx.

Phagocytes take up BVDV or virus-infected cells and transport them to peripheral lymphoid tissues; the virus can also spread systemically through the bloodstream. Viraemia occurs 2-4 days after exposure and virus isolation from serum or leukocytes is generally possible between 3-10 days post infection

During systemic spread the virus is able to gain entry into most tissues with a preference for lymphoid tissues. Neutralising antibodies can be detected from 10-14 days post infection with titres continuing to increase slowly for 8-10 weeks. After 2-3 weeks, antibodies effectively neutralise viral particles, promote clearance of virus and prevent seeding of target organs