Systematic Veterinary Virology

ACUTE INFECTION

• The virus spreads mainly by contact between cattle. Large numbers of BVDV are shed in the secretions and excretions of persistently infected cattle. 
• Infected animals shed virus in nasal and oral secretions, feces and urine. The primary virus entry route is through oral and nasal.
• Other less important routes of entry may include infected semen, biting insects, and contaminated instruments.  Vertical transmission plays an important role in its epidemiology and pathogenesis.
• Noncytopathic BVDV is transmitted transplacentally during the first 4 months of foetal development; therefore, infection is present at birth and lasts for life.
• Clinical disease and reproductive failure are reported in healthy cattle after contact with a persistently infected animal. 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.
• During systemic spread, the virus is able to gain entry to most tissues with a preference for lymphoid tissues. However, the tissues infected may vary between different virus strains.