Systematic Veterinary Virology

PATHOGENESIS

• Hosts affected
  • Horses and other equines are the normal hosts. Humans are also infected by H7N7 and H3N8 viruses. The infection is common in young horses, especially at racetracks, horse shows, and during transport (planes, etc.). The infection is seasonally more prevalent in the summer and autumn.
• Distribution
• The infection is worldwide except in Pacific regions.
• Transmission
  • The disease is highly contagious in nature and can occur as epidemics. Infection spreads by direct contact, aerosol from coughing (up to 30 m), wind (up to 8 km) and indirect contact with infected material.
  • Infected or recovered animals do not act as chronic carriers. There is no vertical transmission or involvement of vectors in spreading the infection. The HA glycoprotein (hemagglutinin) of EIV attaches to respiratory epithelial cell receptors, and enters the cell via endocytosis.
  • Viral replication occurs and new viral particles are released into the airway to infect other cells or become aerosolized.  In 1-3 days this invasion causes necrosis and desquamation of respiratory epithelial cells, exudation of protein-rich fluid into airways, and clumping of cilia, impairing the mucociliary apparatus for up to 4 weeks. 
  • Exudate accumulates and predisposes to secondary bacterial infections. Affected animals shed the virus for 8 days and these animals are important source of infection for healthy stock.
• Incubation period
• The incubation period is very short ranging from 1-5 days.