Equine Influenza

Subtypes

• Two subtypes of Equine Influenza have been described:
  • H7N7 (Equine 1), which was prevalent in the UK between 1963-1977
  • H3N8 (Equine 2), or the European strains, have been circulating since 1965
• Some drift has occurred, causing outbreaks of North-American-like strains of H3N8 in 1998 (EU) and 2003 (South America)

Pathogenesis

• Aerosol and fomite transmission infects the epithelium of the upper respiratory tract, resulting in cell necrosis
• This manifests a bronchiolitis and serous exudation
• 1-3 day incubation period
• Excretion of the virus in nasal secretions peaks at 3-4 days and finishes by 10 days
• Clinical signs:
  • Harsh dry cough
  • Pyrexia (103-106F)
  • Depression
  • Loss of appetite
  • Enlarged submandibular lymph nodes
• Secondary bacterial infection can follow defective muco-ciliary transport, eg Streptococcus zooepidemicus

Diagnosis

• Virus isolation: Deep nasal swabs (12 inches) deposited into 10ml of transport medium and transported at 4C
• Antigen detection via ELISA: tests for type A nucleoprotein
• Serology provides a retrospective diagnosis by Haemagglutination Inhibition (HI):
  • Acute and Convalescent (2 weeks post-acute) samples are tested from the same animal
  • 4-fold increase of HI must be shown to confirm diagnosis

Control

• Isolate coughing horses to minimize spread
• Vaccination:
  • All vaccines include H7 and varieties of the current circulating strains of H3
  • Inactivated egg-grown vaccine is produced with alhydrogel as an adjuvant
  • Live vaccines using purified haemagglutinin do not require adjuvants
  • UK Jockey Club rules regarding timing:
    • Horses must be certified as completing a vaccination course of 3 injections at least 8 days prior to racing
    • The course consists of a primary inoculation followed by a second within 21-92 days and a third after a further 150-215 days
    • Boosters are given annually to maintain immunity, and in the event a year is missed the initial course of three jabs must be repeated

• Causative agent: Orthomyxoviridae
• Rhinitis,
• Pathogenesis:
  • Inhalation -> replication in epithelial cells of upper and lower airways (attaches via haemaglutinin spike and gains entry into cell) -> neuramidase alters efficiency of mucociliary apparatus
• Sloughing of the affected area
• Severity of signs will depend on dose of virus
• Acute tracheobronchitis with coughing, and fever
• May be accompanied by secondary bacterial infections
• No viraemia
• In severe cases may cause bronchointerstitial pneumonia
• Secondary invaders are usually Streptococci