Equine Influenza

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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