Equis ISSN 2398-2977

Equine influenza virus

Contributor(s): Sarah Binns, Susan Dawson, Melissa Kennedy




  • Family: Orthomyxoviridae.
  • Type A influenza viruses affect horses.


  • Latin: influentia - an epidemic.

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Clinical Effects



  • Respiratory tract ciliated epithelium of equids.


  • Aerosol inhalation.

Pathological effects

  • Equine influenza virus Equine influenza virus induces lymphopenia in early stages of infection, as lymphocytes are marginalized in local lymph nodes and respiratory epithelium.
  • Interferon is important in protection against viral replication in primary infection.
  • Mucosal immunity develops in second infections with equine influenza.
  • Recovered horses possess subtype-specific neutralizing antibodies which persist for several months.
  • Incubation period of equine influenza is 1-3 days.
  • Clinical signs: depression, inappetence, pyrexia, harsh dry cough , occasional nasal discharge Equine influenza.
  • Virus spreads by aerosol; small droplets reach the terminal bronchi and alveoli → influenza virus has tropism for ciliated epithelium → epithelium sloughs → lymphocyte infiltration → tracheobronchitis → secondary bacterial pneumonia may occur, especially due to Lancefield Group C streptococci Streptococcus spp.
  • A/equi subtype 2 is more virulent than subtype 1 and may cause primary viral pneumonia in foals; the case fatality rate is high.
  • In some cases the virus may spread into the blood → viremia → myocarditis Heart: endocarditis or liver damage (causing ventral limb edema).
  • Ventral limb edema, muscle soreness, stiffness may be associated with respiratory disease, probably due to viral products and inflammatory mediators, but etiology is unclear.


Control via animal

  • Control relies on vaccination.
  • Clinical cases should be isolated.
  • Complete rest for 1-3 months is essential.
  • Antibiotic cover in foals.

Control via chemotherapies

  • Ampicillin to cover foals against secondary infection Ampicillin.


These vary with the vaccine - the practitioner should note which components are in vaccine to ensure adequate protection.

  • May be combined with tetanus toxoid.

With high risk, boost every 3-4 months.

Antigenic drift (strain differences within a subtype) or shift (change of subtype within a species) may render vaccines ineffective in the future.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Wei G et al (2010) Equine influenza viruses isolated during outbreaks in China in 2007 and 2008. Vet Rec 167 (10), 382-383 PubMed.
  • Daly J M, Sindle T, Tearle J, Barquero N, Newton J R & Corning S (2007) Equine influenza vaccine containing older H3N8 strains offers protection against A/eq/South Africa/4/03 (H3N8) strain in a short-term vaccine efficacy study. Equine Vet J 39 (5), 446-450 PubMed.
  • Newton J R, Verheyen K, Wood J L N, Yates P J & Mumford J A (1999) Equine Influenza in the United Kingdom in 1998. Vet Rec 145 (10) 455-457 PubMed.
  • Chambers T M, Shortridge K F, Li P H et al (1994) Rapid diagnosis of equine influenza by the Directigen Flu-A enzyme immunoassay. Vet Rec 135 (12), 275-279 PubMed.
  • Baker D J (1986) Rationale for the use of influenza vaccines in horses and the importance of antigenic drift. Equine Vet J 18 (2), 93-96 PubMed.