Equis ISSN 2398-2977

Equine influenza

Contributor(s): Sarah Binns, Christopher Brown, Melissa Kennedy, Ruth Morgan, Carla Sommardahl


  • Epizootic disease of the equine upper and lower respiratory tract.
  • Cause: equine influenza virus A (H3N8-American (Kentucky, Florida and Argentina) and European lineage). Most current threat in UK in H3N8-American-Florida clade 2 strain.
  • Signs: harsh cough, pyrexia, lymphadenopathy, depression, inappetence.
  • Diagnosis: virus isolation from nasopharyngeal swabs, ELISA for viral antigen detection, serology.
  • Treatment: supportive.
  • Prognosis: good for adult tracheobronchitis; fair to poor for foal pneumonia.
Print off the Owner factsheets on Equine influenza - 'flu' and Respiratory problems in your horse - not a good wheeze! to give to your clients.



  • Equine influenza A virus Equine influenza virus - an orthomyxovirus.
  • Two main strains: H7N7 (formerly subtype 1) and H3N8 (formerly subtype 2).
  • There have been very few reports of H7N7 subtype virus infections in the last 20 years, H3N8 infections predominate.
  • There are two main lineages of H3N8 viruses: American and Eurasian.
  • Host-species specific: infects horses, donkeys and mules only.

Predisposing factors


  • Contact with infected equids.
  • Stress, eg transport.
  • Compromised local immunity, eg young foals.


  • Lack of effective vaccination against infective strain.


  • Aerosol spread of the organism → tropism for ciliated respiratory epithelium → tracheobronchitis or primary viral pneumonia.
  • Aerosol infection → droplets penetrate into respiratory tract according to their size → tropism of virus for ciliated respiratory epithelium → epithelium sloughs → edema and lymphoid and impaired mucous clearance, infiltration of mucosa → tracheobronchitis in adults or primary viral pneumonia in foals.
  • Incomplete separation of lobes of equine lung → spread of infection by direct extension.
  • May be complicated by secondary infection, eg streptococci Streptococcus spp.
  • Some cases → virus penetrates respiratory tract basement membrane → viremia → myocarditis or hepatic damage → limb edema.


  • Incubation period: 1-3 days.
  • Virus shed in nasal discharge and droplets for 7-10 days.
  • Seroconversion by 8 days following infection.
  • Cough lasts 1-3 weeks.
  • Recovery of uncomplicated infection in 1-2 weeks following onset of clinical signs.


  • Virus cannot survive outside host.
  • Replicates in respiratory epithelium of horses, donkeys and hybrids.
  • Transmitted via nasal discharges and droplets.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Cullinane A, Gildea S & Weldon E (2014) Comparison of primary vaccination regimes for equine influenza: Working towards an evidence-based regime. Equine Vet J 46 (6), 669-673 PubMed.
  • Cullinane A (2014) Equine influenza and air transport. Equine Vet Educ 26 (9), 456-457 WileyBlackwell.
  • Pusterla N, Estell K, Mapes S & Wademan C (2014) Detection of clade 2 equine influenza virus in an adult horse recently imported to the USA. Equine Vet Educ 26 (9), 453-455 WileyBlackwell.
  • Daly J M & Elton D (2013) Potential of a sequence-based antigenic distance measure to indicate equine influenza vaccine strain efficacy. VaccineJul 2. doi: 10.1016/j.vaccine.2013.06.070 PubMed.
  • Murcia P R et al (2013) Evolution of equine influenza virus in vaccinated horses. J Virol 87 (8), 4768-4771 PubMed.
  • Paillot R et al (2013) Whole inactivated equine influenza vaccine: Efficacy against a representative clade 2 equine influenza virus, IFNgamma synthesis and duration of humoral immunity. Vet Microbiol 162 (2-4), 396-407 PubMed.
  • Paillot R et al (2013) Duration of equine influenza virus shedding and infectivity in immunised horses after experimental infection with EIV A/eq2/Richmond/1/07. Vet Microbiol  166 (1-2), 22-34 PubMed
  • Gildea S, Arkins S & Cullinane A (2011) Management and environmental factors involved in equine influenza outbreaks in Ireland 2007-2010. Equine Vet J 43 (5), 608-617 PubMed.
  • Pusterla N et al (2011) Surveillance programme for important equine infectious respiratory pathogens in the USA. Vet Rec 169 (1), 12 PubMed.
  • Patterson-Kane J C, Carrick J B, Axon J E, Wilkie I & Begg A P (2008) The pathology of bronchiointerstitial pneumonia in young foals associated with the first outbreak of equine influenza in Australia. Equine Vet J 40 (3), 199-203 PubMed.
  • Daly J M, Sindle T, Tearle J et al (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, Daly J M, Spencer L & Mumford J A (2006) Description of the outbreak of equine influenza (H3N8) in the United Kingdom in 2003, during which recently vaccinated horses in Newmarket developed respiratory disease. Vet Rec 158 (6), 185-192 PubMed.
  • McCabe V J, Sindle T & Daly J M (2006) Evaluation of the Binax NOW Flu A test kit for the rapid detection of equine influenza virus. Vet Rec 158 (5), 164-165 PubMed.
  • Newton J R, Texier M J & Shepherd M C (2005) Modifying likely protection from equine influenza vaccination by varying dosage intervals within the Jockey Club Rules of Racing. Equine Vet Educ 17 (6), 314-318 VetMedResource.
  • Edlund Toulemonde C, Daly J, Sindle T et al (2005) Efficacy of a recombinant equine influenza vaccine against challenge with an American lineage H3N8 influenza virus responsible for the 2003 outbreak in the United Kingdom. Vet Rec 156 (12), 367-371 PubMed.
  • Peek S F et al (2004) Acute respiratory distress syndrome and fatal interstitial pneumonia associated with equine influenza in a neonatal foal. J Vet Intern Med 18 (1), 132-134 PubMed.
  • Cardwell J, Newton R, Wood J, Geraghty B & Ellis R (2000) Equine influenza in donkeys in the New Forest. Vet Rec 147 (14), 400 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), 449-452 PubMed.
  • Gross D K et al (1998) Effect of moderate exercise on the severity of clinical signs associated with influenza virus infection in horses. Equine Vet J 30, 489-497 PubMed.
  • Rees W A, Harkins J D, Woods W E et al (1997) Amantadine and equine influenza, pharmacology, pharmacokinetics and neurologic effects in the horse. Equine Vet J 29 (2), 104-110 PubMed.
  • Powell D G, Watkins K L, Li P H & Shortridge K F (1995) Outbreak of equine influenza among horses in Hong Kong during 1992. Vet Rec 136 (21), 531-536 PubMed.
  • Chambers T M, Holland R E & Lai A C K (1995) Equine influenza - current veterinary perspectives 1. Equine Pract 17 (8), 19-23 VetMedResource.
  • Chambers T M, Holland R E & Lai A C K (1995) Equine influenza - current veterinary perspectives 2. Equine Pract 17 (10), 26-30 VetMedResource.
  • Baker 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.