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Equine influenza

ISSN 2398-2977


  • Epizootic disease of the equine upper and lower respiratory tract.
  • Cause: equine influenza virus A (H3N8).
  • Signs: harsh dry cough, pyrexia, lymphadenopathy, dull demeanor, nasal discharge, inappetence. In vaccinated horses a mild cough and mild serous discharge may be the only signs.
  • Diagnosis: PCR from nasopharyngeal swabs in acute cases, rising antibody titers on serology.
  • Treatment: supportive - fluid therapy, non-steroidal anti-inflammatory drugs. Treatment of secondary bacterial infection with antimicrobials only where indicated. Antimicrobials are not indicated in uncomplicated cases.
  • Prognosis: good for adult tracheobronchitis; fair to poor for foal pneumonia. Occasionally fatal if secondary bacterial pneumonia develops.
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Presenting signs

Acute presentation

  • Tracheobronchitis or pneumonia may be acute.

Geographic incidence

  • Worldwide.
  • Some countries are free from disease and have strict entry regulations.

Legal: Summary

  • Reportable disease in 21 states.
  • To determine if the disease is reportable in a particular state, visit www.equinediseasecc.org.
  • OIE List B disease.
  • No legislation, but sporting bodies require up to date vaccination for horses to attend competitions and race meetings.

Age predisposition

  • Foals: pneumonia.
  • Adults: tracheobronchitis.

Public health considerations

  • Transmission to humans and dogs has been identified but is extremely rare.

Cost considerations

  • Outbreaks in racing stables, breeding establishments and show barns. Marked impact on the equine industry. Cessation of racing and equestrian competition has a significant economic impact.



  • Equine influenza A virus Equine influenza virus - an orthomyxovirus of the H3N8 subtype.
  • The H7N7 which was recognized in the 1960s is now considered extinct.
  • There are two main lineages of H3N8 viruses: American and Eurasian - the Eurasian is no longer identified and inclusion in vaccines is no longer recommended.
  • The American lineage diverged into Kentucky, South American and Florida lineages in the 1990s. In the 2000s the Florida lineage was identified as two Clades. Clade 2 had been predominant in the Americas and Clade 1 in Europe, however in 2019 a European epidemic was caused by incursion of Clade 2, and Clade 2 now appears to predominate globally.
  • Host-species specific: infects horses, donkeys and mules only.

Predisposing factors


  • Mixing of large numbers of equids, particularly younger animals.
  • Stress, eg transport.
  • Compromised local immunity, eg young foals.


  • Failure to vaccinate at the recommended frequency with a vaccine that is effective against circulating strains.


  • 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 bacterial 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 within 10-14 days of 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.
  • Durham A (2019) Choosing an equine influenza vaccine. In Pract 41 (2), 84-87 BMJ.
  • Paillot R, Pitel P-H, Pronost S, Legrand L et al (2019) Florida clade 1 equine influenza virus in France. Vet Rec 184 (3), 101-101 BMJ.
  • Rendle D, Ivens P, Bowen M, Lawrence H et al (2019) Equine influenza: A current reference for vets in practice in the UK. UK-Vet Equine (Suppl 5), 1-13 MAGOnlineLibrary.
  • Dilai M, Piro M, Harrak M E, Fougerolle S et al (2018) Impact of mixed equine influenza vaccination on correlate of protection in horses. Nato Adv Sci Inst Se (4), 71 PubMed.
  • Paillot R, Garrett D, Lopez-Alvarez M R, Birand I et al (2018) The immunity gap challenge: Protection against a recent Florida Clade 2 equine influenza strain. Nato Adv Sci Inst Se (3), 38 PubMed.
  • Fougerolle S, Legrand L, Garrett D, Birand I et al (2016) Influential factors inducing suboptimal humoral response to vector-based influenza immunisation in Thoroughbred foals. Vaccine 34 (33), 3787-3795 PubMed.
  • Slater J, Borchers K, Chambers T, Cullinane A et al (2014) Report of the International Equine Influenza Roundtable Expert Meeting at Le Touquet, Normandy, February 2013. Equine Vet J 46, 645-650 WileyOnline (pdf download).
  • Cullinane A & Newton J R (2013) Equine influenza-A global perspective. Vet Microbiol 167 (1-2), 205-214 PubMed.
  • Gildea S, Arkins S, Walsh C & Cullinane A (2011) A comparison of antibody responses to commercial equine influenza vaccines following primary vaccination of Thoroughbred weanlings--a randomised blind study. Vaccine 29 (49), 9214-9223 PubMed.
  • Daly J M, MacRae S, Newton J R, Wattrang E & Elton D M (2010) Equine influenza: A review of an unpredictable virus. Vet J 189 (1), 7-14 PubMed.
  • Barquero N, Daly J M & Newton J R (2007) Risk factors for influenza infection in vaccinated racehorses: lessons from an outbreak in Newmarket, UK in 2003. Vaccine 25 (43), 7520-7529 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.
  • Paillot R, Hannant D, Kydd J H & Daly J M (2006) Vaccination against equine influenza: quid novi? Vaccine 24 (19), 4047-4061 PubMed.
  • Newton J, Texier M & Shepherd M (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 WileyOnline.
  • Daly J M, Newton J R & Mumford J A (2004) Current perspectives on control of equine influenza. Vet Res 35 (4), 411-423 PubMed.

Other sources of information

  • OIE World Organization for Animal Health (2020) Equine influenza. OIE Expert Surveillance Panel on Equine Influenza Vaccine Composition, OIE Headquarters, 16 April 2020. Conclusions and Recommendations. In: The Official 2020-1 Bulletin. pp 21-25. Website: www.oiebulletin.com.
  • Paillot R, Marcillaud-Pitel C, D’Ablon X & Pronost S (2017) Equine Vaccines: How, When and Why? In: Report of the Vaccinology Session, French Equine Veterinarians Association 2016, Reims. pp 46.

Further information