ISSN 2398-2977      

Equine influenza



  • 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.
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 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.


This article is available in full to registered subscribers

Sign up now to start a free trial to access all Vetlexicon articles, images, sounds and videos, or Login


This article is available in full to registered subscribers

Sign up now to start a free trial to access all Vetlexicon articles, images, sounds and videos, or Login


This article is available in full to registered subscribers

Sign up now to start a free trial to access all Vetlexicon articles, images, sounds and videos, or Login


This article is available in full to registered subscribers

Sign up now to start a free trial to access all Vetlexicon articles, images, sounds and videos, or Login

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:
  • 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

Can’t find what you’re looking for?

We have an ever growing content library on Vetlexicon so if you ever find we haven't covered something that you need please fill in the form below and let us know!


To show you are not a Bot please can you enter the number showing adjacent to this field