Equine influenza virus: a brief review

dvm360dvm360 April 2021
Volume 54

Although usually not fatal, this devastating respiratory virus causes substantial economic loss and performance impairment.

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Equine influenza virus (EIV), caused by equine influenza A type 2, is an infectious condition of the upper respiratory tract that is endemic worldwide.1,2 It is an RNA virus in the Orthomyxoviridae family; the most commonly identified strain in recent history is H3N8.2,3 There are 3 separate American lineages, with the Florida lineage of most importance in recent outbreaks. Evolution of the virus has led to the classification of 2 Florida sublineages, clade 1 and clade 2.1,2

Pathogenesis and pathophysiology

Although EIV is rarely fatal, it is highly contagious and can lead to significant performance impairment and economic loss when outbreaks occur. Similar to influenza in other species, EIV is spread primarily via the respiratory route. Coughing and nasal discharge from affected animals can lead to transmission via aerosolized particles or direct contact.3 Following a short incubation period of 1 to 3 days, affected animals rapidly develop fever, serous nasal discharge that progresses to mucopurulent discharge, and coughing.1,3

Experimentally infected animals begin shedding the virus in nasal discharge within 24 to 48 hours of infection, and shedding generally persists for 6 to 7 days. Vaccination and partial immunity have been shown to decrease the duration and amount of viral shedding. The virus leads to destruction of ciliated respiratory epithelial cells and disrupts the function of the mucociliary apparatus in the trachea, both of which can predispose affected animals to secondary bacterial pneumonia. Uncomplicated cases last for about 1 to 2 weeks, although coughing can persist for weeks after this time.1

Diagnosing EIV

Young horses in close contact with large numbers of other horses, including racehorses and show horses, are considered the group at highest risk for EIV infection.2 Many racetracks and show organizations, including the United States Equestrian Federation, require EIV vaccination of horses before entering grounds or competing.3,4 However, despite vaccination, outbreaks can still occur, so it is critical that veterinarians be familiar with the condition and know which diagnostics can confirm it.

Diagnostics are aimed at identifying either viral antigen or antibodies directed at the virus. Virus isolation is used to identify the virus itself but can be time-consuming; often, results are not available for upwards of 3 weeks.5

Additional viral antigen testing includes antigen capture enzyme-linked immunosorbent assays and reverse transcription-polymerase chain reaction (RT-PCR) techniques. Both types of test have rapid turnaround times that can be useful in an outbreak situation.5,6 Nasal or deep pharyngeal swabs are the easiest samples to obtain for testing, although bronchoalveolar lavage fluid, fluid from a transtracheal aspirate, or lung tissue can also be used.5

Diagnostics aimed at identifying antibodies against EIV include the hemagglutination inhibition assay and single-radial-hemolysis assay.1,6 Although measuring antibody titers may be useful in disease surveillance and research, confirmation of active infection requires submission of acute and convalescent titers 10 to 21 days apart, which is generally too long to be of benefit when dealing with a potential outbreak.1,5 Many laboratories also offer real-time PCR panels that test for EIV pathogens in addition to other respiratory pathogens, including equine herpesvirus 1 and 4, Streptococcus equi subsp equi infection, equine rhinitis virus, equine adenovirus, and equine arteritis virus.5

Not a core vaccine

The American Association of Equine Practitioners (AAEP) does not consider EIV a core vaccine but instead considers it a risk-based vaccine that is not necessarily warranted for every horse.2 Although it is fairly well known that young horses in frequent contact with other horses are considered high risk, other risk factors that may justify EIV vaccination are less well known.

In addition to being 1 to 5 years of age, having frequent contact with other horses and low concentrations of viral-neutralizing antibodies are considered risk factors for EIV infection.2 It has been suggested that older animals may also be at higher risk for becoming infected than previously thought.2,7 Animals that fall into these categories should be vaccinated at least annually or more frequently, depending on the specific vaccine label.2

Preventive measures

Vaccination and biosecurity

At the time of publication, a plethora of available vaccines contain a number of different strains of H3N8. In addition to vaccination, biosecurity plays a major role in limiting spread during an outbreak.

The 2020 World Organisation for Animal Health (formerly Office International des Epizooties) Expert Surveillance Panel on Equine Influenza Vaccine Composition recommends that manufacturers adhere to previous guidelines for both clade 1 and clade 2 viruses of the Florida sublineage.8

A majority of available EIV vaccines are killed virus products administered intramuscularly.

In addition to killed virus vaccines, there is an intranasal modified-live vaccine. A canarypox-vectored vaccine was previously available but is no longer marketed in the United States.

Vaccine recommendations

The AAEP recommends that horses considered at risk be vaccinated at least annually.

Previously vaccinated at-risk adults should be vaccinated twice yearly. Unvaccinated adults, or horses with unknown vaccination histories, should receive either a single dose of an intranasal modified-live vaccine or a 3-dose series of the killed virus vaccine.2

Boosters should be administered every 6 to 12 months, depending on individual animal risk.2 Pregnant mares should be vaccinated on a similar plan, with the last booster administered no later than 4 weeks prior to parturition. Regardless of vaccine type, vaccination is not recommended before 6 months of age.7 After 6 months of age, they should receive a full 3-dose series of vaccines.2

Refer to the AAEP vaccination guidelines for a more detailed description of vaccine recommendations.2

Kate L. Hepworth-Warren, DVM, DACVIM, is an equine internal medicine specialist working as a clinical assistant professor of equine medicine at North Carolina State College of Veterinary Medicine in Raleigh. Outside of work, she enjoys traveling, reading, running, and the beach.


  1. Landolt GA. Equine influenza virus. Vet Clin North Am Equine Pract. 2014;30(3):507-522. doi:10.1016/j.cveq.2014.08.003
  2. Equine influenza. American Association of Equine Practitioners. 2020. Accessed January 25, 2021. https://aaep.org/guidelines/vaccination-guidelines/risk-based-vaccination-guidelines/equine-influenza.
  3. Gilkerson JR, Bailey KR, Diaz-Méndez A, Hartley CA. Update on viral diseases of the equine respiratory tract. Vet Clin North Am Equine Pract. 2015;31(1):91-104.doi:10.1016/j.cveq.2014.11.007
  4. United States Equestrian Federation. Conduct of licensed competitions. In: 2021 United States Equestrian Federation, Inc. Rule Book. Lexington, KY: United States Equestrian Federation; 2021:83-105. https://www.usef.org/forms-pubs/s9SeSv4S0Sc/gr--general-rules
  5. Animal health diagnostic center. Cornell University College of Veterinary Medicine. Accessed January 25, 2021. https://www.vet.cornell.edu/animal-health-diagnostic-center.
  6. Landolt GA, Townsend HGG, Lunn DP. Equine influenza infection. In: Sellon DC, Long M, eds. Equine Infectious Diseases. 2nd ed. St. Louis, MO: Saunders; 2014:141-150.
  7. Barquero N, Daly JM, Newton JR. Risk factors for influenza infection in vaccinated racehorses: lessons from an outbreak in Newmarket, UK in 2003. Vaccine. 2007;24(43):7520-7529. doi: 10.1016/j.vaccine.2007.08.038
  8. Equine influenza: OIE Expert Surveillance Panel on Equine Influenza Vaccine Composition, OIE Headquarters, 16 April 2020. World Organisation for Animal Health. Accessed January 25, 2021. https://www.oie.int/en/our-scientific-expertise/specific-information-and-recommendations/equine-influenza/
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