Influenza in Horses

Table of Contents

  1. Introduction
  2. Horse Health Risk
  3. Vaccination
  4. Regulatory Information
  5. Recommendations
  6. More Information


Equine influenza is an acute, contagious, respiratory disease caused by two distinct subtypes (subtype 1: H7N7 and subtype 2: H3N8) of influenza A viruses. These are abbreviated to AE-1 and AE-2. In fully susceptible horses, clinical signs include pyrexia (fever) and a harsh dry cough, followed by mucopurulent nasal discharge. In partially immune, vaccinated animals, one or more of these signs may be absent. Characteristically, influenza spreads rapidly in a susceptible population. Influenza is endemic in North America and most of the world. Periodic point explosive outbreaks occur and are quickly spread by a rapidly mobile population of horses. This may be related to the waning of immunity to influenza in the general horse population and/or genetic drift of the virus strain. Only influenza subtype AE-2 has been isolated over the last twenty years worldwide. There are also distinct differences in virus strain lineage between virus isolates in North America and those in Europe. Respiratory disease outbreaks in groups of horses need to be identified as caused by influenza or one of the other common equine respiratory viruses, such as equine herpesvirus 1 & 4, equine rhinovirus 1 & 2 and equine adenovirus.

The equine influenza strains are not known to cause illness in humans and are classified as AE subtype 1: H7N7 and AE subtype 2: H3N8. These virus strains are different from the strains causing the avian influenza outbreaks of recent years:

  • H7N3, which affected poultry in the Fraser Valley of British Columbia in 2004;
  • H7N2, which affected poultry in Maryland in 2003 and 2004;
  • H5N2, which affected poultry in Texas in 2004;
  • HPAI that spread in 2004 to more than ten Asian countries.

Horse Health Risk

The incubation period of equine influenza virus may be as short as 1 to 3 days. Horses can remain infective and shed virus to other horses for up to 10 days in their nasal secretions. A number of factors increase the risk of infection with influenza virus. Horses 1 to 5 years of age are more susceptible to the disease. Those horses with no previous exposure to the virus and/or no previous vaccination to protect them against influenza virus will have a lower immune status and therefore be more susceptible to getting an infection. Those horses that are in barns with a lot of in-and-out traffic will have frequent contacts with a large numbers of horses and therefore are more likely to come in contact with the virus. This is much like children bringing home the common cold from school. Sporadic outbreaks of equine influenza occur throughout all horse areas of the world. The virus infection is spread primarily by aerosolized droplets dispersed by coughing. However, the virus can be spread by contaminated equipment (buckets, grooming aids) and contaminated hands. The virus can survive for hours in the environment. Influenza outbreaks can be avoided by;

  • preventing entry of the virus into horse groups by quarantining newly arrived horses for at least 14 days and
  • appropriate vaccination before exposure.


Equine influenza virus vaccines usually consist of inactivated whole virus subtypes AE-1 and AE-2, or their subunits, with or without adjuvant (to improve immune response). Experts now recommend that the AE-1 component of vaccines should be omitted as no infections with that subtype have been substantiated during the past twenty years.

Using the killed-virus vaccine;

  • Younger horses should receive a primary series of 3 doses of the killed-virus vaccine administered 3-6 weeks apart starting at 6 months of age.
  • Revaccination should be at intervals of 3 to 12 months depending on age and risk of exposure.
  • Revaccination of performance horses should be at intervals of 3 - 4 months.

Using the modified live equine AE-2 influenza vaccine for intranasal administration;

  • A single administration protects for at least 6 months.

Regulatory Information

There are no requirements for horses to be vaccinated against equine influenza. There are no restrictions for infected horses, or horses exhibiting signs of a respiratory virus, from leaving or entering shows, tracks or other facilities. There are no requirements for horse owners, trainers, grooms or veterinarians to notify any official (provincial, federal or racetrack) when influenza is suspected.


  1. Horse owners should consult their veterinarian regarding the need for completing a vaccination program (initial and booster shots) for influenza.
  2. While vaccination does not necessarily prevent influenza, the disease in vaccinated horses is less severe and the signs last a shorter period than when horses are unvaccinated.
  3. Horses traveling to and from horse events such as shows, racetracks and facilities should maintain adequate protection against influenza and the other respiratory viruses.
  4. It is important for attending veterinarians to investigate respiratory disease outbreaks to identify the causal agent. Influenza strain identity is needed to ensure that vaccines are kept current with strains of recent outbreaks.

More Information

Guidelines for Vaccination of Horses. Lexington: American Association of Equine Practitioners, Jan, 2001.

For more information:
Toll Free: 1-877-424-1300
Author: Dr. Bob Wright - Lead Veterinarian, Equine and Alternate Species/OMAFRA; Dr. Dan Kenney - Staff Veterinarian/Ontario Veterinary College/ University of Guelph
Creation Date: 01 February 2005
Last Reviewed: 8 February 2011