Equine influenza

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Equine Influenza

Horse Flu, Equine Influenza A Virus

Equine influenza (EI) is an acute, highly contagious disease of the respiratory tract of horses. It is caused by the orthomyxovirus equine influenza A type 2 (A/equine 2) virus. EI is endemic in the equine population worldwide with the exception of New Zealand and Iceland. Outbreaks occur most often when horses kept in close contact with one another, like at horse shows, racetracks, boarding stables, airplanes, transport vehicles, etc. EI is thought to have originated from birds, and is closely related to avian influenza virus. EI is able to cross species barriers, as it has been reported to cause respiratory disease in dogs in North America and the United Kingdom. The severity of clinical signs of EI in a horse depends on many factors, including the degree of existing immunity. Horses that are partially immune can become subclinically infected and shed virus.

Clinical signs
Usually the first clinical signs observed in horses infected with EI is a high fever (sometimes exceeding 106°F (41.1°C)) which peaks in 2 to 3 days, and again on day 7. During this time most horses also loose their appetite. The virus spreads quickly throughout the horse's respiratory tract, resulting in initially serous discharge, later becoming mucopurulent; coughing typically develops at the same time. Generally, the course of the disease is 7 to 14 days in uncomplicated cases. However in some the coughing persists for another week or so.

Diagnosis of EI is based on virus isolation, virus antigen detection (using an antigen capture enzyme-linked immunosorbent assay (ELISA) or of viral genome using reverse-transcription polymerase chain reaction (RT-PCR) assays), and paired serum testing taken from nasopharyngeal or nasal passage swabs, collected from horses showing signs of respiratory illness. Samples should be obtained as soon as possible from horses suspected of being infected with EI, ideally within 3 to 5 days.

EI is highly contagious and the virus spreads quickly through groups of horses in aerosolized droplets dispersed by coughing. EI can survive for several hours when out in the open, and can contaminate buckets, feeding or grooming equipment and tack. The virus is able to survive in a wet environment such as a water bucket, for up to 72 hours, and for 48 hours on dry surfaces such as grooming equipment, tack, feed, hay, clothing, etc. When horses are vaccinated against EI with inactivated influenza vaccines immunity can be short-lived. This causes recently vaccinated horses to become infected and shed the virus. Therefore, any unvaccinated horses in contact with the horse can become infected with EI.

Incubation Period
EI has a short incubation period, where horses can start to develop clinical signs within 24 hours following exposure to the virus. Horses that are infected are able to shed the virus in their nasal secretions for up to 10 days. In some incidences, horses that are partially immune can become subclinically infected, causing them to shed the virus.

Complications of EI include secondary bacterial pneumonia, myositis (inflammation of the muscles), myocarditis (inflammation of the heart), and leg swelling; in rare incidences, neurological diseases can occur. It has also been suggested that horses infected with EI are predisposed to developing recurrent airway obstruction (RAO) and exercise-induced pulmonary hemorrhage (EIPH).


Nasal discharge
Coughing, acute, sudden onset
Muscle soreness
Loss of appetite
Weight loss
Enlarged lymph nodes


  • History
  • Clinical signs
  • Influenza HI test
  • Influenza PCR
  • Influenza NP ELISA
  • Influenza Directigen
  • APHA Influenza PCR

While waiting for your veterinarian

  • Isolate horses suspected of having EI from other horses, including dogs.



Report diseaseEI is a reportable disease, meaning that if you suspect that your horse has this disease, by law you need to report it to your veterinarian, or a state or federal veterinarian.
Supportive care
RestThe effects of the virus can be significantly exacerbated by even moderate exercise, therefore infected horses should not be asked to perform work until they fully recoverG Landolt et al
Antiviral therapy


Scientific Research

General Overviews

  • nasal discharge icon

Age Range

Most commonly found in young horses between 2 and 3 years of age, but can affect all horses.

Risk Factors

  • Age - Horses 1-5 years of age are most susceptible.
  • Horses that are in frequent contact with large numbers of horses.

Causative agent