Equine influenza virus, or EIV, is an extremely contagious respiratory disease or flu affecting horses, mules, and donkeys.

EIV is characterized by fever, apathy, and lack of appetite, and causes respiratory symptoms such as coughing and nasal discharge. However, some horses infected with EIV have no observable signs of the virus.

Left untreated, the virus can lead to secondary infections and pneumonia. In most cases, influenza is not fatal, however, horses with impaired immune function may be more susceptible to negative outcomes. [1]

EIV most often affects racehorses or show horses because it is easily transmitted between horses at competitions. Influenza outbreaks are commonly linked to horses being transported to another country for competition. [1]

There are several ways to prevent the spread of equine influenza, including vaccination, good hygiene practices, and biosecurity protocols. It is important to quarantine any new horses before introducing them to a herd and to isolate horses that become sick with influenza.

Equine Influenza Virus

Equine influenza is a serious respiratory disease that can affect horses of all ages.

EIV is a member of the Orthomyxoviridae family and Influenzavirus A genus. The virus is related to but distinct from the influenza virus that affects humans and other species. [9]

There have been reports of other species, such as pigs and dogs, becoming infected with EIV during outbreaks. Some evidence suggests that humans can catch EIV, but this is rare and is unlikely to result in clinical signs. [4]

Care should be taken when handling infected horses, but you are likely not at risk of EIV if your horse has the virus. [4][5][6][7]

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Strains of EIV

There are two strains of the equine influenza virus – H7N7 (subtype 1) and H3N8 (subtype 2) – both of which are sub-types of influenza A.

The equine influenza virus is a single-stranded RNA virus, which readily mutates into different sub-strains of influenza. [9] As different variants of EIV emerge, they vary in severity and in how easily they spread. [8]

The H7N7 strain, or equi-1, was the first strain of equine influenza identified in 1956 in Europe. This strain is now considered extinct because the last outbreak of H7N7 occurred in 1979, and few cases have been reported since then.

The H3N8 strain (equi-2) was first detected in Florida in 1963 and is a greater concern for horses today. [5][8]

The H3N8 strain affects horses more severely than H7N7, putting them at greater risk of contracting a secondary infection. [9]


The virus replicates in the upper airways of the horse’s lungs, damaging cilia and epithelial cells. These are important structures within the airways that filter debris from inhaled air.

This causes the horse to develop a dry, unproductive cough and become congested. [8][11] The destruction of the epithelium in the respiratory tract also puts the horse at risk of secondary bacterial infections, including pneumonia and bronchitis.

Infected horses remain contagious for 7-10 days, during which period they shed the virus through exhaled air and nasal secretions. [10]

Most horses recover within 2-3 weeks. You can support your horse’s recovery by giving them time to rest, providing them with a well-ventilated environment, reducing exposure to dust and implementing good hygiene practices. [21]

Secondary Infections

Equine influenza is usually not fatal on its own and has a good prognosis for most horses.

However, EIV can cause the susceptible horse to develop other conditions such as secondary bacterial infections, which can lower the horse’s likelihood of survival.

Some secondary infections or conditions that have occurred in horses with EIV include: [8][17]

  • Bronchitis or bronchiolitis (inflamed bronchi and bronchioles, which are airways in the lungs)
  • Bronchointerstitial pneumonia, which is particularly prevalent and dangerous in EIV-infected foals
  • Bacterial infection and inflammation of the larynx – a structure in the trachea (windpipe)
  • Myocarditis (inflammation of the heart muscle)
  • Pleuritis (infection of the membrane covering the lungs)

Horses at the greatest risk of mortality from the equine influenza virus include: [1][11]

  • Foals, particularly at 2-3 months old when maternal antibodies are wearing off
  • Older horses
  • Horses with pre-existing health conditions

Signs & Symptoms

The equine influenza virus has an incubation period of 1-3 days. Horses are contagious during the incubation period but do not show outward signs of infection. [8]

Some horses never develop signs of the virus and are asymptomatic carriers of EIV. Asymptomatic horses may have a low viral load, may have been exposed to a mild variant of the virus, or may have antibodies from a previous infection or vaccination that suppress clinical signs. [13]

In symptomatic horses, symptoms usually become noticeable after three days and persist for 1-2 weeks. [9][10] Symptoms may include any or all of the following: [1][8][11][13][14][15]

  • Fever with a temperature greater than 102°F/38.9°C
  • Lethargy
  • Depression
  • Lack of appetite
  • Nasal discharge which is initially serous or watery, and becomes mucopurulent (a thick, opaque mucus)
  • Harsh, dry cough
  • Laboured breathing
  • Aching muscles
  • Swollen submandibular lymph nodes (located between the lower jaw bones)
  • Conjunctiva (swollen, oozing, pink eye membrane)
  • Inflamed, sore throat

In some cases, horses can also develop distal limb edema (swelling or accumulation of fluid in the lower legs) and cardiomyopathy (disorders of the heart muscle).

High fever is typically the first symptom to appear and will usually last 4-5 days, although this varies depending on the strain of the virus. Coughing may persist for longer than 2 weeks after the horse has recovered. [8]

One study reported that donkeys and mules became more ill from EIV than horses. Care should be taken to minimize the risk of transmission to your non-horse equine companions. [2]

If you notice your horse showing signs of influenza, contact your veterinarian to obtain a diagnosis.


Equine influenza is spread through direct contact with infected horses or through contact with contaminated surfaces such as tack, equipment, feed buckets, or clothing.

The virus can spread through respiratory secretions such as mucous or nasal discharge. Inhalation of contaminated droplets expelled by coughing or sneezing is another way the virus can spread. [16]

The UC Davis School of Veterinary Medicine reports that a coughing horse can spread the virus up to 150 feet in aerosolized particles, potentially infecting an entire barn. [22]

This makes quarantine procedures very important when a case of EIV is diagnosed to prevent the virus from spreading to barn mates.

Outbreaks of EIV can occur when horses are transported long distances or between countries for competition. Horses can carry the influenza virus but exhibit no clinical signs, causing the virus to be unknowingly transmitted to fellow competitors.

If the competition is held in an area that where local horses have no antibodies against a novel strain of EIV, there is potential for an outbreak or epidemic to occur.

Furthermore, horses travelling to the competition can become infected and transmit EIV after being transported home.

Horses may remain contagious and continue to shed the virus for 7 – 10 days after the infection has resolved.

Risk Factors

Numerous factors affect each horse’s risk of contracting EIV. Knowing these risk factors will enable horse owners to take precautions to reduce their horse’s risk of infection.

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