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