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What is Equine Influenza?

Equine Influenza Virus (EIV), which causes Equine Influenza, is a highly contagious virus. Equine Influenza is one of the most common equine infectious respiratory diseases. Endemic outbreaks of EIV occur sporadically worldwide and can be severely disruptive to equestrian events and local economies.

EIV is subject to antigenic drift, a mechanism of mutation that results in the development of new viral strains. Consequently, there are several distinct sublineages of EIV. In North America, surveillance suggests that most cases of Equine Influenza are caused by virus strains belonging to the Florida sublineage Clade 1 (e.g. Ohio/2003).

Nasal discharge is a sign of Equine Influenza

How is Equine Influenza contracted?

Direct contact between horses via respiratory secretions

The time between exposure to the infection and the appearance of symptoms for EIV ranges from 24 hours to five days. Most commonly, EIV is transmitted directly from horse to horse via respiratory secretions (droplets from coughing or nasal discharge). Viral shedding is likely to last for up to 7–10 days, although this period may be shorter in horses who have been vaccinated.

EIV can also be transmitted indirectly via contaminated equipment and other objects, or by people on contaminated hands or clothing. The virus can survive for up to two days in the environment and on hard surfaces, and for three days in water.

Clinical signs and symptoms of Equine Influenza

Clinical signs and symptoms of Equine Influenza:


Behavioural Changes; Depression

Muscle Stiffness

Lack of Appetite

Nasal Discharge,

initially serous, but may become mucoid


Less commonly, horses with EIV may develop distal limb edema and/or cardiomyopathy. Donkeys and mules may develop more severe symptoms of EIV than horses.

Prevention and treatment of Equine Influenza

Equine Influenza will typically resolve on its own without treatment other than symptom management. Fatalities resulting from EIV infection are rare. A horse showing symptoms of Equine Influenza should be isolated immediately, even before a diagnosis is confirmed.

To minimize the risk of EIV spreading, precautionary biosecurity measures should be followed, including:

Isolation Isolating all horses in the direct vicinity (e.g. the same barn or herd) of a horse with EIV
Following isolation protocols for
21 days
following the resolution of the last case of EIV infection

Although treatment for EIV is simple (rest and waiting for the disease to run its course), the impact on your training and showing schedule can be considerable. EIV can keep your horse out of competition for three or more weeks.

Vaccination is the most effective way to reduce the risk of a horse developing Equine Influenza infection. Typically, annual vaccination is recommended for horses at risk; however, the ideal frequency may vary depending on the horse’s age, sex, and potential exposure.

Learn more about how

VETERA® and CALVENZA® vaccines can help protect your horse.

Find out how