Influenza in Horses

Key Takeaways

Equine influenza is a highly contagious viral disease that can spread quickly among young, at-risk, and unvaccinated horses. 

• Symptoms of equine influenza include fever, nasal discharge, lethargy, enlarged lymph nodes, and cough

• Horses become infected with the virus through inhalation of respiratory secretions from an infected horse, most commonly during competitions, or from newly introduced horses

• Diagnosis of influenza typically requires a physical exam, blood work, and a nasal swab to identify the virus

• There are no specific treatments for influenza, although anti-inflammatories are used to reduce fever and muscle soreness

• The prognosis is good, unless there are secondary complications such as pneumonia

• Vaccination and biosecurity measures are imperative in equine influenza prevention

A Closer Look: What is Influenza in Horses?

Influenza viruses are a specific class of virus that are transmitted between individuals through respiratory secretions. Different strains of the virus infect specific mammalian species. Influenza viruses usually cause respiratory symptoms and effective vaccinations are widely available.

Equine influenza is a common condition in horses. Infected horses typically recover within 1-3 weeks, although it can take up to 3 months for horses to return to athletic performance.

Risk Factors

In severe cases, horses can develop pneumonia or pleuropneumonia, which has a poorer prognosis than uncomplicated influenza.

Horses that develop secondary complications from influenza virus such as pneumonia may have additional symptoms, such as:

Difficulty breathing

• Rapid breathing

• Persistent fever

• Moist or wet cough

• Severe lethargy

Influenza is rarely fatal.

Possible Causes

Equine influenza is caused by infection from equine influenza virus. Horses become infected by inhaling viral particles shed in respiratory secretions from an infected horse. The virus can also be spread by handlers and/or equipment shared between horses.

Main Symptoms

The main symptoms of equine influenza include:

• Fever

• Clear nasal discharge

• Swollen lymph nodes

• A dry, non-productive cough

• Lethargy

• Muscle soreness

Horses may also have a reduced appetite.

Testing and Diagnosis

While a presumptive diagnosis of equine influenza is made based on a quickly spreading respiratory illness among a group of horses, diagnostics to identify influenza include:

• Physical examination

• Bloodwork

• Nasal swab

If a secondary pneumonia is suspected, diagnostic imaging may also be performed.

Steps to Recovery

There are no specific treatments for equine influenza. Supportive care such as anti-inflammatories to aid with fever and muscle soreness and several weeks of rest may be recommended. Horses should be quarantined during their treatment period to prevent further spread of the disease. Antibiotics may also be prescribed if pneumonia is present. 

If there are no secondary complications, infected horses have a good prognosis with resolution of symptoms in 1-3 weeks, although it can take up to 3 months for horses to return to full athletic performance.

Horses that develop secondary complications have a poorer prognosis. These horses can take several months to recover from infection, and may be unable to return to athletic performance. Complications can be minimized through exercise restriction/rest, good air quality, and a hygienic environment during the recovery period.


Vaccinations are recommended for competition horses, and some competition regulating bodies may have specific vaccination requirements. 

Vaccinating horses against influenza is not a guarantee against infection, but vaccination can help reduce disease spread and reduce the severity of symptoms. 

Another way to prevent disease is by implementing biosecurity measures. Appropriate measures include: 

• A 2-week isolation period when introducing a new horse or when returning from competition

• Minimizing contact with unfamiliar horses

• Keeping equipment separate between horses 

• Avoiding communal water troughs 

• Isolating sick horses

• Practicing good hygiene

Is Influenza Common in Horses?

Equine influenza is common in horses.

Typical Treatment

• Anti-inflammatories

• Stall rest

• Antibiotics

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