Equine influenza is a serious, fast-spreading respiratory disease that affects horses, donkeys, and mules. It is caused by strains of the influenza A virus, most commonly H3N8, that continue to evolve globally. Although mortality is low in healthy horses, outbreaks can significantly impact equine populations, particularly among young or unvaccinated groups.
Prompt recognition and diagnosis, combined with supportive care, strict biosecurity, vaccination, and quarantine, are key to controlling outbreaks, minimizing complications, and ensuring rapid recovery.
Clinical Signs
Clinical signs in equine animals usually develop 1-3 days following exposure and often include:
- Dry, harsh cough, which can persist for weeks after other signs resolve
- Sudden high fever (up to 106 °F)
- Lethargy, anorexia, muscle weakness
- Nasal discharge, initially clear and watery that may become mucopurulent (yellow or white) with secondary bacterial infection
- Enlarged lymph nodes around the head and neck area
- Infection without (or with mild) clinical signs is possible, even in vaccinated animals, but these animals can still shed and spread the virus
Donkeys, mules, young horses, and animals with weakened immune systems are more likely to experience more serious illness. Some animals may develop secondary bacterial infections such as pneumonia or pleuropneumonia. In rare cases, affected animals might show swelling in the lower legs or experience inflammation of the heart, blood vessels, or muscles. Neurologic signs are also possible but very unusual. Increased fetal losses may be seen in some animals that become ill late in pregnancy.
Prevention & Biosecurity
The following actions are important for disease prevention and control:
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Vaccination
- Quarantine and isolation
- Newly introduced equine animals should be isolated for at least 14 days and monitored for signs of illness.
- During outbreaks: isolate sick/exposed animals for up to 14–21 days after resolution of the last suspected case (coughing infected horses can spread the virus as far as 150 feet).
- Hygiene and disinfection
- Practice strict cleaning of hands, equipment, and clothing
- The virus is inactivated by common disinfectants, including bleach, alcohol, quaternary ammonium, aldehydes, and phenols
The virus can survive on contaminated surfaces for 2 days, in water for up to 3 days (longer in cold conditions), and in aerosols for hours.
If you suspect equine influenza, report it immediately:
Contact your veterinarian.
Recommendations & Treatment
Your veterinarian can help diagnose equine influenza, usually by taking nasal swabs and submitting to a laboratory for testing. Your veterinarian also will be able to provide guidance on supportive care and biosecurity. Rest is essential and medications may be necessary. Additional measures may be necessary to control or prevent outbreaks. Animal health officials may work with your veterinarian to quarantine affected and exposed animals.
Resources
ODA Resources
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Equine Influenza
For Oregon equine influenza cases and outbreak information, see the main Equine Influenza page.
Other Resources