What is Equine Influenza?
Equine influenza, or “horse flu,” is a highly contagious respiratory virus that affects horses of all ages. Similar to the human flu, it spreads through:
- Direct contact, especially nose-to-nose
- Inhalation of airborne droplets from coughing
- Contaminated hands, grooming tools, and shared surfaces
Common Symptoms
Infected horses typically develop:
- Fever (over 101.5°F / 38.6°C)
- Dry, persistent cough
- Crusty nasal discharge
- Lethargy and reduced appetite
Recovery is expected, but full healing of the respiratory system may take weeks to months.
How Does It Spread?
Equine Influenza Virus (EIV) spreads rapidly in environments where horses are in close proximity, such as barns, shows, and during transportation. Transmission occurs through:
- Coughing/sneezing
- Contact with contaminated objects (brushes, tack, clothing)
- Indirect contact (surfaces, trailers, human handlers)
Infected horses can spread the virus before showing symptoms, making early detection and isolation critical.
Signs of Illness in Horses
Horses usually begin to show signs within 1 to 3 days after infection. Watch for:
- Fever (39–41°C / 102.2–105.8°F)
- Dry cough
- Wheezing (in severe cases)
- Runny nose
- Loss of appetite
- Fatigue
Note: Vaccinated horses may show milder symptoms or none at all.
Prevention & Vaccination
Vaccination is the most effective way to prevent equine influenza. The flu vaccine is classified as a “Risk-Based Vaccine” by the American Association of Equine Practitioners (AAEP), meaning it should be used based on a horse’s lifestyle and exposure risk.
Who Should Be Vaccinated?
- Horses traveling or frequently exposed to new horses
- Horses living in active barns or show facilities
- Young horses, who are more vulnerable to severe illness
During Outbreaks
If your barn experiences a flu outbreak:
- Isolate infected horses
- Consult your vet about administering intranasal flu boosters to uninfected horses. This can provide immunity within 5 days.
- Maintain strict hygiene and avoid contact between sick and healthy horses.
Regional Considerations
In places like Florida, where mosquito season arrives early, it’s especially important to keep horses up to date on vaccinations, not only for flu but also for diseases like:
- Eastern Equine Encephalitis (EEE)
- West Nile Virus (WNV)
Travel Tips for Horse Owners
- Take your horse’s temperature twice daily while traveling.
- Monitor for coughing, fatigue, or appetite loss.
- Immediately report any unusual signs to your veterinarian.
Equine Influenza in Other Species
- EIV H3N8 has adapted and infected dogs in the past (canine influenza), but the canine strain is now considered extinct.
- Despite genetic similarities, the virus strains in horses, dogs, and birds have diverged into separate lineages.
Final Thoughts
Equine influenza is serious but preventable. Stay vigilant, vaccinate wisely, and work closely with your veterinarian to keep your horses safe and healthy.
Dr. Sally DeNotta, Clinical Associate Professor at UF Large Animal Clinical Sciences
