Equine Influenza Information for Veterinarians

Equine influenza virus (EIV), specifically influenza A (H3N8), is a highly contagious respiratory pathogen affecting horses and other equids. It is endemic in most countries, with outbreak potential in equine populations. Most clinical cases exhibit low mortality, but the disease can severely impact performance and lead to complications, especially in naive or immunocompromised animals.

Potential Hosts

Primary hosts: Equine species (horses, donkeys, mules); EIV has also been reported in zebras; donkeys and mules often experience more severe disease. 

Other species: While uncommon, other mammals (e.g., dogs) can occasionally be infected by equine influenza viruses. EIV has not been shown to cause disease in humans, but serological evidence suggests humans can be infected.

Clinical Signs

The incubation period for equine influenza is usually 1–3 days but can range up to 5 days. Clinical signs can vary from mild to severe depending on the age and immune status of the animal, asymptomatic infection (and viral shedding) is possible.

Common presentations:

• Sudden onset fever (up to 106 °F), depression, anorexia, muscle pain/weakness
• Serous nasal discharge that may progress to mucopurulent if secondary infections occur
• Dry, harsh, often paroxysmal cough that typically precedes fever and can persist for weeks post-recovery
• Possible wheezing and/or tachypnea
• Mild submandibular and retropharyngeal lymphadenopathy

Less common or complication signs:

• Secondary bacterial pneumonia or pleuropneumonia
• Swelling in distal limbs, myocarditis/cardiomyopathy, vasculitis, myositis, and neurologic signs
• Increased fetal losses (stillbirths, dystocia, 'red bag' deliveries) may be seen in animals that become ill late in pregnancy

Healthy adult horses usually recover within 1-3 weeks; recovery may take months in severely affected animals.

Report Equine Influenza

Equine influenza is an immediately reportable disease. If you suspect equine influenza, please contact the State Veterinarian at 503-986-4711. ODA can assist in getting samples properly submitted and with potential biosecurity needs.

Diagnosis & Treatment

Diagnostic confirmation of equine influenza is usually obtained from early nasal or nasopharyngeal swabs submitted for PCR testing.

Treatment usually involves:

• Rest and supportive care
• Medications if needed to control fever and discomfort or to treat secondary bacterial infections
• Optimizing environmental factors: improve ventilation, reduce dust and stress

Prevention