Equine influenza (EI) is caused by influenza A virus strains H3N8 and H7N7, with H3N8 responsible for virtually all contemporary outbreaks. It spreads via respiratory aerosols and contaminated equipment. The OIE Expert Surveillance Panel monitors strain evolution and issues biannual vaccine composition recommendations. Current AAEP guidelines recommend vaccines matching OIE recommendations.
Clinical Signs
- Fever: 38.5 to 41 degrees C, often the first sign
- Dry, harsh cough: persistent, may last 2 to 3 weeks
- Nasal discharge: watery initially, may thicken with secondary infection
- Depression and anorexia
Treatment
Rest is the most critical intervention — one week of rest per day of fever. NSAIDs reduce fever. Antibiotics not indicated unless secondary bacterial pneumonia develops. Isolate affected horses immediately.
Vaccination
AAEP recommends biannual vaccination for horses with regular contact with other horses. Modified live intranasal vaccines (Flu Avert, Merck) stimulate local mucosal immunity. Killed virus intramuscular vaccines require a two-dose primary series then 6-month boosters. New arrivals should be isolated 3 weeks before joining the herd.
Biosecurity During an Outbreak
Isolate affected horses immediately. Restrict horse movement. Use separate equipment; handlers wash hands between barns. Monitor all horses for fever twice daily for 10 to 14 days (the same protocol used when managing strangles outbreaks).
Prognosis
Excellent for uncomplicated disease with adequate rest. Prognosis worsens with premature return to work or secondary bacterial pneumonia.
Sources: AAEP Infectious Disease Guidelines; OIE Expert Surveillance Panel; Pusterla N et al. in Equine Infectious Diseases, 2nd ed. (2014).