MSD Manual

Please confirm that you are a health care professional

honeypot link
Professional Version

Equine Influenza


Kara M. Lascola

, DVM, MS, DACVIM, Auburn University College of Veterinary Medicine

Reviewed/Revised Jul 2023

Equine influenza virus is a highly infectious RNA virus and is a common cause of acute respiratory disease in horses and other equids. Clinical signs are similar to those associated with other respiratory viruses and most often include fever, cough, and nasal discharge. Subclinical disease is possible. Definitive diagnosis can be made by viral detection (PCR array, viral isolation) or paired serologic testing. Treatment is primarily supportive in infected animals. Biosecurity practices and vaccination are critical in prevention and control of disease outbreaks.

Etiology and Epidemiology of Equine Influenza

Equine influenza is highly infectious and spreads rapidly among naive horses. Outbreaks in susceptible horse populations are associated with considerable economic impacts. China, Japan, and Australia experienced devastating epidemics of equine influenza affecting tens of thousands of horses in 2007. Horses 1–5 years old are the most susceptible to infection, particularly when housed in large groups. Horses that are immunosuppressed for various reasons (traveling, training) may have an increased risk for infection and more severe disease.

Equine influenza (Orthomyxovirus) A/equine type-2 H3N8 was first recognized in 1963 and has subsequently become endemic in many countries. New Zealand and Iceland are currently free of equine influenza. Antigenic divergence of equine influenza virus (EIV) over time has resulted in the emergence of distinct lineages (European and American). The European lineage is currently not recognized as a cause of infection in horses. Since 2010, the Florida H3N8 clades 1 and 2 sublineage strains (from American lineage) have predominated in EIV outbreaks worldwide. The World Organization for Animal Health (formerly called "OIE") monitors antigenic divergence of EIV strains associated with disease outbreaks and bases annual vaccine strain recommendations on their findings.

Endemicity is maintained by sporadic clinical cases and by inapparent infection in susceptible horses introduced into the population by birth, through waning immunity, or after movement from other areas or countries. A carrier state is not recognized for equine influenza. The clinical outcome after viral exposure largely depends on immune status; clinical disease varies from a mild, inapparent infection to severe disease in susceptible animals. Influenza is rarely fatal except in donkeys, zebras, and debilitated horses.

Transmission is most commonly direct via inhalation of infective respiratory secretions from coughing horses. Indirect transmission via fomites (clothing, hands, shared water) should also be considered an important mechanism of disease transmission. Shedding is longest in naive horses and lasts ~7–10 days after the time of infection. Epidemics arise when one or more acutely infected horses are introduced into a susceptible group. Vaccinated horses can act as subclinical shedders. The epidemiologic outcome depends on the antigenic characteristics of the circulating virus and the immune status of a given population of horses at time of exposure. Frequent natural exposure or regular vaccination may contribute to the extent of antigenic drift seen with specific strains of A/equine-2 virus in some parts of the world.

Clinical Findings and Lesions of Equine Influenza

  • High fever (up to 106°F [41.1°C]) with depression, anorexia, and weakness

  • Serous nasal discharge that may become mucopurulent due to secondary bacterial infection

  • Submandibular or retropharyngeal lymphadenopathy (slight)

  • Cough (dry, harsh, and nonproductive)

The incubation period of influenza is ~1–3 days, with clinical signs developing ~3–5 days after exposure. Overt clinical signs begin abruptly and usually last < 3 days in uncomplicated cases. Clinical signs are generally more severe in younger horses (1–5 years). Mildly affected horses demonstrate complete and uneventful recovery in 2–3 weeks; severely affected horses may convalesce up to 6 months. Recovery may be hastened by complete restriction of strenuous physical activity.

Clinical signs of respiratory disease associated EIV infection are often difficult to distinguish from those associated with EHV-1 or EHV-4 infection. The presence of a rapidly spreading respiratory infection in a group of horses characterized by rapid onset, high fever, depression, and cough is presumptive evidence of equine influenza.

Cough develops early in the course of infection and may persist for several weeks. Cough as a clinical sign is more common in influenza compared to equine herpesvirus infection Equine Herpesvirus Infection Equine herpesviruses are ubiquitous worldwide, with EHV-1 and EHV-4 having the greatest clinical importance in causing respiratory disease in horses. Fever, nasal discharge, malaise, pharyngitis... read more Equine Herpesvirus Infection . The virus replicates within respiratory epithelial cells, resulting in destruction of tracheal and bronchial epithelium and cilia. Respiratory tract epithelium takes ~21 days to regenerate; during this time, horses are susceptible to development of secondary bacterial complications such as pneumonia, pleuropneumonia Pleuropneumonia in Horses Pleuropneumonia represents a severe manifestation of bronchopneumonia and is defined as an infection of the lungs that extends to include the pleura and pleural space. Clinical signs of pleuropneumonia... read more Pleuropneumonia in Horses , and chronic bronchitis. Complications are minimized by restricting exercise, controlling dust, providing superior ventilation, and practicing good stable hygiene. Vasculitis and distal limb edema, myositis, and myocarditis can occur as infrequent complications of EIV infection.

Diagnosis of Equine Influenza

  • RT-PCR array

  • Viral isolation

  • Rapid tests

  • Serology

Definitive diagnosis is most frequently determined by virus isolation or RT-PCR array from nasopharyngeal swabs. These samples should be obtained soon after the onset of illness (~1–2 days) and submitted according to specific laboratory requirements. Virus isolation can also be performed from swab samples.

Additional diagnostic tests include commercially available stall-side immunoassay or antigen-capture ELISA kits.(1 References Equine influenza virus is a highly infectious RNA virus and is a common cause of acute respiratory disease in horses and other equids. Clinical signs are similar to those associated with other... read more ) Serologic testing can also be performed, on paired serum samples collected shortly after the appearance of clinical signs and then again approximately 2 weeks later.

Treatment and Prevention of Equine Influenza

  • Supportive care

  • Biosecurity

  • Vaccination

Horses that do not develop complications require rest and supportive care. Horses should be rested 1 week for every day of fever, with a minimum of 3 weeks rest (to allow regeneration of the mucociliary apparatus). Judicious use of NSAIDs is recommended for control of fever. Antimicrobials are indicated in horses with secondary bacterial pneumonia.

Control and prevention of influenza requires biosecurity practices and vaccination. Exposure can be reduced by isolation of newly introduced horses for 2 weeks. During an outbreak, sick horses should be isolated following standard biosecurity guidelines for 21 days after resolution signs in last newly infected horse. Environmental contamination is possible and the virus can remain viable for ~2–3 days on fomites and in water. Survival of the virus in aerosolized droplets lasts for a few hours.

Numerous vaccines are commercially available for prevention of equine influenza. The current recommendation is that vaccines should offer protection against the strains of the clade 1 and clade 2 Florida lineage. Most influenza vaccines are inactivated, adjuvanted vaccines recommended primarily for intramuscular administration. An intranasal modified-live influenza vaccine, designed to induce mucosal (local) antibody protection, also exists. This vaccine is temperature sensitive and is not capable of replicating beyond the nasal passages (ie, inactivated by core body temperature). Vaccine schedules should follow manufacturer guidelines and will vary according to age, risk of disease, and previous vaccination history. Because the duration of protection provided by current vaccines is limited, booster injections for at-risk adult horses are recommended every 6 months. Vaccination in an outbreak may be of value especially when performed early in the course of the outbreak and in horses previously vaccinated. The intranasal vaccine can achieve more rapid protection (~5–7 days) and thus is recommended for horses unvaccinated before the outbreak. Fédération Equestre Internationale (FEI) and United States Equestrian Federation (USEF) sponsored events require documentation of vaccination against EIV within 6 months of entering event grounds.

Key Points

  • Equine influenza virus is responsible for sizable disease outbreaks and substantial losses to the equine industry.

  • Clinical signs of EIV infection may be difficult to distinguish from other common viral respiratory pathogens.

  • EIV is rarely fatal but secondary bacterial pneumonia is an important complication of infection.

  • Diagnosis of EIV relies primarily on PCR array of nasopharyngeal swabs during the acute phase of clinical illness.

  • Supportive care is the mainstay of treatment in horses with uncomplicated EIV infection.

  • Vaccination is essential for reducing the spread of EIV among horses and biosecurity is critical for the control of outbreaks.


  • Yamanaka T, Tsujimura K, Kondo T, Matsumura T. Evaluation of antigen detection kits for diagnosis of equine influenza. J Vet Med Sci. 2008 Feb;70(2):189-92. doi: 10.1292/jvms.70.189. PMID: 18319581.

For More Information

quiz link

Test your knowledge

Take a Quiz!