EI is an acute respiratory disease that can cause rapidly spreading outbreaks in horses that are congregated together.

Epidemiology and Aetiology

Equine influenza is highly contagious and spreads rapidly among susceptible horses.

Two immunologically distinct influenza viruses (Orthomyxovirus A/Equi-1 and Orthomyxovirus A/Equi-2) have been found in horse populations worldwide.

Transmission occurs by inhalation of respiratory secretions. Infected horses excrete the virus in expired air for up to 8 days after infection. Influenza is rarely fatal except in donkeys, zebras, and debilitated horses.

Epidemics arise when one or more acutely infected horses are introduced into a susceptible group. 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.

Clinical features

The incubation period of influenza is 1-5 days. Clinical signs begin abruptly and include high fever (up to 106°F [41.1°C]), serous nasal discharge, swollen and enlarged sub-mandibular lymph nodes, and coughing. Depression, anorexia, and weakness are frequently observed. Clinical signs usually last less than 3 days in uncomplicated cases. Influenza virus replicates within respiratory epithelial cells, resulting in destruction of tracheal and bronchial epithelium and cilia.

A dry, hacking cough develops early in the course of infection and may persist for several weeks. Nasal discharge, although scant and serous initially, may become muco-purulent due to secondary bacterial infection.

Mildly affected horses recover uneventfully in 2 to 3 weeks; but severely affected horses may take up to 6 months to recover. Respiratory tract epithelium takes about 21 days to regenerate and during this time horses are susceptible to development of secondary bacterial complications such as pneumonia, pleuropneumonia, and chronic bronchitis.

Pathogenesis and Lesions

Gross and microscopic lesions are not highly specific to the disease. There may be some inflammation of the mucosa of the upper respiratory tract. Acute lobular pneumonia or bronchopneumonia is usually present in fatal cases.

Specimen Collection

Nasopharyngeal swabs or nasal washings are obtained for virus isolation and antigen detection.
Blood samples may be collected from horses in the febrile stage and from the same horse 3 weeks later. Samples should be chilled and forwarded with water ice or frozen gel packs. If delays over 48 hours are expected, they should be frozen and sent on dry ice.


The presence of a rapidly spreading respiratory infection in a group of horses characterised by rapid onset, high fever, depression, and cough. Definitive diagnosis can be determined by virus isolation, influenza A antigen detection, or paired serology (hemagglutination inhibition). EI is isolated from swabs or nasal washings by the inoculation of 9-11 day old embryonated chicken eggs. Serological diagnosis is carried out by hemagglutination-inhibition tests using antigen of the appropriate hemagglutinin type. Cell cultures may be used for differential diagnosis. The acute-phase blood samples may be used for bacteriological culture.

Differential Diagnosis
The following diseases must be considered in the differential diagnosis:

  • Strangles
  • Equine rhinopnuemonitis
  • Equine viral arteritis
  • Equine rhinovirus infections
  • Pasturellosis
  • Equine adenovirus infections


Horses that do not develop complications require rest and supportive care.

Prevention and control

Prevention of influenza requires hygienic management practices and vaccination. Exposure can be reduced by isolation of newly introduced horses for 2 weeks. Vaccines are commercially available for prevention of equine influenza.

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