Equine influenza (EI) is an acute, highly contagious disease of the respiratory tract of horses. It is caused by the orthomyxovirus equine influenza A type 2 (A/equine 2) virus. EI is endemic in the equine population worldwide with the exception of New Zealand and Iceland. Outbreaks occur most often when horses kept in close contact with one another, like at horse shows, racetracks, boarding stables, airplanes, transport vehicles, etc. EI is thought to have originated from birds, and is closely related to avian influenza virus. EI is able to cross species barriers, as it has been reported to cause respiratory disease in dogs in North America and the United Kingdom. The severity of clinical signs of EI in a horse depends on many factors, including the degree of existing immunity. Horses that are partially immune can become subclinically infected and shed virus.

Clinical signs
Usually the first clinical signs observed in horses infected with EI is a high fever (sometimes exceeding 106°F (41.1°C)) which peaks in 2 to 3 days, and again on day 7. During this time most horses also loose their appetite. The virus spreads quickly throughout the horse's respiratory tract, resulting in initially serous discharge, later becoming mucopurulent; coughing typically develops at the same time. Generally, the course of the disease is 7 to 14 days in uncomplicated cases. However in some the coughing persists for another week or so.

Diagnosis
Diagnosis of EI is based on virus isolation, virus antigen detection (using an antigen capture enzyme-linked immunosorbent assay (ELISA) or of viral genome using reverse-transcription polymerase chain reaction (RT-PCR) assays), and paired serum testing taken from nasopharyngeal or nasal passage swabs, collected from horses showing signs of respiratory illness. Samples should be obtained as soon as possible from horses suspected of being infected with EI, ideally within 3 to 5 days.

Transmission
EI is highly contagious and the virus spreads quickly through groups of horses in aerosolized droplets dispersed by coughing. EI can survive for several hours when out in the open, and can contaminate buckets, feeding or grooming equipment and tack. The virus is able to survive in a wet environment such as a water bucket, for up to 72 hours, and for 48 hours on dry surfaces such as grooming equipment, tack, feed, hay, clothing, etc. When horses are vaccinated against EI with inactivated influenza vaccines immunity can be short-lived. This causes recently vaccinated horses to become infected and shed the virus. Therefore, any unvaccinated horses in contact with the horse can become infected with EI.

Incubation Period
EI has a short incubation period, where horses can start to develop clinical signs within 24 hours following exposure to the virus. Horses that are infected are able to shed the virus in their nasal secretions for up to 10 days. In some incidences, horses that are partially immune can become subclinically infected, causing them to shed the virus.

Complications
Complications of EI include secondary bacterial pneumonia, myositis (inflammation of the muscles), myocarditis (inflammation of the heart), and leg swelling; in rare incidences, neurological diseases can occur. It has also been suggested that horses infected with EI are predisposed to developing recurrent airway obstruction (RAO) and exercise-induced pulmonary hemorrhage (EIPH).