Equine influenza is highly contagious and spreads rapidly among susceptible horses. Two distinct influenza viruses have been found in horse populations worldwide, although only one of these strains has been seen since 1980. Disease varies from a mild, undetected infection to severe disease in susceptible animals. Influenza is rarely fatal except in sick, weakened, or stressed horses. Transmission occurs by inhalation of respiratory secretions. Epidemics can arise when one or more infected horses are introduced into a susceptible group.

The incubation period of influenza is about 1 to 3 days after exposure to an infected horse. Signs develop rapidly and include a high fever (up to 106°F [41°C]), clear nasal discharge, swollen lymph nodes, and coughing that is dry, harsh, and nonproductive. Depression, loss of appetite, and weakness are frequently seen. These signs usually last less than 3 days in uncomplicated cases, although the cough can persist for several weeks. Nasal discharge may become filled with mucous and pus due to bacterial infection. Disease is rare in foals less than 9 months of age. Mildly affected horses recover in 2 to 3 weeks, but severely affected horses may take up to 6 months to fully recover. Complications are minimized by restricting exercise, controlling dust, providing superior ventilation, and practicing good stable hygiene.

The presence of a rapidly spreading respiratory infection with high fever, depression, and cough in a group of horses suggests the possibility of equine influenza. However, equine influenza cannot be differentiated from other causes of equine respiratory disease, such as equine herpesvirus infection (see Lung and Airway Disorders of Horses: Equine Herpesvirus Infection (Equine Viral Rhinopneumonitis)), solely on the basis of signs. A definite diagnosis can be made by identifying the virus in samples obtained from the nose and throat early in the course of the infection.

Horses that do not develop complications generally require no treatment aside from rest and supportive care. Horses should be rested 1 week for every day of fever with a minimum of 3 weeks rest, to allow healing of damaged respiratory tissues. Nonsteroidal anti-inflammatory drugs (NSAIDs) are usually given to horses with a fever higher than 104°F (40°C). Antibiotics may be prescribed when fever persists beyond 3 to 4 days or when pus-containing nasal discharge or pneumonia are present.

Prevention of influenza requires hygienic management practices and vaccination. Exposure can be reduced by isolation of newly introduced horses for 2 weeks. A number of vaccines are commercially available for the prevention of equine influenza; however booster vaccines are recommended every 6 months to maintain immunity. Vaccination should not begin in foals until 9 months of age. As is done with human influenza vaccines (flu shots), vaccine manufacturers monitor continuously to ensure that the content of the currently available vaccine reflects, as closely as possible, the strains of virus causing infection at that time.

Last full review/revision July 2011 by Bonnie R. Rush, DVM, MS, DACVIM; Neil W. Dyer, DVM, MS, DACVP; Joe Hauptman, DVM, MS, DACVS; Ned F. Kuehn, DVM, MS, DACVIM; Stuart M. Taylor, PhD, BVMS, MRCVS, DECVP; Wendy E. Vaala, VMD, DACVIM; Maureen H. Milne, BVMS, MVM, DCHP, MRCVS