In unvaccinated horses the classic signs of equine influenza include a harsh dry cough, pyrexia and labored breathing. This is typically accompanied by lethargy, depression and a loss of appetite. Horses may also develop a mild serous or mucoid nasal discharge. The infection will usually spread very rapidly through a naive horse population with close to 100% infection rate.
Horses that have only partial protection, either due to irregular vaccination or the use of outdated vaccine strains, will typically show signs of milder non-specific respiratory disease.
Damage to the epithelium and cilia of the upper respiratory tract caused by the virus leaves horses with increased susceptibility to secondary opportunistic infections. The development of secondary bacterial infections is a very important complication of equine influenza. Prolonged pyrexia, lethargy and malaise are usually accompanied by a more profuse mucopurulent/purulent nasal discharge.
In compromised horses this can lead to the development of pneumonias and even death. This increased susceptibility typically lasts for 50-100 days post infection. It is critical to recognise this and not over stress or over work the horse during this period as they run a higher risk of developing complications.