Equine Influenza (EI) is a highly contagious though rarely fatal respiratory disease of horses, donkeys and mules and other equidae. The disease has been recorded throughout history, and when horses were the main draft animals, outbreaks of EI crippled the economy. Nowadays outbreaks still have a severe impact on the horse industry. EI is caused by two subtypes of infl uenza A viruses: H7N7 and H3N8, of the family Orthomyxoviridae. They are related to but distinct from the viruses that cause human and avian infl uenza. Equine Infl uenza is a disease listed in the OIE Terrestrial Animal Health Code and countries are obligated to report the occurrence of the disease according to the OIE Code. The disease is entrenched in most of the world, with the exceptions of Australia (where an important outbreak occurred in 2007), New Zealand, and Iceland.
How is the disease transmitted and spread?
Highly contagious, EI is spread by contact with infected animals, which in coughing excrete the virus. In fact animals can begin to excrete the virus as they develop a fever before showing clinical signs. It can also be spread by mechanical transmission of the virus on clothing, equipment, brushes etc carried by people working with horses. Once introduced into an area with a susceptible population, the disease, with an incubation period of only one to three days, spreads quickly and is capable of causing explosive outbreaks. Crowding and transportation are factors that favour the spread of EI.
Clinical signs In fully susceptible animals, clinical signs include fever and a harsh dry cough followed by a nasal discharge. Depression, loss of appetite, muscle pain and weakness are frequently observed. The clinical signs generally abate within a few days, but complications due to secondary infections are common. While most animals recover in two weeks, the cough may continue longer and it may take as much as six months for some horses to regain their full ability. If animals are not rested adequately, the clinical course is prolonged.
Prevention Vaccination is practiced in most countries. However, due to the variability of the strains of virus in circulation, and the diffi culty in matching the vaccine strain to the strains of virus in circulation, vaccination does not always prevent infection although it can reduce the severity of the disease and speed recovery times. The OIE also convenes an Expert Surveillance Panel on Equine Infl uenza Vaccine that examines the strains of virus in circulation making recommendations on which strains should be included in the vaccines.