Equine Influenza

Respiratory disease is of great importance in horses, as it has a direct and often substantial impact upon their performance.

Equine influenza is endemic in all horse populations, and influenza outbreaks are prevalent throughout the world (with the exception of New Zealand and Iceland), and as such are a major source of disruption of equestrian activities. These events, where large numbers of horses are congregated, are conducive to the spread of influenza virus, which is transmitted by the respiratory route. The disease is highly contagious, and viral particles may be transmitted directly from horse to horse, or via contamination of equipment and clothing. An unvaccinated horse population, with no previous exposure to the virus, has a nearly 100% infection rate.

Equine influenza is classified as an “Influenza A” type of virus, with two distinct sub-types of this virus: Equine-1 (H7N7) and Equine-2 (H3N8). The viral disease causes an inflammation of the respiratory membrane, and after an incubation period of approximately 1-5 days, ‘flu symptoms start to become apparent. The clinical disease as a whole can be extremely debilitating to the horse, lasting for about 10 days, although a cough may persist for longer. As with humans, horses which recover from influenza are not immune from subsequent exposure to the virus

Vaccination is a major defence against the disease, and is an important aspect of protection against influenza outbreaks. Vaccination is also a stringent requirement for highly mobile horses, such as those competing in international competitions.

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    • High fever (103° - 106° C)
    • Lethargy
    • Poor performance
    • Anorexia
    • Harsh, dry hacking cough
    • Rapid breathing
    • Mandibular lymph node enlargement
    • Limb oedema
    • Nasal discharge – may change from watery to yellow-green in colour
    • Ocular discharge
  • Equine influenza symptoms are also seen in other respiratory diseases, including Strangles, equine herpesvirus-1 and -4, streptococcus infection and bacterial pneumonia. However, once more serious causes of respiratory infection have been eliminated, diagnosis of influenza in unvaccinated horses is usually made on the basis of history and the clinical symptom of the characteristic dry cough. As vaccinated horses may only show quite mild influenza symptoms and shed very little virus, in order to determine an exact diagnosis in these horses, nasal swabbing is necessary for viral isolation testing.

  • Affected horses should be isolated to prevent further spread of the virus. Complete rest in a dust-free and well-ventilated stall is the most essential treatment component. Feed may need to be made more palatable in order to assist in swallowing, for example, soft feed or soaked good quality hay might be given. Anti-inflammatory drugs may be used to reduce fever, and if a secondary bacterial infection, such as pneumonia develops, antibiotics may also be administered. No strenuous exercise until 2 weeks after influenza symptoms have ceased is recommended.

  • Vaccination is a major tool in the prevention or reduction of equine influenza symptoms. Horses are given a primary vaccination series, followed by an annual booster. However, other factors which may increase the risk of exposure, such as competing in international events, may necessitate vaccination on a 6-monthly, rather than annual, basis.

    Good management and hygiene procedures are also essential to reduce the spread of virus particles. Hygiene measures, such as sterilising transport vehicles, buckets, and other equipment must be undertaken regularly. Handlers should wash their hands frequently. Any horses showing influenza symptoms must be isolated immediately to reduce the risk of influenza spread to other horses.

  • Do current vaccines on the market protect against new or diverging equine influenza strains?

    The OIE/WHO makes formal recommendations when there is a need to update vaccine strains in the light of new or diverging influenza strains. Additionally, many currently-marketed vaccines have proven cross-protection of their existing vaccines against newer equine influenza strains, such as the “South Africa/4/03” or the “Sydney/07” strains which were responsible for devastating influenza outbreaks.