Equine Influenza

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Also known as: Equine Flu


There are two subtypes of Equine Influenza described in the world today. These are:

  • H7N7 (Equine 1), which was prevalent in the UK between 1963-1977
  • H3N8 (Equine 2), or the European strains, have been circulating since 1965.

Aerosol and fomite transmission infects the epithelium of the upper respiratory tract, resulting in cell necrosis, which then manifests as bronchiolitis and serous exudation. There is a 1 - 3 day incubation period, with excretion of the virus in nasal secretions peaking at 3-4 days and finishing by 10 days.

The disease usually occurs as an outbreak with many horses on the yard becoming infected. The most common age to encounter the disease is around 2 years and stress is a predisposing factor, so this disease is most commonly seen in racing yards.

Clinical Signs

There will be a harsh dry cough with pyrexia (39-410>C / 103-106F). The horse will appear generally depressed with a loss of appetite and decreased activity. Submandibular lymph nodes will be enlarged on physical examination. Secondary bacterial infection can follow defective muco-ciliary transport, eg Streptococcus zooepidemicus.


Virus isolation by deep nasal swabs (12 inches) deposited into 10ml of transport medium and transported at 40C. An antigen detection ELISA should then be performed. Blood samples for serology should also be taken on day one of the disease (acute phase) and then two weeks later (convalescent stage). A 4-fold increase of haemaglutination inhibition (HI) must be shown to confirm diagnosis.

Treatment and Control

Isolate coughing horses at once to minimize spread and perform thorough disinfection of all shared tack, stables, vehicles, boot and feed buckets. Treat any secondary bacterial infections with suitable antibiotics.

Control for this disease is by vaccination. All vaccines include H7 and varieties of the current circulating strains of H3. There are strict rules from the Jockey Club regarding timing of vaccinations: Horses must be certified as completing a vaccination course of 3 injections at least 8 days prior to racing. The course consists of a primary inoculation followed by a second within 21-92 days and a third after a further 150-215 days. Boosters are given annually on or before the anniversary of the third injection to maintain immunity, and in the event a year is missed, the initial course of three injections must be repeated.


Brown, C.M, Bertone, J.J. (2002) The 5-Minute Veterinary Consult- Equine', Lippincott, Williams & Wilkins

Knottenbelt, D.C. A Handbook of Equine Medicine for Final Year Students University of Liverpool

Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) Equine Internal Medicine (Third Edition), Saunders.

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