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Equine influenza is a disease we all fear for the consequences it brings: it can lead to temporary shut-down of a racing yard.

It is an endemic disease in the UK, maintained by sporadic cases and by inapparent infection in susceptible horses introduced into the population by birth, through waning immunity, or after movement from other areas or countries.

Clinical disease varies from a mild, subclinical infection to severe disease in susceptible animals.

Influenza is rarely fatal except in weakened horses.

Transmission occurs by inhalation of respiratory secretions containing the virus (i.e., an infected horse coughs on another horse’s face, or if they share the same water trough etc).

Influenza virus replicates within the respiratory cells, resulting in destruction of the surface layer of trachea and bronchi. These epithelial cells take about 21 days to regenerate: during this time, a bacterial infection can develop on top of flu, leading to pneumonia. In such case, the watery nasal discharge becomes thick and mucous. Vaccinated horses typically display only mild and transient symptoms.

Mildly affected horses recover uneventfully in 2–3 weeks; severely affected horses may take a lot longer, and have more serious symptoms. Restricting exercise, controlling dust, providing and practicing good stable hygiene will improve the outcome.

A rapidly spreading respiratory infection in a group of horses, having high fever, depression, and cough should raise an alarm bell. However, equine flu cannot be differentiated from other causes of respiratory disease, such as equine herpesvirus, solely on the basis of clinical signs.

Diagnosis is achieved by nasopharyngeal swab testing: the goal is to isolate the virus in a swab sample, similar to what we do for Covid-19.

Horses that do not develop complications only require rest and supportive care. Old textbooks used to advise to rest horses for 1 week for every day of fever, with a minimum of 3 weeks rest: this makes sense cause it allows regeneration of the respiratory cells. Anti-inflammatories are recommended for horses with a fever. Antibiotics are indicated when fever persists or in cases with nasal discharge or pneumonia. Severely affected horses might need more intensive support therapy.

Prevention requires hygiene practices and obviously vaccination. There are different brands of commercially available vaccines: as is done with human influenza vaccines (flu shots), manufacturers attempt to ensure that the currently available vaccine reflects, as closely as possible, the strains of virus circulating at that time.

Because the duration of protection provided by current vaccines is limited, booster injections should be administered every 6 months. Sedentary horses can be vaccinated annually. Foals should be vaccinated beginning at 6 months, with booster vaccination in 3–6 weeks and again in 6 months. Broodmares should be vaccinated 4 to 6 weeks before foaling.

What is it?

A highly contagious viral disease that produces respiratory signs.

Time window:

Symptoms develop in 1 to 3 days. Clinical signs last a week in uncomplicated cases. Horses need to be isolated immediately if equine flu is suspected or diagnosed.


  • High Temperature (above 40C)

  • Watery nasal discharge, which can become purulent.

  • Enlarged lymph nodes

  • Dry, non productive cough - this can persist for weeks

  • Depression

  • Loss of appetite

  • Weakness

  • Severely affect horses recover in 3 weeks to 6 months

What should I do?

  • If you suspect flu, you must isolate the horse, disinfect everything the horse has been in contact with (including yourself) and ring the vet.

  • It is good practice, when introducing new horses to the yard, to isolate them for 2 weeks and perform a blood test to check their health

  • Always make sure vaccinations are up to date

  • All confirmed racehorse cases MUST be reported to the BHA.

What should I NOT do?

Do not underestimate this disease because it is highly contagious and will spread quickly through the yard.

What will the vet do?

  • Nasopharyngeal swab - to establish if respiratory disease is due to a virus

  • Formulate a treatment plan - this might only include painkillers and electrolytes if the horse has a mild form


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