Equine influenza

Equine influenza adversely affects the respiratory tract and is a highly infectious disease. Sometimes known as horse flu, this disease is mainly caused by two strains of Influenza A. These viruses are equine subtypes A1 (or H7H7) and A2 (or H3N8).  Horses, ponies, donkeys and mules can all be infected by these viruses but transmission can also occur to dogs.

The viruses can be found in the epithelial cells in the respiratory tract where they rapidly reproduce. Unfortunately some foals may develop pneumonia which can be fatal Young foals, elderly horses and those with compromised immunity are more susceptible to catching the disease due to their relatively weaker immune systems. They are also more likely to have more severe symptoms and so vaccinations are recommended.

Transmission results from contaminated aersol droplets and symptoms include coughing, nasal discharge, a high temperature, loss of appetite and depression. The disease is not zoonotic which means it cannot be transferred to humans. Vaccines are thought of as essential in the UK to reduce or prevent the risk of infection and annual boosters are strongly encouraged.


The virus is transmitted via aerosol droplets from an infected horse coughing and the healthy horse breathing them in. Inhalation of infected nasal discharge can also spread the disease as well as infected manure and urine. Indirect transmission can also occur as a result of contact with contaminated surfaces such as feed buckets, rugs and brushes. It is a close certainty that if an unvaccinated horse is exposed to the viruses responsible then it will become infected.  

After infection, the horse can shed the virus for another five days thus increasing the chance of the disease spreading. Outside of the host, the influenza virus can survive for as long thirty-six hours but fortunately most disinfectant methods can destroy it. Correct hygiene and ventilation is necessary to prevent and reduce the risks of infection.


The symptoms begin with a dry, hacking cough, nasal discharge, ocular discharge, loss of appetite and depression. The legs become swollen from limb oedema and the horse may shiver. The horse may wheeze during inhalation and the horse can experience weight loss.

The symptoms progresses to a discharge of yellow mucus following the coughing. It usually takes the horse to regain full health within a period of two to three weeks.  Foals can develop pneumonia following infection of the equine influenza virus and occasionally this can prove to be fatal.

Treatment and Prevention

There is no cure for equine influenza. The horse should stop working and rest until fully recovered. Antibiotics are administered to prevent or treat any secondary infections, especially if infection from pneumonia is suspected. Anti-inflammatory medicines are administered for horses with high temperatures.

Vaccinations are strongly recommended to prevent the spread of the disease. Boosters should also be given to provide the horse with long term protection. Most horse events require proof of vaccination or else the horse cannot enter. Horses with the disease should be isolated thus stopping further transmission.

Diagnosis and Prognosis

Horses are diagnosed with the disease following the observation of the symptoms and then by obtaining nasopharyngeal swabs where the virus can be isolated and identified. Serology and the Polymerase Chain Reaction (PCR) test are other methods which may be used to identify the bacteria. Most horses will return to full health from this disease in two or three weeks provided there are no serious complications. Foals may die if the disease leads to pneumonia.


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