Equine influenza is a highly contagious viral disease of horses. Equine influenza viruses belong to the orthomyxovirus family and has two subtypes.

The virus can alter itself in two ways; antigenic shift or antigenic drift. Antigenic shift is when there is a major change to the antigenic structure (this is quite rare) or antigenic drift when there is a minor change to the antigenic structure (more common). It is these changes that allow for reinfection and new outbreaks to occur and is the reason that vaccines have to be updated to allow protection against new strains. A similar change occurs with the human influenza virus.

Horses of all ages can be affected but infection is most common in young (two to three years old) unvaccinated horses. Infection occurs via inhalation and has a short incubation period of one to three days, and horses remaining infective for three to six days after the last signs of disease.

Outbreaks can occur and are most common when large numbers of young susceptible horses are brought together at shows or sales for weaning or training. Spread is rapid and up to 100% affected in immunologically naïve populations.

Clinical signs can vary and can include a cough (dry, harsh, sudden onset), fever, lethargy, inappetence, depression, tenderness of lymph nodes under the jaw and/or a serous bilateral nasal discharge.

The nasal discharge can become thick with pus due to any secondary bacterial infection. Other clinical signs include an increase in the heart rate, increase in respiratory rate, congestion of the mucous membranes, excessive tearing, lower limb swelling and muscle stiffness.

If secondary bacterial infection of the lower respiratory tract occurs then a pneumonia can develop causing difficulty breathing, chest pain and a reluctance to move.

In a small number of cases infection can spread to the heart causing inflammation which can result in an increase in heart rate, irregular heart rhythm and severe exercise intolerance.

In partially immune/vaccinated horses’ infections may be subclinical or if clinical signs are present are usually only mild. Performance horses may only present with exercise intolerance.

In naïve/unvaccinated horses clinical signs can be more severe. Mortality rate is usually low in uncomplicated cases with the exception of foals.

Foals who have not received enough antibodies from their dams’ milk at birth (the colostrum) show severe clinical signs of viral pneumonia which can lead to death within 48 hours.

Diagnosis is based on clinical signs, changes on blood samples, virus isolation or detection of the virus from nasal swabs or detection of increases in antibody levels to the virus in the blood.

Treatment is based on complete rest and good general nursing care.

A week off for every day of fever is often recommended followed by a gradual return to work. Antibiotics are only required if secondary bacterial infections are present and ideally should be based on culture and sensitivity results.

Non-steroidal anti-inflammatory drugs such as ‘bute’ can be helpful in horses with high fevers or muscle stiffness.

Bronchodilators and mucolytics may also be helpful in some cases. As outbreaks can occur, horses should be isolated with separate feeding, cleaning, grooming equipment etc and exercise stopped to minimise stress. In the face of an outbreak all healthy horses should be vaccinated.

Prevention includes isolation of new arrivals for three weeks, maintaining good airway hygiene/ventilation and vaccination. Jockey Club Rules advise a primary course of two doses not less than 21 days or more than 92 days apart.

The first booster must be given not less than 150 days and nor more than 215 days after the second injection of the primary course. Thereafter, annual boosters are to be given not more than 12 months apart.

If horses are competing under FEI rules, then they must have received a booster within six months of the competition.

Owners are always advised to check with the organisers prior to attending a competition to ensure that they are fully compliant with the rules.

Vaccines are usually given in the neck or chest and side-effects can occur and include localised pain/swelling at the injection site, stiff/sore neck, reluctance to put head to ground, fever or flu like symptoms.

In most cases side effects are usually mild, transient and respond well to treatment. If a side effect does occur, then a form can be completed and reported to the veterinary medicines directorate (VMD) as part of a pharmacovigilance scheme.