“Equine flu doesn’t kill horses, but it does kill horse events,” is a well-known saying among vets.

Over the last two years there have been 52 recorded outbreaks of equine flu in the UK. The disease is highly contagious and is acquired mainly by the inhalation of viruses, via aerosols (airborne particles), from other infected horses or via virus-containing droplets in the horse’s environment. In short, affected horses coughing and spluttering will spread the virus.

Unlike strangles and some other infections, the flu virus is not spread solely by direct contact. It can travel more than 2km in the air and can be spread indirectly, on tack, other equipment or personnel, to affect a whole yard and beyond.

Equine flu is difficult to control, especially in horses that are transported frequently and mixed extensively. Outbreaks are most common when young, susceptible horses are brought together at sales and shows, or for training.

Vaccination is key for disease control and is compulsory under British Horseracing Authority (BHA) and International Equestrian Federation (FEI) rules, as well as being required for many national equestrian shows and organisations. Despite this, many horses travel widely without ever having the vaccination records in their passports checked — which is of concern when so many animals remain unvaccinated.

This month has been designated flu awareness month to raise awareness of these risks.

Understanding equine flu

A virus particle virion

A virus particle virion

Equine flu is caused by an influenza virus similar to those that affect people (horses cannot be infected by human influenza, however, or vice versa).

The virus has an ability to mutate the proteins on its surface, a process known as antigenic drift. These surface proteins are the structures recognised by the immune system — if they change, a vaccine produced to recognise proteins on older virus strains will be less effective.

To overcome this problem, the World Organisation for Animal Health (OIE) is using continual analysis of flu infection data to recommend suitable vaccine strains for inclusion in commercial vaccines.

The aim of vaccination is two-fold: to protect an individual from becoming ill and to limit the spread of infection by reducing the amount of virus such a horse will shed if he does become infected.

As all influenza viruses undergo frequent genetic changes to produce different strains, it is difficult and expensive to produce up-to-date commercial vaccines against the current strain. As a result, available vaccines may not provide full protection against new strains. Vaccinated horses are much less severely affected by the disease, however, and develop a degree of cross protection between flu strains, so vaccination is worthwhile.

An approaching epidemic?

Studies conclude that the majority of outbreaks in vaccinated groups of horses are of limited size and that vaccination reduces the occurrence of epidemics.

There is the alarming suggestion that a large outbreak occurs every 10-15 years. The last one in the UK was in 2003, so vigilance is vital.

This means vaccinating all horses, ponies and donkeys, and contacting your vet if you notice any signs of flu. Tests can then be carried out to determine the cause, ensuring that the vaccines used match the currently circulating strains as closely as possible.

Ref: Horse & Hound; 21 April 2016