Equine Flu Outbreak – what do you need to know?

Once again, the UK has been hit by an outbreak of infectious disease – but this time it’s horses that are affected. As a result, the headlines are full of “Horse Flu!!!” stories, racecourses are closed, and some people are recommending horse owners have their horses vaccinations “topped up” to minimise the risk. So what’s going on? How severe is the risk? And what can we do to minimise it?

 What is Equine Flu?

Equine Influenza is, as its name would suggest, a close relative of the human influenza virus. And just like that one, it affects the respiratory tracts of susceptible animals – in this case, horses. Fortunately, however, unlike bird flu or swine flu, the equine version does not cause disease in humans (although there is some evidence that humans can carry it without becoming infected). The virus is rarely fatal in horses or donkeys, but affected animals require prolonged convalescence, and complications such as pneumonia aren’t uncommon. Although the disease is not notifiable in the UK, it is so contagious that outbreaks should ALWAYS be reported to your vet, who will notify the Animal Health Trust’s EquiFluNet (who are monitoring this outbreak).

The strain that appears to be present in the current outbreak is H3N8 – all flu viruses are characterised by their haemagglutinin and neuraminidase receptors, in this case H number 3 and N number 8. This is a Clade 1 virus that is a direct descendent of the Florida strain that first emerged in 1999. It is also a strain that has, from time to time, been reported to infect dogs, although fortunately this is rare – the Equine Flu strains don’t jump the species barrier easily.

 What horses have been affected by this outbreak?

So far, there are 12 reported clusters of cases, in Essex, Cheshire, Derbyshire, Lincolnshire, Leicestershire, Yorkshire, Somerset and Middlesex. However, several of these have been in racehorses, and at least one affected yard reportedly sent runners to racing meetings (in all good faith) within the incubation period of the disease (but before it was diagnosed), meaning that it is likely that horses from distant areas are at least potentially at risk. The trouble with Equine Influenza is that it usually has a 3 day incubation period, in which the horse is infectious, but shows few or no signs of illness.

Of these, 7 of the 12 clusters involved unvaccinated horses, and vaccinated horses appear to be less at risk.

 Why didn’t the vaccines work?

Unfortunately, flu viruses are very unstable things. Because they’re part of a group known as RNA viruses, they don’t copy themselves very efficiently when they replicate, and so they mutate and change very fast. It seems likely that this particular strain has mutated slightly (a process known as antigenic drift) to the point where the virus is capable of triggering an active infection.

It isn’t just one brand of vaccine either – 2 brands have been challenged in this outbreak, and the third challenged on the continent. All three have shown some breakdowns in protection, in the face of a very heavy virus challenge.