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The news that four horses died in a recent outbreak of the equine herpes virus (EHV) has caused considerable alarm among horse owners.

Although it is believed that the outbreak is contained in one yard in the Hertfordshire and Bedfordshire area, the speed and severity of the infection is a sobering reminder of the potential of such diseases to wreak havoc in the equine community.

We asked biosecurity expert Professor Josh Slater, of the Royal Veterinary College, for the lowdown on this highly contagious disease.

Why is EHV so serious?

Equine herpes virus is very common, according to Josh.

“The virus is constantly present,” he explains. “A large majority of horses are exposed to the virus early in life and become lifelong carriers — the same as humans with cold sores, chicken pox and shingles.”

Josh explains that the virus is a “silent risk”, residing in the nerves or immune cells of these apparently healthy horses until it becomes reactivated by stressful episodes such as travel, competition, illness or weaning.

“The majority of horses that reactivate show no clinical signs of the disease,” he says. “But these ‘carriers’ shed the virus via nasal droplets, becoming contagious to others and causing an outbreak such as the recent occurrence in the UK. Sometimes they develop the disease themselves, which can lead to sporadic, isolated cases.

“Exactly why some horses should succumb to the full-blown illness is not always clear,” adds Josh. “Much depends on the horse’s immunity, which can fade with time, or the virulence of a circulating strain. There’s also a genetic influence over how an individual horse may cope, but this is not yet fully understood.”

How does EHV show itself?

Of the different equine herpes viruses, EHV-1 and EHV-4 are of most concern to UK horse owners.

“Both are associated with respiratory infection, but EHV-1 is the more problematic as it can enter the bloodstream and spread to other organs,” says Josh. “If it reaches the uterus of a pregnant mare it can cause abortion; if it affects the spinal cord and brain it can cause neurological disease. There are some particularly high-virulence strains of this version, which can have devastating consequences.”

The EHV-4 version typically causes little more than respiratory disease. While most infections with EHV-1 progress no further than this, in certain cases the neurological effects can result in problems ranging from hindlimb weakness to more significant loss of co-ordination. Inflammation within the spinal cord can lead to paralysis, triggering a cascade of indirect complications such as colic, pneumonia and an inability to urinate or defecate. Some horses recover, but the damage may be long-term and career-ending.

Which horses are at risk?

“ALL breeds, types and ages are susceptible,” says Josh. “While youngstock are more susceptible to infection, neurological disease can occur in all age groups. It appears that geriatric horses are more likely to develop the neurological form after infection.”

Should we be worried?

The facts are alarming, yet experts insist that there is no increased risk at present and no need to panic.

“With proper biosecurity and simple hygiene, it is possible to contain the virus,” explains Josh, adding that the appearance of a few cases at this time of year is not out of the ordinary. “We should not be thinking about stopping what we do, but about what we can change in our routine. A small amount of very simple things, such as keeping hands clean by using alcohol hand gels and avoiding horse-to-horse contact, will significantly reduce risk.”

Are there early warnings?

One of the first signs is a fever, so contact your vet immediately if your horse’s temperature exceeds 38.5ºC. Respiratory signs including coughing or nasal discharge may also be present, along with lethargy or wobbliness.

The incubation period is typically four to six days, although signs can appear as little as 24 hours after contact with an affected horse.

How to stop it spreading?

“The virus spreads via aerosol [very small droplets] and will travel for several metres,” says Josh. “Nasal discharge [larger droplets] of infected animals will also transfer infection — typically by nose-to-nose contact between horses, or through being smeared on things such as hands or shared tack and equipment. The virus may survive for some time after it has been shed, so sensible hygiene precautions and isolation are a must [see panel, right].”

Josh points out that our large and very mobile equine population provides significant potential for the spread of disease.

“Without movement there’s no horse industry,” he says. “There will always be risk, but the goal is to reduce this and do all we can to safeguard individual horses, yards and the industry as a whole.”

Is vaccination worthwhile?

“Vaccines against EHV-1 are available, but these are licensed for the respiratory and abortion forms of the disease,” says Josh. “They are not licensed for the neurological form and there is no information about their ability to protect against this.

“Vaccines do reduce the severity of the disease, however. They shorten the duration of virus shedding from infected horses, helping reduce disease spread, and decrease the amount of virus that enters the bloodstream. Most studies have shown that vaccination reduces the risk of abortion.”

It takes several weeks for vaccines to generate immunity in unvaccinated horses, Josh adds. To provide any protection, a full vaccine course must have been administered before an outbreak occurs.

What’s in the future?

Ongoing research aims to deepen our understanding of immunity to the disease, which is complex compared to the flu virus. Josh urges awareness of its devastating effects.

“Until there are reports of abortion or paralysis, equine herpes virus tends to drop off the radar,” he says. “It’s vital that owners are vigilant to its early signs and take appropriate steps to prevent its spread.”

 

Protect your horse

Good general yard and travel management should reduce risk of disease, so follow these basic rules:

  1. Be hygienic: Keep separate tack and grooming items for each horse and disinfect stables and shared equipment periodically.
  2. Monitor movement: Hand and boot washing is a good idea for anyone moving between yards, including farriers, physiotherapists and dental technicians.
  3. Take his temperature: Take your horse’s temperature daily — it’s a useful indication of his general health. Anything above 38.5ºC should prompt an immediate call to your vet.
  4. Isolate newcomers: Organise a remote paddock corner or an unused stable where yard newcomers can be quarantined. Three weeks of isolation should be adequate to identify any horses incubating disease.
  5. Show sense: Travel to shows, clinics, studs and racecourses, where many horses mix, will always carry a risk of transmission of infectious diseases. Ensure that the event or venue has adequate health checks in place for horses on arrival, and that everyone is vigilant for signs of disease.
    Take simple, practical steps, such as washing your hands regularly (using an alcohol hand gel in the absence of facilities) and reducing muzzle-to-muzzle contact (between temporary stables, for example). Don’t touch other people’s horses, or let others touch yours. Avoid sharing water buckets or hand-grazing areas.
  6. Stay vigilant: Disinfect transport upon returning from a show and monitor your horse for signs of ill health.
  7. If the worst happens: Strict biosecurity measures should be implemented — under veterinary supervision — should an outbreak occur. This will reduce the risk of spread on the premises or to other yards.

Find out more about EHV on the Horserace Betting Levy Board (HBLB) website, or via your smartphone or tablet with the new HBLB “EquiBioSafe” app

Ref Horse & Hound; 1 December 2016