Equine Herpesvirus Q&A

May 19, 2011

What is Equine Herpesvirus (EHV)?

As the name suggests, it is a family of viruses that affects horses, each resulting in a different set of clinical signs.


Electron micrograph of equine herpesvirus-1.
Why the widespread concern in the current outbreak?

Nine EHVs have been identified worldwide, but the one at the centre of the current outbreak poses a particularly serious health risk to horses. It is the neurological form of Equine Herpesvirus-1 (EHV-1). These so-called neuropathogenic strains of EHV-1 are known by the shorthand, nEHV-1. EHV-1 strains can actually cause four manifestations of disease, the neurological form being but one. Other EHV-1 strains can cause respiratory disease, abortion and neonatal death. Worryingly, in recent years, there has been a marked increase in the number of cases of the neurological form of EHV-1 reported in the United States.

What is Equine Herpesvirus Myeloencephalopathy (EHM)?

This is another name for the neurological disease associated with EHV infections. The neurological signs appear as a result of damage to blood vessels in the brain and spinal cord associated with infection. Interference with the blood supply leads to tissue damage and a loss in normal function of areas in the brain and spinal cord.

How common are EHV-1 infections?

Equine herpesvirus-1 is widespread in most horse populations around the world. (See footnote re Australia and New Zealand).

By the age of two, according to US data, nearly all horses have been infected with some form of EHV-1. Usually, the exposure occurs in foals from contact with their mother. The virus can then become latent, or inactive, in the horse's body, setting up a carrier state that is life-long. Horses of any age that are carriers of EHV-1 do not show any external signs of disease when the virus is in a latent form. The virus can be reactivated during times of stress, such as strenuous exercise, long-distance transport, or at weaning.

How is EHV-1 transmitted?

Transmission occurs after an exposure to an adult horse or foal with an active EHV-1 viral infection. This is mostly likely to occur through respiratory shedding of the virus. The infected animal may or may not be showing outward signs of disease. Infection via the respiratory tract can be direct or indirect. EHV-1 can be spread indirectly through contact with contaminated objects, such as tack, rags, feed or water buckets, or people's hands and clothing. The virus can become airborne, but it is difficult to establish the distance the virus can spread in this manner under typical horse-management conditions.

What kind of ability does EHV-1 have to spread?

There have been several outbreaks of the neurological form at large horse facilities and events at racetracks, show grounds, veterinary clinics, and boarding stables. The large number of horses that can be exposed on such premises and the serious nature of the disease has caused significant concern within the animal health community and the US horse industry.

Recent occurrences of the neurological form appear to meet the criteria of an emerging disease. A disease is considered to be "emerging" when it satisfies at least one of three criteria: the disease is identified for the first time in a region or country; it changes in severity, type of animal that can be infected, or other changes in pathogen behaviour; or there is a change in its geographic range or in its incidence within that range. In the case of the neurological form, recent outbreaks point to a change in the severity and behavior of the virus.

Are scientists certain that the severity of the neurological form is worsening?

It is possible the reporting of EHV-1 cases has increased, rather than an actual increase in the number and severity of cases; however, more information is needed to make such a distinction.

When should I suspect my horse might have the neurological form of EHV-1?

Signs include fever before the neurological signs, which can appear as decreased co-ordination, urine dribbling, loss of tail tone, hind limb weakness, leaning against a wall or fence to maintain balance, lethargy, and inability to rise.

How is EHV-1 diagnosed?

Nasal swabs can be tested at a laboratory for the virus. Blood samples can also be tested for antibodies specific to EHV-1. A vet will typically collect both nasal swab and blood samples to maximise the chances of diagnosing the disease. If the horse is dead, the vet may perform a necropsy to determine infection by the virus.

Is there a treatment for the neurological form of the disease?

Care for infected horses centres around support to ease the symptoms and problems associated with the disease. These may include intravenous fluids or anti-inflammatory drugs. Antibiotics may be used to treat a secondary bacterial infection if one develops. Antibiotics, however, have no effect on the equine herpesvirus itself. Antiviral drugs have also been used to treat cases. Research on the effectiveness of these drugs, their cost effectiveness, and the optimal dosing is still under way.

How can I prevent EHV-1 spreading to other horses?

Stop horse movement if your animals may be infected with EHV-1. This is the most important first step horse owners can take. Horses should neither enter nor leave premises where the disease has been diagnosed until cleared by a veterinarian. Do not allow exposed horses to have contact with unexposed horses on the premises. Sick horses should be isolated. Horses that have aborted or shown signs of fever, respiratory disease, or neurologic disease should be separated from healthy horses. Ideally, sick horses should be moved into a separate building or paddock on the premises, or be taken to a vet hospital with an isolation facility. Do not share equipment among horses on the facility. Practice proper biosecurity measures to prevent people from spreading the virus, as people can transfer this virus from horse to horse via their hands and clothing. People should wash their hands after handling one horse before handling another. They should also change their clothes and footwear after working with a sick horse.

Ideally, anyone who takes care of a sick horse should not work with healthy horses. If this is not possible, deal with healthy horses first, then the sick ones. Wearing gloves and using disinfectant to sanitize footwear can also help minimize the risk of people spreading the virus between animals.

Can I vaccinate my horses against this threat?

Vaccines exist to control the respiratory and abortion manifestations of EHV-1. Unfortunately, currently licensed vaccines are not labelled for the prevention of the neurological form. University and private researchers are looking into several existing vaccines to determine if they can provide such protection.

Can animals other than horses catch EHV-1?

EHV-1 cannot be transmitted to humans or cattle, as the virus is very specific to equine species such as horses, mules and donkeys. Llamas and alpacas can be affected as well.