Isolation, vaccines effective against equine herpes virus

The economic impact of equine herpes virus 1 infection in horses, and in particular its most serious form, equine herpes myeloencephalopathy, is significant.

EHM outbreaks or the perceived threat of an outbreak have resulted in the closure of veterinary hospitals and racetracks and the cancellation of equestrian events.

Blood antibody levels can be tested to confirm exposure to EHV-1. The latest molecular diagnostic technique, polymerase chain reaction (PCR), rapidly identifies virus DNA in nasal secretions. Post mortem diagnosis involves the detection of inflamed blood vessels in the spinal cord and confirmation of virus with special labelling antibodies.

Treatment for EHM is supportive. Horses unable to stand are supported in a sling and their bladders frequently emptied. Anti-inflammatory steroids are commonly administered.

The latest treatment and prevention option is the antiviral drug acyclovir, which was developed for treating human herpes infections. This drug has been given to valuable horses during an outbreak of EHM to prevent them from developing the disease. It has also been used to treat affected horses. While the drug shows promise, efficacy has not been proven, and it is expensive.

Most horses under supportive care begin to improve after a week, although a full recovery may take months. Some severely affected horses never regain full function of their hind end. Those that have rapidly progressing disease or those that lose the ability to stand are more likely to die or be euthanized.

Isolating horses with EHM early in the disease is important to limiting spread.


Several types of EHV-1 vaccines are available with variable efficacy. The major applications for this vaccine include young horses susceptible to the respiratory form and prevention of abortion storms in pregnant mares. Well-established vaccination protocols exist for pregnant mares.

No vaccine claims to protect against EHM and there have been several reports of vaccinated horses developing the neurological form.

Vaccines do not completely prevent infection, but vaccinated horses have fewer outbreaks of respiratory disease and less virus shedding.

Some equestrian events are requiring vaccination in an attempt to decrease the chance of an EHM outbreak, but mature horses are likely already carrying the virus and the disease is rare.

The concept of herd immunity, which involves vaccinating a large percentage of the population, is important in managing this disease at a herd level. If the majority of horses are vaccinated, theoretically there should be reduced virus shedding and less chance that uninfected horses will contract the virus.

Control measures aimed at de-creasing the spread of the virus must address the ways it can be transmitted. These include aerosolized respiratory secretions, nose-to-nose contact and inanimate objects, called fomites, which had contact with the horse and now carry the virus.


Common barn fomites include water troughs, feed buckets, grooming equipment, halters and bridles.

New horses or those recently returned from outside events should be kept isolated for at least three weeks and carefully monitored for signs of respiratory disease: nasal discharge and fever.

Pregnant mares should be kept as isolated as possible to prevent abortions. There are no control measures specific to preventing the EHM form.

Ideally, a new vaccination will be developed that can be administered to foals before infection and that results in long-term immunity.

For now, however, the message is to keep horses and equipment away from others at horse events and talk to veterinarians about an appropriate vaccination protocol.