By Margaret Evans

In February 2012, Equine Canada’s Health and Welfare Committee advised horse owners on its website that the neurologic form of equine herpesvirus, equine herpesvirus myelitis (EHM), caused by equine herpesvirus-1 (EHV-1) had been confirmed in horses in southern Ontario and California. A blood sample from a horse in Ontario with severe neurological signs tested positive for the virus in early January, 2012. After the animal’s condition deteriorated, it had to be euthanized. In December of 2011, another horse with similar symptoms at a different location in Ontario was also euthanized but because the required samples had not been taken, it could not be confirmed as having EHV-l.

Then in January, the California Department of Food and Agriculture confirmed two new cases of the illness. According to Equine Canada, California has had 16 cases of EHV-l and two are believed to have had the neurologic condition EHM.

Last year, several horses that attended the National Cutting Horse Association’s Western National Championship in Utah were diagnosed with the neurologic form of EHV-1. Many horses were exposed at that event to this highly contagious disease. With this year’s competition season about to start, some riders, especially those who plan on travelling to the United States on competition circuits, might have some concerns.

According to the American Association of Equine Practitioners, equine herpesvirus-1 (EHV-1) is a common virus that is found in horse populations worldwide. EHV-1 causes respiratory diseases, abortion, and can sometimes lead to the neurologic disease EHM.

“In 2006, a paper was published in which increased risk of the neurologic form of equine herpesvirus-1 (equine herpesvirus myelitis or EHM) was reported to be associated with a particular genetic variant of the virus,” explained Professor Hugh Townsend with the Department of Large Animal Clinical Sciences, Western College of Veterinary Medicine, University of Saskatchewan. “Although this designation may prove to be of use to research scientists in the field, the fact is that many cases of EHM have been associated with the so called ‘non-neuropathic’ form of the virus. Currently, experts agree that the genetic characterization of isolates of EHV-1 is not a clinically useful procedure. The important point is that any outbreak of neurological disease is to determine if the EHV-1 is present along with the typical signs and pathology of the disease.”

Clinical signs for the onset of EHV-l invariably start with fever and the respiratory disease can also present with a cough and a nasal discharge. These signs, of course, can also be symptoms of influenza and diagnostic testing is needed to confirm the illness.

EHV-1 can cause a broodmare to abort late in pregnancy (after eight months) and it can happen with little or no warning. The virus infects the placenta, fetal membranes, and birth fluids. Aborted fetuses are infectious and mares that have lost foals continue to shed the virus through nasal secretions.

Foals and young horses are at higher risk as well as horses in their senior years. Neonatal foals may be infected before birth and, if so, their prognosis is poor. Signs include weakness, jaundice, and respiratory difficulty. The outcome for foals is often fatal.

The route of infection is through the respiratory system but the virus does not remain in there for more than three weeks.

EHV-1 is transmitted directly by horse to horse contact or by a horse coughing or snorting so that infected droplets are taken up by another horse close by (within about 10 metres). It can also be transmitted by a person working with or around an infected horse, then introducing infected spores to another horse via unclean hands, clothing, boots, and grooming or barn tools. It is therefore essential when working around a sick horse to take all the sanitary precautions of hand washing, changing clothes and boots, and disinfecting tools, equipment, towels, rags, and all implements that have come in contact with the infected horse before working with others.

The virus can remain in the environment for several weeks and an infected horse on the road to recovery can continue to shed the virus for 10 days or more which is why it is often recommended that the horse have a 28-day isolation period. But even once recovered, the horse will permanently carry the virus.

“Once infected with this virus, and many horses are, they may remain infected for life and, at least in some cases, they can pass the infection to other horses,” cautioned Townsend. “However, the impact of this process upon infection and disease occurrence is not well understood and is still being studied. For this reason, horses known to have been infected with the virus are not considered to provide significant risk for other horses.”

EHM, the neurological form of EHV-l, results from widespread damage to blood vessel endothelium, the thin layer of cells lining the interior surface of blood and lymphatic vessels. There is also damage to the blood-brain barrier. This neurological form of the disease usually affects the hindquarters and the urinary tract. Signs include a lack of co-ordination, incontinence, and bladder distension. The severely affected horse may lie down then be unable to get up. Some might sit in a dog-like position. Treatment is a matter of consultation between the horse owner and their veterinarian.

EHM, the neurologic form of EHV-1, has been around a long time.

“Nobody knows when or where the first case of this disease occurred,” said Townsend. “What we do know is that the first case was reported in the literature in 1885. Since that time, outbreaks of EHM have been reported regularly. It is not clear whether these outbreaks are becoming larger or more frequent but with the aid of the Internet they are certainly being reported more commonly and in more detail.”

Townsend added that while EHM has been reported widely, it is most commonly reported in North America and Europe. It does not appear to be influenced by climate change or weather variations. Since it is a contagious disease transmitted directly from horse to horse, insects are not involved in the spread of this condition and there is no risk of transmission to humans.

Right now, while there are vaccines for EHV-1, there is no effective vaccine for EHM, the neurologic form. However, Townsend said that there are some very good research scientists who are working on the problem and there is some preliminary evidence to suggest that it may be possible to develop an effective vaccine over the next ten years.
“Current research in Canada is focused upon basic research related to the persistence of the virus in the horse and other related questions as well as ongoing efforts to learn as much as possible about the epidemiology of the disease through the detailed study of natural outbreaks,” he continued.

While EHM is not a reportable disease in Canada, Ontario has introduced a new Animal Health Act and consideration is currently being given as to whether the disease should be mandatory reporting in that province.

EHM doesn’t have to be a death sentence. With a quick response, full veterinary care, medication, and supportive nursing, horses can recover after a few months and go on to lead normal lives. But during the time of the infection and for a period of time afterwards according to a veterinarian’s advice, the infected horse must be kept isolated, the handler must change clothes and scrub before handling other horses, and all traffic on and off the premises where infected horses are housed should be eliminated.

For horses attending shows this season, some “best practices” protocols should be followed. A full list of the guidelines is outlined in an excellent brochure on biosecurity available through Horse Council BC (www.hcbc.ca).

The core advice is to:
•    Trailer your own horse in your own scrubbed and disinfected trailer;
•    Don’t trailer other horses;
•    Keep your horse away from any other horse;
•    Don’t allow strangers to pet your horse;
•    Keep hand sanitizers and disinfectants available; and
•    Clean and disinfect all tack and boots before leaving the show grounds so germs are not transported home.

By adopting comprehensive biosecurity protocols, the risk of spread of the infection can be minimized. Your vet will be able to help with establishing those protocols and providing information to others on the farm or at the facility as to what guidelines should be followed.

Main Article Photo - Once infected with equine herpesvirus, a horse can remain infected for the rest of its life, and, even if it appears to be healthy, may be able to pass the infection to other horses.