Neurological Herpes, aka EHM – equine herpesvirus myeloencephalitis – is usually caused by a specific strain of the EHV-1 virus which is equine herpesvirus 1. EHV-1 typically causes mild respiratory tract disease or “colds” in young horses. Most adults develop immunity but at least 75% continue to carry the virus which may be shed in nasal secretions during times of stress.
The EHV-1 virus can also cause abortions and illness of infected foals that survive. The neurological form is caused by a specific mutation of the EHV-1 virus which causes damage to the blood vessels of the spinal cord and/or brain. The horse may be infected with that strain or it may mutate from a more common respiratory strain after infection. It has a strong tendency to attack areas of the spinal cord which control the hind legs and ability to urinate.
Country fairs, rodeos, shows, racetracks and other places where horses congregate are ideal for spreading viral infections
EHM outbreaks occur after large numbers of horses have congregated, such as at shows or racetracks, but the virus can also be carried back to infect home barns. There have always been scattered cases of EHM seen during outbreaks of respiratory EHV-1 disease but in the last decade or so it has become clear that outbreaks of EHM, often without respiratory disease, are becoming more common. Anywhere from approximately 29% to 71% of EHM cases are euthanized due to the severity of their disease.
Despite the fact that EHV-1 vaccinations are often required by barns, shows or racetracks, there are no vaccines that can prevent EHM although preliminary research has suggested the modified live vaccine, Rhinomune, may be somewhat better than inactivated vaccines. The good news is that state officials do a spectacular job of controlling spread of the virus when a case has been reported.
Your best protection is strong biosecurity measures such as:
- Isolate new horses on the premises for at least 2 weeks
- Pick boarding facilities with little to no horse movement if your horse rarely travels off the farm
- Always thoroughly strip and disinfect stalls before using them the first time, at home and when away
- Disinfect trailers between trips
- Bring your own buckets, bedding, pitchforks, etc when you travel
- Use a small mesh wire muzzle or completely sealed muzzle when using turnout facilities or temporary holding stalls away from home to prevent contact of the muzzle or tongue with surfaces
- Do not allow direct muzzle contact with other horses away from home
The individual’s best defense is a strong immune system and antioxidant defenses. This must start with a quality, well balanced diet with adequate protein. Pay attention to the antioxidant and homeostatic nutrients often low in unsupplemented diets such as selenium, zinc and copper as well as vitamin E and omega-3 fatty acids if not on fresh pasture. Avoid added iron, and minimal to no manganese is needed in most areas. Lysine is also very important for immune function in general and because a low lysine level facilitates growth of EHV-1. Supplement 10 grams/day routinely, with a second or third dose daily if the horse has been exposed.
Exposed or symptomatic horses may also benefit from higher than normal doses of vitamin E to help them maintain normal levels of this key antioxidant in the nervous system. Plant based antioxidants, particularly resveratrol from grapes and citrus bioflavonoids, support antioxidant activity and structural integrity in blood vessels.
EHM is not preventable by vaccination but you can limit exposure by good biosecurity measures. As with any infection, a robust immune system will limit the activity of the organism in the horse’s body. If your horse does get infected, good antioxidant levels will help control damage and sound nutrition contributes greatly to recovery.
Eleanor Kellon, VMD