WSDA Field Veterinarian
Spring is just around the corner and horses are going to be coming together under stressful conditions whether for a weekend trail ride, jackpot roping, or a large, nationally-organized event – prime conditions for equine herpes virus (EHV).
Just this month, a horse in King County tested positive for non-neuropathogenic EHV-1 neurologic strain. Unfortunately, the horse had to be euthanized.
There are multiple strains of EHV-1 in horses. A diagnosis of EHM is made when a horse is infected with EHV-1 and showing neurological signs independent of what strain of the virus the horse is infected with.
To be clear, the horse in Washington was not infected with the strain of virus currently causing an EHM outbreak in California.
The outbreak in California is caused by the neuropathogenic strain of EHV-1 which is more highly contagious and more deadly than the non-neuropathogenic strain.
The horse diagnosed in Washington was infected with the non-neuropathogenic strain which usually causes only mild respiratory diseases. Unfortunately, this horse also developed neurological signs (EHM) and had to be euthanized.
With this recent case and the recent cases in California, it’s a good time to review your knowledge of EHV-1 and equine herpes meyloencephalopathy (EHM), and make sure you have a robust biosecurity program to prevent the spread of disease associated with commingling of horses.
What to watch for
Most horses have been exposed to, or infected with, the equine herpes virus by the time they are two years old.
There are several equine herpes viruses, with EHV-1 and 4 posing the greatest risk to the horse community. Symptoms of EHV-1 infection in horses include:
- respiratory disease (rhinopneumonitis)
- abortions
- neonatal death
- neurological disease
EHV-4 produces a mild to moderate respiratory disease in foals, occasionally causes abortions, and in extremely rare cases develops into EHM.
After an initial EHV-1 infection, the virus remains in the body in its latent form, essentially hiding from the immune system, waiting for the opportunity to revert to its active form. Stress, caused by traveling, training, overcrowding, or competition, is often the trigger for the virus to revert.
Once active, EHV-1 is highly infectious and spreads quickly through direct contact with nasal secretions or aerosol droplets. Direct horse-to-horse contact does not require the horses to have actual physical contact, only direct contact with the nasal discharges of an infected horse.
People can’t be infected by the virus, but they often facilitate its spread through contamination of hands, clothing, equipment, tack, trailers, stalls, feed buckets, water buckets, and many other items. In addition, aerosol droplets can travel several feet to infect other nearby horses.
Once infected with EHV-1, horses will usually exhibit signs of illness within four to six days. Clinical signs range from mild respiratory to severe neurological deficits. Typical signs of respiratory disease include fever, discharge from the eyes and nose, slight cough, depression, going off feed, and swollen lymph nodes. Clinical signs of EHM include fever, depression, hind end weakness and incoordination, loss of tail tone, head tilt, urine dribbling, and may present as down and unable to rise.
EHM cases often have minimal to no respiratory signs. Unlike respiratory disease, from which most horses recover, EHM is a life-threatening disease with no cure. Treatment is limited to supportive care and the prognosis is poor with a fatality rate as high as 30 percent.
Even though EHV-1 is highly contagious and can quickly spread among horses, it doesn’t typically persist long in the environment and can be neutralized efficiently with good hygiene practices. In fact, EHV-1 is estimated to persist in the environment for less than 7 days and no more than 30 days under ideal conditions.
Available vaccines can prevent the respiratory and abortion forms of EHV-I, but none are labeled as preventing EHM. Maintaining a vaccination protocol for the respiratory and abortion forms of EHV-1 decreases viral shedding and may decrease the incidence of EHM.
Strong biosecurity measures, which should be in place even in the absence of an outbreak, are essential for limiting the spread of EHV-1 and EHM.
Protecting your horse
Here are some recommendations to minimize the spread of EHV-1 before, during, and after commingling at a show or event:
- Have a strong biosecurity plan and practice it at all times to prevent the spread of infectious agents.
- Keep horses current on all recommended vaccines and have health papers in order.
- Prevent horse-to-horse contact and provide each horse with its own equipment including tack, grooming equipment, water bucket, feed bucket and all other items. Do not share the equipment with other horses.
- Keep people from touching your horse, especially around the face, nose, and neck. If people do touch your horse (groomers, hair braiders, veterinarians, etc.), insist they first wash their hands with soap and water, or use an alcohol-based hand sanitizer, and again after touching your horse.
- Bring several gallon jugs of your own water to water your horse so you won’t need to use a common hose.
- If a common hose is used for watering, make sure the nozzle and any other part which could touch the water bucket is disinfected. After the hose is disinfected, hold it high above the water bucket, do not touch any part of the bucket with the hose. Do not immerse the hose end into your water bucket when filling it.
- Disinfect your shoes or boots right after the show and before returning to your barn.
- Wash your clothes immediately after attending a show.
- Don’t stable your horse with other horses. If you can, keep your horse in its trailer. If you must stable it, clean, and disinfect the area thoroughly.
- Check your horse’s temperature twice daily for at least two weeks after returning home and monitor them for the respiratory or neurological signs mentioned above. If a fever develops or signs appear, contact your veterinarian immediately and let them know you have recently traveled with your horse, and the particulars of the show
Reportable diseases
EHV-1 and EHM are reportable diseases in the state of Washington. Visit our Reportable Diseases webpage to report cases.
After contacting your veterinarian, it is likely they will take nasal swabs and blood samples to check for EHV-1. It is important to isolate your horse and treat them as if they were infected until laboratory results come in.
Prevention, through good planning and good biosecurity plans, is always better than dealing with an outbreak. The Equine Disease Communication Center (EDCC) at equinediseasecc.org/alerts provides a good online tool for horse owners interested in checking out the current equine disease alerts in the United States and Canada. It’s always a good idea to check before you go.