Equine Herpesvirus Myeloencephalopathy Outbreak Update

On May 16, the American Association of Equine Practitioners (AAEP) notified its members of an outbreak of equine herpesvirus myeloencephalopathy (EHM) in the U.S. and Canada. According to the AAEP, the outbreak “appears related to initial cases” at the National Cutting Horse Association (NCHA) Western National Championships, held April 29 through May 8 in Odgen, Utah.  Horses at this event may have been exposed to the equine herpesvirus-1 (EHV-1) that causes EHM, and could spread the infection to other horses.  Horses from 18 states attended the Championship and were potentially exposed.

The virus can be spread through contact with infected horses (particularly their nasal secretions) and contaminated people and equipment, as well as via aerosol transmission. Therefore, the key aspect of controlling this outbreak is proper attention to biosecurity measures, including quarantine of exposed or potentially exposed horses. To limit the potential spread of EHV-1 and to alleviate horse owners’ fears of exposure, a number of events in many states have been postponed or canceled. The Washington State University College of Veterinary Medicine’s veterinary teaching hospital enacted a voluntary quarantine and is closed to non-emergency equine and camelid cases as of May 13 after discovering that a hospitalized patient had competed at the NCHA Western National Championships; the horse tested positive for EHV-1 but was not showing signs of disease.  In addition, the hospital at the Colorado State University College of Veterinary Medicine was closed to non-emergency equine and camelid cases on May 16 as a preventive measure after EHV-1 infection was confirmed in two Colorado horses that had attended the NCHA Western National Championships.

The AAEP has developed resources about EHV-1, EHM and the current outbreak, including updates and a FAQ for veterinarians and FAQs and resources for horse owners.

Veterinarians are urged to contact their state veterinarian or state/provincial animal health department to report suspect cases of EHV-1 or EHM. On May 19, the AAEP announced that it is working with the American Horse Council and the U.S. Department of Agriculture (USDA) to collect data on affected horses and to coordinate communications about the outbreak.

The USDA has also provided information on the outbreak, including situation reports that provide state-based details on the number of horses exposed, suspected and confirmed cases, and deaths. As of May 19, a total of 997 horses were reportedly exposed to EHV-1 (either at the NCHA event or through contact with horses exposed at the event); 21 cases of EHV-1 infection and 12 cases of EHM have been confirmed; and 7 horses died or were euthanized.

Below are some messages you can use when talking to clients about this outbreak:

  • Equine herpesvirus myeloencephalopathy (EHM) is caused by equine herpesvirus-1 (EHV-1). The virus most commonly causes respiratory infection, and not all infected horses will develop EHM;
  • EHV-1 is a normally occurring virus found in the equine population; this outbreak is not being caused by a new virus or a new strain of a virus;
  • Signs of EHM in horses may include nasal discharge, incoordination, weakness of the rear limbs and hind end, lethargy, urine dribbling, and decreased tail tone;
  • Llamas and alpacas can also be infected with EHV-1 and may develop neurologic disease;
  • Currently, there is no equine vaccine that has a label claim for protection against EHM;
  • There is no specific treatment that has been proven effective for EHM;
  • If your horse has potentially been exposed to an infected horse (or through contact with people or equipment that have been in contact with an infected horse), or is showing signs that could indicate EHM, quarantine your horse and other potentially exposed horses and contact your veterinarian.

8 thoughts on “Equine Herpesvirus Myeloencephalopathy Outbreak Update

  1. @Lori Edwards
    Good question, Lori! We at the AVMA don’t have an answer to it and haven’t seen any published research yet, but someone with the AAEP might know more. There’s been a lot of research on EHV-1, and particular interest in the neurologic form because it can be so devastating. We hope to see some good news coming out of that research.

  2. I have recently heard that treating Equine Herpes with Lymphocyte T-Cell Immunomodulator (LTCI-originally for feline leukemia) has shown to be of significant help in treating horses with Herpes and other viral mediated infections….any comment?

  3. thankyou for keeping us so informed …. us out here in the country have a need to know….

  4. @wendy brodhead
    Thanks for the reply, Wendy, and your attention to your horses’ health. One of your best sources of info about horse health is the AAEP’s website, http://www.aaep.org. They have a lot of good information specifically for horse owners and you can sign up for their email newsletters. They also have a partnership with The Horse (thehorse.com), which also sends out some good email newsletters. Good luck with your horses!

  5. Hello, Please keep me informed of all outbreaks of any kind. We here at Shylah Farm want to be aware for anything that affects our horses well being. Thank you and God Bless Wendy B

  6. If you’re an AVMA member and you opted in to receive email alerts about public health or animal health crises in which veterinarians play a role, you’ve either already received or will soon receive an email alert about this. If you aren’t signed up to receive these alerts, you can sign up for newsletters and alerts by going to “My AVMA” and clicking on the “Subscribe to receive veterinary newsletters and updates” link.