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Understanding Equine Herpes Virus

March 02, 2022
Understanding Equine Herpes Virus

Equine Herpes Virus has reared its ugly head recently in California and continues to spread throughout the state, posing extreme health risks to horses. Great efforts are being made to protect horses and prevent further spread, and understanding this virus and how it is transmitted is the first step horse owners can take to protect their horses.

Update as of March 21st per the CDFA: No additional cases of EHV or EHM have been confirmed since March 21, 2022. All USEF Horse Shows in California have been postponed through March 31st in an effort to stop the spread of EHV and EHM according to the CDFA's recommendations. Once horse shows resume, USEF recommends the following requirements and best practices for biosecurity are implemented on showgrounds:

  • Check and record temperatures twice daily and post on stall door
  • Any suspicion of illness in horses, including a temperature over 101.5°F, should be immediately reported to the show office and veterinarian.
  • Immediately isolate and/or quarantine any horse exhibiting symptoms of illness
  • DO NOT travel to another competition for 14 days
  • Avoid animal-to-animal contact
  • Do not share equipment between horses. IF YOU MUST SHARE, scrub and clean equipment with detergent and dry completely between shared use.
  • When filling water buckets, DO NOT dip the end of the hose in each bucket. Hold the hose above the water when filling.
  • Wash/sanitize hands thoroughly before and after direct horse contact
  • Limit human-horse contact
  • Because humans can be a means of disease transmission, avoid moving between barns unless absolutely necessary

What is Equine Herpes Virus?

Equine Herpes Virus (EHV) is a highly contagious Alphaherpesviridae with 9 known variants. EHV-1 is one of three of these variants that is most commonly found in the US, and is the specific variant that currently plagues California.

EHV-1 can cause upper respiratory disease, neurological disease, abortions, and neonatal death. The neurologic disease syndrome caused by EHV-1 is known as Equine Herpes Myeloencephalopathy (EHM), which is a result of damaged blood vessels in the brain and spinal cord that lead to neurologic clinical signs.

While all equids are susceptible to EHV-1, mules are asymptomatic shedders. This means that mules often do not show clinical signs of infection but can still transmit the virus. For this reason, it is important to separate mules and horses on the same property.


How is EHV-1 spread?

EHV-1 is spread through direct horse-to-horse contact via nasal secretions. The virus can also be transmitted through indirect contact via contaminated objects such as clothing, equipment, stable supplies, and even human hands. EHV-1 can also become airborne for short distances, which is why isolation of infected animals is imperative to preventing infection of healthy animals.

EHV-1 can live on contaminated surfaces such as hands and clothes for 4-6 hours and is viable for 7-10 days in nasal discharge, requiring extensive cleaning and quarantine practices to prevent its spread.

The California Department of Food and Agriculture defines an exposed horse as “a horse which has been in close contact with a confirmed case of the Equine Herpes Virus (EHV-1) within the last 14 days”. Any exposed horses should be monitored closely and handlers should take serious biosecurity measures to prevent potential transmission.

What are the clinical signs of EHV-1 and EHM?

EHV-1 infection can cause a range of clinical symptoms including fever, nasal discharge and cough, lethargy or depression, difficulty urinating, loss of tail tone, and swelling of the legs. EHV-1 and EHM can both present neurologic symptoms such as stumbling or weakness in the hind limbs. These symptoms can rapidly progress to all of the limbs and make it difficult for horses to lie down or stand up.

Fever is the most common symptom of EHV-1 and EHM. If your horse has been exposed or in close contact with an infected horse it is important to take their rectal temperature TWICE per day. If your horse’s temperature rises above 101.5°F, isolate your horse immediately and contact your veterinarian. If your horse is presenting any clinical signs of infection, your veterinarian should obtain both nasal swabs and blood samples to be tested for infection.

How can you prevent EHV-1 and EHM?

Vaccines are available to prevent the respiratory and abortion forms of EHV; however, there are no vaccines availabe to protect against the neurologic form, EHM.

Horses with compromised immune systems are most likely to become infected and experience clinical signs, regardless of vaccination status.

Vaccinated horses can still contract EHV-1 but the vaccine can reduce the severity of the clinical signs and possibly reduce transmissibility. The American Association of Equine Practitioners (AAEP) recommends a 6 month vaccination schedule for EHV, and some vaccine suppliers even recommend vaccination every 3 months. All recognized USEF competitions require an EHV vaccine within 6 months of attending the horse show for all competing horses. Until now, non-showing horses were not required to provide proof of vaccination; however, the extreme situation in California is forcing show management companies to either require proof of vaccination for ALL horses on the show grounds or prohibit any non-competing horses from being brought onto the property.

Is there a treatment for EHV and EHM?

Supportive treatment for any infected horses includes the administering of IV fluids, anti-inflammatory drugs, antiviral drugs and other supportive therapies to reduce fever and other clinical symptoms.

At owner and trainer discretion, immune boosting supplements can be administered if risk of exposure is suspected.

T.H.E. Equine Edge’s EHV supplement is one option horse owners can introduce to horses as a preventative measure or as a supportive treatment to infected horses. Their all natural formula is made up of a combination of L-Lysine, Echinacea and Garlic to help provide a natural relief of Equine Herpes Virus symptoms affecting their respiratory system and overall immune health when under stress or weakened immune system. While this supplement does not aid in the treatment of neurologic symptoms, early action and implementation can help prevent EHV-1 infection or the development of EHM once infected. Always consult with your veterinarian before adding any new supplements to your horse’s diet, especially if they are showing clinical signs of infection or have confirmed infection.

What biosecurity measures should be implemented to prevent the spread of EHV-1?

If a horse is suspected to be infected, has been exposed to other infected animals, or is confirmed positive, ISOLATE IMMEDIATELY. All isolated horses should be cared for after healthy horses, and handlers need to follow strict sanitization protocol. When handling isolated horses, wear protective clothing such as coveralls, rubber or plastic boots, and rubber gloves so that all apparel can either be disposed of or properly sanitized before coming into contact with other horses. Maintain disinfectant-saturated foot mats or foot baths at the entry and exit of the isolation area to allow for immediate sanitization of footwear. Be sure to change the disinfectant solution frequently as the presence of organic matter such as soil or manure may make the solution ineffective. All bleach and alcohol disinfectants are inactivated by the presence of this matter.

Other disinfectants are proven to be more effective as they are not deactivated by organic matter. These products include Intervention ®, 1 Stroke Environ®, SynPhenol-3®, or Virkon®. It is still good practice to regularly change any sanitizing solution regardless of proclaimed efficacy, and be sure to follow all product manufacturer guidelines when using.

Lastly, be sure to use SEPARATE grooming, feeding, and handling equipment for each horse to prevent possible transmission.

To stay up to date on the status of EHV-1 and EHM in California, refer to the The California Department of Food and Agriculture website.

All information in this article is based off of the The California Department of Food and Agriculture website as well as the U.S. Department of Agriculture Animal and Plant Health Inspection Service website.