Two horses touching noses

Limiting the Spread of Equine Herpesvirus-1

Samantha Shields Animal Health, Livestock diseases Leave a Comment

Both voluntary and official quarantines are used to limit the spread of equine herpesvirus-1 (EHV-1). For a month beginning the week prior to Thanksgiving 2019, both types of movement restriction were implemented in several parts of the country. Case reports can be found on the disease outbreak alert page maintained by the Equine Disease Communication Center (EDCC).

The Impacts of EHVs on Horses

Equine herpesviruses are associated with respiratory disease, abortions, neonatal death, and neurologic disease in horses. Because of these different manifestations of disease, the virus is also referred to as equine rhinopneumonitis (because it attacks the linings of the nose and lungs) or viral abortion (for causing abortion). The neurologic disease associated with EHV-1 infection is called equine herpesvirus myeloencephalopathy (EHM). The neurologic form is the most devastating because affected horses are often euthanized. However, any type of infection with EHV may lead to movement restrictions.

The increasing number of outbreaks with high fatality rates and the identification of more virulent strains of EHV-1 in the latter half of the 2000s led to considering EHM as a potentially emerging  disease (AVMA, 2007). In every state, the neurologic form of EHV-1 is reportable[JMS2]  to the state animal health authority (EDCC). When a quarantine is placed on a premises, release typically requires all horses test negative for the virus twice a week apart and a full 14 days has passed since the last exposure to an infected horse with no clinical signs observed.

Transmission and Clinical Signs of EHV-1

Older horses are frequently infected with equine herpesviruses without showing signs of illness, but can still transmit the virus to other horses. Horses can become latent carriers that have not fully cleared the virus after infection and are not showing signs of illness. In fact, 80 to 85 percent of the global equine population is latently infected with EHV-1 or EHV-4 (CFSPH, 2014). Reactivation of the virus by stresses such as transportation or illness can lead to viremia meaning the virus can be found in the bloodstream. A horse that is viremic after being latent or after an initial infection can have an abortion or develop neurologic signs.

Transmission of EHV-1 in Horses


The clinical signs associated with EHV infection include (AAEP, 2017):

  • Fever
  • Nasal discharge
  • Lethargy
  • Lack of coordination
  • Difficulty urinating
  • Hind limb weakness
  • Loss of tail tone
  • Head tilt
  • Leaning
  • Inability to rise

EHV-1 has an incubation period of up to 14 days, but usually disease will be evident within 8 to 12 days of exposure. Nose-to-nose contact spreads EHV-1 through nasal and respiratory secretions. It can also be spread through fomites, which are objects that came into contact with infected sources. These objects can include hands, clothes, equipment, vehicles, and feed and water buckets. The virus can survive in the environment up to seven days.

At present, there is no specific treatment for EHM; only supportive care can be given. However, there is a novel therapy being studied involving heparin (Bentz, 2018). Horses that are down are usually euthanized. Vaccination is not guaranteed to protect against EHM, although it may reduce shedding. So how can EHV-1 be prevented?

Preventing EHV-1 Infection

The best way to prevent EHV-1 is to actively implement biosecurity measures. The following recommendations can reduce the chance of your horse acquiring EHV infection or many other diseases (Munson, 2019):

  • Isolate animals upon returning to a premises after traveling.
  • Disinfect and sanitize vehicles and equipment used off the premises.
  • Never share equipment between farms without sanitizing.
  • Start with healthy horses and end with ill and isolated horses when completing daily chores and feedings.

And, of course, always monitor your horse for clinical signs of disease.

Through biosecurity practices and movement restrictions, the spread of EHV-1 can be minimized. Reporting any clinical signs or potential contact with an infected animal to a veterinarian can increase awareness of the disease. Increased awareness can allow producers to take initiative and protect the horses on their premises.


Nevada Department of Agriculture. [JMS3] May 2019. All EHV-1 Quarantines and Restricted Travel Recommendations Lifted. Retrieved from

American Association of Equine Practitioners (AAEP). 2017. FAQ: Equine Herpesvirus (EHV). Retrieved from

United States Department of Agriculture, Animal and Plant Health Inspection Service (USDA, APHIS). December 2018. Equine Herpesvirus (EHV). Retrieved from

Bentz, Amy I. April 8, 2018. A promising novel treatment for equine herpesvirus myeloencephalopathy, American Veterinarian. Retrieved from

Equine Disease Communication Center (n.d.) Reportable Diseases. Retrieved from

American Veterinary Medical Association (AVMA). March 1, 2007. Neurologic form of EHV-1 a potentially emerging disease.  JAVMA News. Retrieved from

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About the Author
Samantha Shields

Samantha Shields

Samantha Shields is a junior undergraduate student at the University of Vermont, studying for a Bachelor of Science in Biological Science, and a minor in Animal Science. She worked with ADBCAP Director Julie M. Smith, DVM, PhD, as an online outreach assistant intern. Samantha assisted with content development for the Healthy Farms Healthy Agriculture website by evaluating, summarizing, and presenting information about protecting animal health. At the University of Vermont, she is an active member of many programs including RALLYTHON, University of Vermont Program Board, Campus Recreation, and the Women’s Club Hockey. Samantha is interested in pursuing the field of epidemiology after graduation. She has a particular interest in the research of different factors that result in diseases, as well as public health emergency planning and response.

About the Editor

Joanna Cummings

Joanna Cummings received a Bachelor of Science in Horticulture from The Pennsylvania State University, with a specialization in vegetable crop and greenhouse production. At PSU, she worked for the Professor of Plant Nutrition as a research technician on no-till vegetable crop experiments at the horticulture research facility, and as a greenhouse assistant in the All-American Selections Research Gardens. Her career in the agriculture industry includes work on dairy and vegetable farms, and as a greenhouse manager, estate gardener, landscaper and market garden entrepreneur. Joanna transitioned into the communications field after receiving a Master of Science in Environmental Studies, with a major in Communications, from Antioch University New England. At Antioch she worked as a field botany laboratory teaching assistant and manager of the herbarium. Joanna’s communications work experience includes agriculture education and outreach coordinator, marketing manager, director of communications, public information officer, webmaster, training program manager and project manager for nonprofit, government, academic and commercial organizations. She is currently working with Animal Disease Biosecurity Coordinated Agricultural Project (ADBCAP) Director Julie M. Smith, DVM, PhD, as a communications professional in the University of Vermont Animal and Veterinary Sciences Department. She is also the webmaster for the Healthy Farms Healthy Agriculture website.

About the Editor

Dr. Julie Smith

Julie Smith DVM, PhD, is a research associate professor at the University of Vermont. Julie received her B.S. in Biological Sciences, D.V.M., and Ph.D. in Animal Nutrition at Cornell University. Since joining the Department of Animal and Veterinary Sciences in 2002, she has applied her veterinary background to programs in the areas of herd health, calf and heifer management, and agricultural emergency management. She is responsible for teaching the undergraduate Animal Welfare class required of majors in her department. Julie has conducted trainings for Extension educators, livestock producers, and community members on the risks posed by a range of animal diseases, whether they already exist in the United States, exist outside of the United States, or pose a risk to both animal and human health. In all cases, she emphasizes the importance of awareness and prevention. As a veterinarian and spouse of a dairy farmer, Julie is well aware of the animal health and well-being concerns of dairy animals. She is currently leading the Animal Disease Biosecurity Coordinated Agricultural Project (ADBCAP), a multi-species, multi-state project looking at the human behavioral aspects of implementing practices to protect animal health and food security.


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