Equine Herpesvirus 1: Virus, Disease, Economics and Containment

Equine herpesvirus 1 (also known as Equine Herpes Virus 1, or EHV-1) is similar to other strains of the herpes virus in structure (double stranded DNA virus with a lipid-envelope, approximately 150 – 250 nm size) and in its ability to lie dormant in a population and be reactivated, thus becoming transmissible again after a period of time in a host. EHV-1 is unique in its ability to attack multiple different organ systems of the horse. The symptoms of infection can manifest as respiratory disease, abortion of fetuses, infection of newborn foals, and severe neurological disease. Because the virus can be shed by “silent carriers” - asymptomatic infected horses - and because of its effect on multiple organ systems, EHV-1 poses an economic threat to multiple facets of Equine- based business1.

In its mildest form, EHV-1 can cause acute upper respiratory infections of young and mature horses. The infected animals are able to shed the virus to other horses via nasal discharge. These infections are usually resolved with anti-virals and supportive therapy. During times of stress, the dormant virus may be reactivated in infected horses and spread to other horses through contact with nasal secretions1.

When the virus is passed to a pregnant mare, it can cause late-term abortion of the fetus, which poses the biggest economic threat to the Equine-based industry. Foals that are carried to full term, but are infected with the EHV-1 virus, are born with severe clinical signs of the virus, and generally die within a few days, even with extensive supportive care1.

Equine Herpes Myeloencephalopathy (EHM) or neurological Equine Herpes Virus-1 is the most severe clinical form of the virus. EHM is the form of the virus involved in the 2011 EHV-1 outbreak in Utah2. The symptoms of EHM include severe and rapid degeneration of hind-limb function, gait abnormalities, inability to stand, and paralysis. This form of EHV-1 is resistant to vaccination and generally results in death or euthanasia of the infected horse3.

Currently, EHV-1 is deeply rooted in the world’s horse population in the form of silent carriers of the virus. Because the virus can remain dormant and continue to reactivate over the lifetime of the horse, it has an unlimited potential to infect additional horses, and has a huge impact on equine populations in all countries and regions of the world. The current outbreaks of EHV-1 in several western U.S. states can be traced back to the National Cutting Horse Association’s Western National Championships in Ogden, Utah where 54 California horses were confirmed to have been exposed to the virus. As a result, many western states are advising the cancellation of local and national horse shows for an indefinite portion of the show season. The loss of valuable horses and revenue from these major shows has the ability to devastate the horse showing industry. Outbreaks in the racehorse populations, similarly, could shut down the multi-billion dollar racing industry1.

Containment practices of EHV-1 are similar to the containment of other viral pathogens in horse populations. Because EHM is resistant to vaccination, immunization is used solely to protect horses from the other forms of EHV-1. However, vaccinations are not effective for horses already confirmed to be exposed to the virus. The quarantine period for infected or suspected at-risk horses is 28 days. Sound herd management is crucial to stop the spread of the disease. If possible, infected horses or suspected at-risk horses should be quarantined on a property separate from healthy or unexposed horses. Any horses coming onto the property should be quarantined away from healthy herds to confirm their health status4.

All equipment used for the infected horse, including cleaning equipment, tack, rags, feed and water tubs should be kept for use solely for that horse. Appropriate biosecurity measures should be taken when handling an infected horse. Caretakers should care for healthy horses first and sick horses last, using primary protective barriers such as gloves, gowns, and rubber boots that are discarded or disinfected before leaving the quarantine area to prevent the spread of the virus through contact with infected clothing, hands, or other objects. Footbaths with disinfectant should be used at the entrances to all quarantine areas to disinfect footwear and ensure that the virus is not spread to other areas of the facility, and all caretakers should use disinfectant on their hands prior to handling another horse. All facility surfaces should be cleaned of all organic matter and then disinfected prior to use by a healthy horse4.

EHV-1 can live in the environment under ideal conditions for up to a month, so it is essential to properly disinfect all surfaces that the infected horse has come in contact with. Several disinfectants such as bleach, phenolics, quaternary ammonium compounds, and accelerated hydrogen peroxide products, etc. are effective against EHV-1, but some are deactivated by the presence of organic matter found in a farm environment, such as manure or soil. Therefore, if those agents are used to disinfect contaminated articles or surfaces in the barn, all organic matter must be removed using a cleaning agent4. Other products are being developed specifically for use around horses.

As a result of the EHV-1 outbreaks, businesses that manufacture farm and veterinary disinfectant/cleaning products have a need to validate product efficacy against EHV-1. Microbac Laboratories, Inc., offers efficacy testing for antimicrobial and disinfectant products against a wide range of microbial and viral agents, including EHV-1. 



  1. "Diseases of the Horse Caused by Equine Herpesvirus Type 1 (EHV-1) – A Bibliography of Source Materials." Gluck Equine Research Center Department of Veterinary Science. Web. 05 June 2011. http://www.ca.uky.edu/gluck/BiblioEHV1.asp.
  2. "2011 Outbreak of Equine Herpes Virus-1 (EHV-1)." California Department of Food and Agriculture. 02 June 2011. Web. 05 July 2011. http://www.cdfa.ca.gov/AHFSS/Animal_Health/pdfs/EHV-1QA.pdf.
  3. "Equine Herpesvirus 1 (EHV-1) Myeloencephalopathy - A Guide to Understanding the Neurologial Form of EHV Infection." USDA - Animal and Plant Health Inspection Service. United States Department of Agriculture, 2008. Web. 5 June 2011.
  4. http://www.aphis.usda.gov/vs/nahss/equine/ehv/equine_herpesvirus_brochure_20 09.pdf.
  5. "Biosecurity Guidelines." American Association of Equine Practitioners, 2006. Web. 05 June 2011.
  6. http://www.aaep.org/pdfs/control_guidelines/Biosecurity_instructions%201.pdf.


Authored by Lindsey Davis, Associate Scientist - Microbac Laboratories, Inc.