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Equine Herpes Virus Type 1 (EHV-1) Induced Myeloencephalopathy (EHM) Risk Testing Released by Etalon

Equine herpesviruses are DNA viruses that are found in most horses all over the world. It is believed that almost all horses have been infected with the viruses and have, most of the time, no serious side effects. Recently, there has been a trend of increasing numbers of outbreaks of a devastating form of EHV-1. Following infection, some horses then suffer Equine Herpesvirus Myeloencephalopathy (EHM), which is accompanied by serious and sometimes fatal neurological effects. Researchers have identified a genetic variant that correlates to a 1.43x increased risk of developing that neurological form (Equine Herpesvirus Myeloencephalopathy, EHM) if infected by Equine Herpes Virus type 1 (EHV-1). Testing for this genetic variant is now available at Explore how to better understand your horse’s risk of developing the neurological form (Equine Herpesvirus Myeloencephalopathy, EHM) if infected by Equine Herpes Virus type 1 (EHV-1).

Equine Herpesvirus Myeloencephalopathy (EHM) risk

Gene or region: intron of TSPAN9

Reference allele: G

Mutant allele: A

Affected Breeds: Diverse breeds from the US, Canada and Europe

Research Confidence: Discovery; findings require further validation & larger study is in progress

What will my horse’s test results look like?

G/G or G/A EHM Higher Risk - Two or one Equine Herpesvirus Myeloencephalopathy risk (EHMR) variants detected. Horse has 1.43x increased risk of developing the neurological form (Equine Herpesvirus Myeloencephalopathy, EHM) if infected by Equine Herpes Virus type 1 (EHV-1).


EHM Lower Risk - No Equine Herpesvirus Myeloencephalopathy risk (EHMR) variants detected. Horse has a lower risk of developing the neurological form (Equine Herpesvirus Myeloencephalopathy, EHM) if infected by Equine Herpes Virus type 1 (EHV-1).

What is Equine Herpes Virus Type 1(EHV-1)?

EHV-1 stands for Equine Herpes Virus type 1. Equine herpesviruses are DNA viruses that are found in horses all over the world, the majority of which will have no serious side effects. EHV-1 can cause four manifestations of disease in horses, including the neurological form, respiratory disease, abortion and neonatal death.

What is Equine Herpesvirus Myeloencephalopathy (EHM)?

Equine Herpesvirus Myeloencephalopathy (EHM) is the neurologic disease associated with Equine Herpes Virus type 1 (EHV-1), representing one outcome of infection. This occurs to a subset of infected horses and its symptoms are painful and sometimes fatal. Neurologic signs result from inflammation of the blood vessels, blood clots, and death of neurologic tissue. EHM can affect one or multiple infected horses in a group.

What are some of the signs of EHV-1?

After infection, the incubation period is typically 4-6 days, but may be as short as 24 hours. EHV-1 typically causes a two-phase fever peaking on day 1 or 2 and again on day 6 or 7. Respiratory infections often present nasal and ocular discharge, and minimal coughing. There may also be some persistent enlargement of lymph nodes under the jaw. The neurologic form of EHV-1, EHM, presents with minimal respiratory signs, and fever (rectal temperature greater than 101.5 degrees F) is considered a warning sign. Sudden neurologic disease progresses rapidly and reaches peak intensity within 24 to 48 hours from onset of initial neurologic signs. Horses may develop EHM even without any preceding fever and/or respiratory signs. Clinical signs of EHM may include:

  • Incoordination

  • Hind limb weakness

  • Loss of tail tone

  • Lethargy

  • Urine dribbling

  • Head tilt

  • Leaning against a fence or wall to maintain balance

  • Inability to rise

How does EHV-1 spread?

EHV-1 is contagious and spread by direct horse-to-horse contact via the respiratory tract through nasal secretions. It is important to know that this virus can also be spread indirectly through contact with physical objects that are contaminated with the virus, including but not limited to: human contaminated hands or clothing, contaminated equipment and tack, contaminated trailers used for transporting horses, contaminated wipe rags or other grooming equipment, or contaminated feed and water buckets. The air around the horse that is shedding the virus can also be contaminated. Although it is known that the virus can be airborne, it is difficult to establish the spread distance under typical horse management and environmental conditions.

What does this variant correlate to? A genome-wide association study (GWAS) investigated host genetic variations associated with EHM, resulting in a variant (BIEC2_946397) within an intron of the tetraspanin 9 (TSPAN9) gene, which is expressed in endothelial cells and platelets, correlated with a recessive protective effect against EHM in EHV-1 affected individuals. Understanding the contribution of host genetic variation to the development of EHM can help with strategies for treating individual cases and managing outbreaks.

How to order testing: all it takes is a hair sample!

1) Log in to your Etalon account or create an account

2) Choose your horse, or "Add" your horse to your account and add Horse name

3) Select “Single Test - Equine Herpesvirus Myeloencephalopathy Risk (EHM) - related to Equine Herpes Virus type-1 (EHV-1)" from the Packages dropdown (scroll down the list)

4) Place your order, and follow the directions for submitting hair sample. 

Please note, we do not send out a kit. After you place your order, you will have the opportunity to print out a pre-filled hair sample submission form with instructions on how to submit a sample for testing.

If you have any additional questions, do not hesitate to reach out to Team Etalon at or by giving us a call at (650) 380-2995.

For additional resources, please visit the Equine Disease Communication Center (EDCC), Equine Herpesvirus Resources via the American Association of Equine Practitioners (AAEP), and the United States Equestrian Federation.


Brosnahan, M. M., Al Abri, M. A., Brooks, S. A., Antczak, D. F., & Osterrieder, N. (2019). Genome-wide association study of equine herpesvirus type 1-induced myeloencephalopathy identifies a significant single nucleotide polymorphism in a platelet-related gene. The Veterinary Journal, 245, 49-54.

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