A Guide To Understanding the

Neurologic Form of EHV Infection

Equine

Herpesvirus (EHV)

Myeloencephalopathy
What is EHM?

Equine herpesvirus myeloencephalopathy 

(EHM) is another name for the neurologic disease 

associated with equine herpesvirus (EHV) infections. Neurological signs appear as a result of 

damage to blood vessels in the brain and spinal 

cord associated with EHV infection. Interference 

with the blood supply leads to tissue damage and 

a subsequent loss in normal function of areas in 

the brain and spinal cord. 

What are the equine herpesviruses? 

To date, nine EHVs have been identified worldwide. Three of these—EHV-1, EHV-3, and EHV4—pose the most serious health risks for domesticated horses and can have significant economic 

impacts on the U.S. equine industry.

•	EHV-1: Can cause four manifestations of 

disease in horses, including a neurological form, respiratory disease, abortion, and 

neonatal death. EHM is most often due to 

mutant or neuropathogenic strains of EHV-1, 

so called because of a particular mutation in 

the genome.

•	EHV-3: Causes a venereal disease called 

equine coital exanthema that affects the 

external genitalia.

•	EHV-4: Causes a nonfatal upper respiratory 

tract disease in foals and is uncommonly 

associated with abortion and rarely with

neurologic disease.

Neurological signs appear

as a result of damage to

blood vessels in the brain

and spinal cord associated

with EHV infection.
How common are EHV-1 infections?

By 2 years of age, almost all horses have been 

infected with EHV-1. The initial exposure generally occurs in foals from contact with their dams. 

The virus can then become latent, or inactive, in 

the horse’s body, setting up a carrier state that 

is life-long. Horses of any age that are carriers of 

EHV-1 do not show any external signs of disease 

when the virus is in a latent form. The virus can 

be reactivated during times of stress, such as 

strenuous exercise, long-distance transport, or at 

weaning.

The transmission of EHV-1 occurs after an exposure to an 

adult horse or foal with an active EHV-1 viral infection. This 

exposure generally occurs via respiratory shedding of the 

virus. The infected adult horse or foal may or may not be 

exhibiting clinical signs of disease.

Transmission of EHV-1 

Latently

Infected Horse Stress

Latency Established Virus Reactivation

Infection of Foal/Adult Horse 

Infection of Other Foals/Adult Horses 
Why should I be concerned

about EHV-1?

EHV-1 is the primary cause of EHM. In recent 

years, there has been a marked increase in the 

number of EHV-1 cases, especially EHM, reported 

in the United States. There have also been several 

outbreaks of EHM at large horse facilities and 

events—at racetracks, horse show grounds, 

veterinary clinics, and boarding stables. The large 

number of horses that can be exposed on such 

premises and the serious nature of the disease 

have caused significant concern within the animal 

health community and the U.S. horse industry. 

Furthermore, these recent occurrences of 

EHM appear to meet the criteria of an emerging 

disease. A disease is considered to be “emerging” 

when it satisfies at least one of three criteria: 

1.The disease is identified for the first time in a 

region or country; 

2.A disease changes in severity, type of animal 

that can be infected, or other changes in 

pathogen behavior; or 

3.There is a change in geographic range of a 

disease or in its incidence within a range. 

EHM likely meets the second criterion of an 

emerging disease, as the recent EHV-1 outbreaks 

seem to be associated with a change in the severity and behavior of the virus. It is possible that the 

reporting of EHV-1 cases has increased, rather 

than an actual increase in the number and severity of cases; however, more data are needed to 

make such a distinction.

The virus can be reactivated 

during times of stress, such 

as strenuous exercise, longdistance transport, or at 

weaning.
How is the virus spread?

The most common way for EHV-1 to spread 

is by direct horse-to-horse contact. This virus 

is shed from infected horses via the respiratory 

tract or through direct or indirect contact with 

an infected aborted fetus and fetal membranes. 

Horses may appear to be perfectly healthy yet 

spread the virus via the secretions from their 

nostrils. 

It is important to realize that EHV-1 can also 

be spread indirectly through contact with physical objects contaminated with infectious virus. 

Examples of such objects include:

•	 Tack,

•	 Wipe rags or other grooming equipment,

•	 Feed and water buckets, and

•	 People’s hands or clothing.

The air around a horse that is shedding the 

virus can also be contaminated with infectious 

virus. Although we know that the virus can 

become airborne, it is difficult to establish the distance the virus can spread in this manner under 

typical horse management and environmental 

conditions.
When should I suspect my horse might 

have EHM?

Signs of EHM include:

•	 Fever preceding neurologic signs (either in a

horse diagnosed with EHM or in horses that 

have been exposed to a horse diagnosed with 

EHM),

•	 Decreased coordination,

•	 Urine dribbling,

•	 Loss of tail tone,

•	 Hind limb weakness,

•	 Leaning against a wall or fence to maintain

balance,

•	 Lethargy, and

•	 Inability to rise.

Horses may appear to be

perfectly healthy yet spread 

the virus via the secretions 

from their nostrils.
How is the disease diagnosed?

Diagnostics your veterinarian might perform for

a live horse include:

•	 Nasal swab collection for laboratory examination and detection of virus by polymerase 

chain reaction (PCR) assay or by virus isolation; and

•	 Blood collection to detect the virus by PCR

assay or by virus isolation. Blood samples 

should be collected 2 to 3 weeks apart for 

levels of antibodies specific to EHV-1.

Ideally, your veterinarian will collect both nasal 

swab and blood samples to optimize arriving at a 

diagnosis.

Diagnostics your veterinarian might perform for

a deceased horse include:

•	 Necropsy examination of a horse with neurologic disease that dies or has to be euthanized, or necropsy examination of an aborted 

fetus/fetal membranes. Necropsy provides an 

important means of confirming the presence 

of EHV-1 disease.

Is there a treatment for EHM?

The standard form of care for EHM is primarily 

supportive. Treatments may include intravenous 

fluids or anti-inflammatory drugs. Antibiotics may 

be used to treat a secondary bacterial infection if 

one develops; however, antibiotics have no effect 

on the equine herpesvirus itself. Antiviral drugs 

have also been used to treat EHM cases. Research

on the efficacy of these drugs, their cost effectiveness, and the optimal dosing regimen for EHM is 

still underway.
How can I prevent EHV-1, the primary 

cause of EHM, from spreading to other 

horses?

There are many steps you can take to help

prevent the spread of EHV-1.

•	 Stop horse movement

if your animals may be 

infected with EHV1. This is the most 

important first step 

horse owners can take. 

Horses should neither 

enter nor leave a premises where EHM has 

been diagnosed until 

cleared by the veterinarian. 

•	 Do not allow horses

exposed to EHM 

case(s) to have contact 

with unexposed horses 

on the premises.

•	 Isolate sick horses.

Horses that have 

aborted or shown signs 

of fever, respiratory 

disease, or neurologic 

disease should be 

separated from healthy horses. Ideally, the 

sick horse(s) should be moved into a separate building or paddock on the premises, or 

be transported to a veterinary hospital with an 

isolation facility.

•	 Do not share equipment among horses on the

facility. Since this virus can be spread from 

horse to horse via contaminated objects such 

as water/feed buckets or bridles, equipment 
should not be shared among horses.

•	 Practice proper biosecurity measures to

prevent people from spreading the virus. 

Since people can transfer this virus from 

horse to horse via their hands and clothing, 

personnel should wash their hands after handling one horse and before handling another. 

They should also change their clothes and 

footwear after working with a sick horse. 

Optimally, a person who takes care of a sick 

horse should not work with healthy horses. 

When this is not practical, healthy horses

should be handled first and sick horses last. 

Wearing gloves and using disinfectant to

sanitize footwear can also help minimize the 

risk of people spreading the virus between 

animals.
Can I prevent EHM with vaccines?

Vaccines exist to control the respiratory and 

abortion manifestations of EHV-1; however, the 

currently licensed vaccines are not labeled for 

the prevention of EHM. University and private 

researchers are looking into several existing vaccines to determine if they protect against EHM. 

New vaccines for EHM are also being studied.

Useful Resources

For more information related to EHV, EHM, and 

biosecurity, please refer to the following documents and Web sites:

•	 United States Department of Agriculture

(USDA)

USDA:APHIS:VS:CEAH. 2008. Equine Herpesvirus Myeloencephalopathy: Mitigation 

Experiences, Lessons Learned, and Future

Needs. N#522.0708. http://www.aphis.usda.

gov/vs/nahss/equine/

•	 USDA:APHIS:VS:CEAH:CEI. Emerging

Animal Disease Notice. 2007. Equine Herpes

Virus Myeloencephalopathy: A Potentially

Emerging Disease.

http://www.aphis.usda.gov/vs/ceah/cei/taf/

emerging diseasenotice.files/ehv.pdf

•	 USDA:APHIS. Biosecurity—the Key to Keeping Your Horses Healthy.

http://www.aphis.usda.gov/publications/

animal_health/content/printable_version/

HorseBioSecurity_final.pdf

Since this virus can be spread 

from horse to horse via contaminated objects such as 

water/feed buckets or bridles, 

equipment should not be 

shared among horses.
•	 University of California Center for Equine

Health (CEH)

http://www.vetmed.ucdavis.edu/ceh/topics.htm

•	 American Association of Equine Practitioners

(AAEP)

http://www.aaep.org/control_guidelines_nonmember.htm

http://www.aaep.org/vaccination_guidelines.

htm

•	 Iowa State University Biological Risk Management (BRM)

http://www.cfsph.iastate.edu/BRM/equineresources.htm

http://www.cfsph.iastate.edu/BRM/disinfects.

htm

For More Information

To find out more about EHM, please contact:

USDA–APHIS-VS-CEAH

NRRC Building B, M.S. 2E7

2150 Centre Avenue

Fort Collins, CO 80526-8117

(970) 494-7000

E-mail: NAHMS@aphis.usda.gov

http://nahms.aphis.usda.gov

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