Identification of Equine Herpes Myeloencephalopathy (EHM) (neurological form of herpes virus infection) occurs across the country every year. Occasionally clusters or outbreaks are identified. The most concerning outbreaks for the horse industry are those which occur in large shows with horses from many states and/or countries, as there is the potential for a small cluster to become a multiple location outbreak.  Outbreaks lead to the cancellation of multiple shows, quarantine of multiple farms, and voluntary closure of some private and teaching hospitals. This article by our very own Ben Buchanan DVM, DACVIM, DACVECC will focus exclusively on Equine Herpes Myeloencephalopathy; detailing the clinical signs, diagnosis, transmission, prevention, and biosecurity. 


Five different herpes virus are commonly found in domestic horses. Equine Herpes Virus (EHV) infection is ubiquitous in the equine population and most horses are infected in the first few months of life. Following infection with an EHV, the virus can essentially hide from the immune system in the lymphoid or neurologic tissues; horses are then said to have a “latent infection.” A latent viral infection can become reactivated via currently unknown mechanisms often during times of stress. Horses with re-activated latent infections are a source of infection for other horses. The isolate of EHV we are concerned about is EHV-1. The neurologic form of EHV-1 infection is generally referred to as Equine Herpes Myeoloencephalopathy (EHM). This form of herpes virus infection can cause clinical signs in any age, breed, or gender of the horse.Our understanding of how and when an EHV-1 infection will lead to EHM is poor, but improving. It is likely that many environmental, host, and viral factors not yet elucidated are involved. How the degree of viremia is related to EHM is not known., but other factors are important in the development of EHM. Both varieties of the virus establish latency and can be detected or recrudesce in subsequent years.

Clinical Signs 

Infection usually develops following exposure to a horse shedding the virus but in a small percentage of cases, infection occurs by reactivation of latent virus. EHV-1 typically causes a biphasic fever peaking on day 1 or 2 and again on day 6 or 7. With respiratory infections there is often significant nasal and ocular discharge, but not a lot of coughing. There may be some enlargement of submandibular lymph nodes. With the neurologic form there is typically minimal respiratory signs, with fever (rectal temperature greater than 102°F) being the only warning. Neurologic disease appears suddenly and is rapidly progressive reaching its peak intensity in 2-3 days. The degree of neurologic signs depend on the number, size, and location of the ischemic lesions in the spinal cord. In horses infected with EHM, clinical signs may include: nasal discharge, incoordination, hind end weakness, recumbency / paralysis, lethargy, urine dribbling, decreased tail tone, and/or head tilt. Bladder paralysis is common in the early stages of the disease.


Horses that show a fever and any of these signs should be isolated and examined on the farm. Definitive diagnosis of EHM may only be possible with histologic and post mortem examination of CNS tissue. A presumptive diagnosis can be made when EHV-1 is isolated from nasal secretions or blood in combination with appropriate clinical signs. Once exposed and infected with EHV-1 the virus may be detected in blood and nasal swabs for 21 and 14 days respectively. Virus shedding maybe more transient and difficult to detect in the latter stages of disease. Horses can develop clinical signs as early as 1 day after exposure to the virus but clinical signs can be delayed up to 10 days after exposure. Nasal shedding typically peaks within 24 to 48 hours of EHV-1 infection and can quickly become undetectable.


Horse to horse transmission of the herpes viruses is significant when horses are kept in close contact. However, contaminated equipment (e.g., water buckets, water hose handles, cleaning and grooming equipment etc) can be a source of infection and people can transmit the virus on their hands or clothes. An infected horse will excrete and aerosolize the virus in respiratory secretions.  All horses with clinical signs are expected to be contagious, although horses not showing any clinical signs can shed EHV-1.  Neurologic horses shed large quantities of virus and should be securely isolated. The virus is estimated to be viable for up to 7 days in the environment under normal circumstances but may remain viable for a maximum of one month under perfect conditions. The virus is easily killed in the environment by most disinfectants.


Preventing exposure is likely to be of the greatest value as, for all the equine herpes viruses, vaccination is not fully protective. Based on a study of the 2011 Cutting Horse outbreak, individuals who had more “at risk” activities or competed in more events had an increased chance of developing EHM. Things such as sharing water, grazing on the show grounds, using wash racks, and being tied in a barn were all considered risks.

Unfortunately, boostering well vaccinated horses during an outbreak is not helpful. While some clinicians believe that vaccination may facilitate development of EHM, evidence for this belief is lacking. Vaccinations 14 days prior to exposure is not likely to be harmful, and may help limit the spread of the disease.


Horses with confirmed EHM should be isolated. Strict hygiene and biosecurity measures should be implemented because the virus can be aerosolized. The exposed horse must also be isolated to control the spread of the virus. Based on an analysis of an outbreak at Colorado State, the authors concluded horses with active nasal EHV-1 shedding should be isolated in an airspace that is separate from other horses by strictly enforced biosecurity and isolation procedures. Personnel should wear protective clothing and adhere strictly to hand sanitation. Rectal temperatures should be taken on every potentially exposed case twice daily.

Maintain isolation procedures (primary perimeter) for 28 days after the last suspected new infection resolves. Rectal temps should be taken and recorded on every horse at least once a day. Any horse with a fever should be tested by both nasal swab and whole blood PCR. In the absence of clinical disease, the risk of exposure decreases with time. There should be compliance with requirements by state animal health officials for the duration of quarantine and testing.

EHM is a preventable disease with common sense measures. A veterinarian should be consulted early with any suspected cases. Horses that ultimately develop EHM have a good chance of survival if treated correctly.

If you have any questions about Equine Herpes Myeloencephalopathy, please contact us.

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