Overview of Equine Herpesvirus and Equine Herpesviruses
Equine herpesvirus (EHV) infection is a highly contagious DNA virus that primarily causes respiratory illness, reproductive failure (abortion), and severe neurological disease (equine herpesvirus myeloencephalopathy, or EHM) in horses. The most common strains of equine herpesvirus are EHV-1 and EHV-4, which are responsible for the majority of outbreaks. EHV-1 is particularly associated with respiratory disease, abortion, and the neurologic form of the disease (EHM), while EHV-4 mainly causes respiratory illness. Another important strain, EHV-3, is responsible for a venereal disease affecting the external genitalia of horses. Each form of the disease—respiratory, abortion, neurologic, and venereal—can have significant impacts on horse health and management, as outlined in the AAEP Equine Herpesvirus 1 & 4 Disease Guidelines and New Mexico State University’s guide to Equine Herpesvirus in Horses.
EHV is not transmissible to humans. Young horses are particularly vulnerable to EHV infection, and outbreaks can result in substantial economic losses. The incubation period for EHV is typically 2 to 10 days before symptoms appear, as described in the Texas A&M AgriLife Extension fact sheet on Equine Herpesvirus (EHV-1 & EHV-4).
Transmission of EHV occurs through direct contact, nasal shedding (aerosol droplets from coughing), contaminated equipment, and exposure to other horses that may be infected or have come into contact with contaminated environments. The virus can be transmitted from horse to horse before any clinical signs appear, making early detection and quarantining essential during outbreaks. The virus can survive in the environment for prolonged periods under the right conditions, so it is crucial to avoid sharing grooming equipment, water, and feed sources between infected and healthy horses to reduce the risk of spreading the virus, as noted by Texas A&M AgriLife Extension and New Mexico State University.
After initial infection, equine herpesvirus may become latent, allowing horses to carry the virus without showing signs until it is reactivated by stress, travel, or strenuous exercise. A substantial percentage of horses harbor latent EHV-1, which can reactivate and cause disease. The immune system plays a key role in controlling herpetic infections, and a compromised immune system can increase the likelihood of virus reactivation and disease manifestation, as explained by New Mexico State University and the University of Florida Large Animal Hospital’s page on Equine Herpesviruses (EHV-1 & EHV-4).
Clinical Signs, Diagnosis, and Testing
Core clinical signs of equine herpes virus (EHV-1) infection to watch for include fever, depression, loss of appetite, and neurological symptoms such as ataxia or weakness. Additional common symptoms are nasal discharge, watery eyes, and swollen lymph nodes under the jaw; coughing is usually not present. Symptoms of EHV-1 typically develop within four to six days after exposure, but can appear as soon as 24 hours or take up to ten days, as described by Texas A&M AgriLife Extension and New Mexico State University.
Fever is the most consistent sign of EHV-1 infection, but it is usually not persistent and can fluctuate throughout the day. EHV-1 typically presents with a biphasic fever, with the first spike occurring on the first or second day and a second peak around day six. Monitoring for elevated temperature is crucial, as it serves as an early warning sign of infection and possible neurological disease onset, according to Texas A&M AgriLife Extension.
Most horses that exhibit symptoms of EHV-1 do not develop the neurologic form, equine herpesvirus myeloencephalopathy (EHM), but when they do, elevated temperature is the primary warning sign. Neurologic symptoms of EHV-1 include rapid onset of hind limb weakness or incoordination, urine dribbling, and inability to stand, all of which are emphasized in the Texas A&M AgriLife Extension guide.
It is recommended to monitor horses’ temperatures twice daily to detect any elevated temperature early. If EHV-1 is suspected, nasal swab and whole blood PCR testing should be performed to confirm the diagnosis, as recommended by Texas A&M AgriLife Extension and the University of Florida’s Equine Herpesviruses resource.
EHV-1 vs EHV-4: EHV 1 and EHV 4 Differences
Equine herpes virus (EHV) includes several types, with EHV-1 and EHV-4 being the most common. EHV-1 can cause a range of outcomes, including respiratory disease, abortion in pregnant mares, and a serious neurological form known as equine herpesvirus myeloencephalopathy (EHM). The neurological form is particularly significant in disease outbreaks and can lead to severe symptoms such as incoordination, weakness, and even paralysis. For more detailed information on this manifestation, refer to the New Mexico State University guide Equine Herpesvirus in Horses.
EHV-4 typically causes respiratory disease, especially in young horses, but is less commonly associated with abortion or neurological complications, as described by New Mexico State University.
Additionally, EHV-3 is responsible for a venereal disease that affects the external genitalia of horses, impacting reproductive health, also covered in the New Mexico State University resource.
Vaccination can reduce respiratory symptoms and the risk of abortion but is not highly effective against the neurological form of the virus, according to Texas A&M AgriLife Extension.
Equine Herpes Myeloencephalopathy (EHM, equine herpes myeloencephalopathy ehm)
Equine herpesvirus myeloencephalopathy (EHM) is the neurological manifestation caused by specific strains or mutated forms of equine herpes virus type 1 (EHV-1), involving viral mutation, latency, and reactivation. EHM develops when the virus causes blood vessel damage and neural tissue injury in the brain and spinal cord. The pathogenesis involves inflammation and blood clots in the small blood vessels surrounding these structures, leading to neural damage, as described in New Mexico State University’s EHV guide and the AAEP guidelines.
Neurologic signs of EHM can include high fever, lack of coordination (ataxia), limb weakness, urinary dribbling or incontinence, and decreased sensation in the tail and perineal area. Loss of tail tone is also common. As the disease progresses, some horses may become recumbent and unable to rise, which is a severe progression sign. These neurologic signs result from the virus affecting neural tissue in the brain and spinal cord, as outlined by Texas A&M AgriLife Extension.
Currently, there is no effective treatment that directly eradicates equine herpesvirus myeloencephalopathy. Treatment options are limited to supportive therapy and the use of anti-inflammatory agents such as flunixin meglumine or corticosteroids. The prognosis for horses with EHM varies widely, and survival rates depend on severity and the ability to provide intensive supportive care, according to Texas A&M AgriLife Extension and New Mexico State University.
Management of Exposed Horses and Monitoring Protocols
- isolate exposed horses immediately. Infected horses must be isolated and quarantined for at least two weeks after the last signs of the disease, and strict quarantine for at least 21 days is essential to manage the spread of EHV, as infected horses can remain contagious for an extended period, following guidance from Texas A&M AgriLife Extension.
- implement twice-daily rectal temperature checks
- restrict handler access to a single trained person
- maintain detailed contact and movement logs
Monitoring Protocols for Exposed Horses
- record temperatures and clinical signs on a chart
- collect daily nasal swabs if symptoms appear
- notify veterinarian upon any abnormal reading
Guidance for Horse Owners
- Separate sick horses from healthy horses. EHV can be shed for prolonged periods of time, including by clinically healthy horses, which increases the risk for continued disease spread, as highlighted by the Colorado Department of Agriculture’s Equine Herpes Virus Outbreak page.
- Contact your private veterinarian promptly for fever.
- Avoid sharing tack, buckets, and water sources.
- Owners and horse handlers should change their clothes and thoroughly wash their hands and arms after caring for infected horses to reduce transmission risk, as recommended by Texas A&M AgriLife Extension.
- Follow animal health officials’ quarantine directives.
A horse’s risk of developing EHV-1 infection or equine herpesvirus myeloencephalopathy (EHM) is influenced by factors such as vaccination status, exposure to infected horses, stress levels, age, and immune system health. Pregnant mares are at particular risk, as EHV can cause abortions and other health complications, leading to significant disruptions in breeding operations, as described by New Mexico State University.
Guidance for Veterinarians and Animal Health Officials
- Report suspected EHM cases to the state veterinarian’s office, as both EHV-1 and EHM are reportable diseases and must be reported to state animal health officials, as indicated by the Colorado Department of Agriculture’s Equine Herpes Myeloencephalopathy (EHM) page.
- Reference the AAEP Equine Herpesvirus 1 & 4 Disease Guidelines for authoritative recommendations on EHV management.
- Veterinary medicine provides essential guidance for the diagnosis, management, and prevention of EHV-1 infections in horses.
- Collect paired nasal swab and whole blood samples.
- Advise quarantine length based on testing and symptoms.
- Coordinate testing with accredited diagnostic labs.
- Many racetracks, shows, and other equine venues require vaccination and a recent booster for EHV-1 to help control outbreaks, as discussed by Texas A&M AgriLife Extension.
- In the event of an EHV-1 outbreak, it is crucial to avoid traveling with horses until the outbreak is resolved and local animal health officials have announced it is safe, as also emphasized by Texas A&M AgriLife Extension.
Biosecurity for Event Organizers
- require health attestations and recent vaccination proof
- implement pre-entry temperature screening stations
- designate isolation stalls for suspected cases
- restrict attendees who had recent exposure
Movement, Reporting, and Legal Requirements
- check destination-state movement and import rules
- halt movement of exposed horses until clearance
- notify state animal health officials of confirmed cases
Treatment and Supportive Care for Equine Herpesvirus EHV Cases
- provide supportive care per veterinary orders, as this remains the primary approach for treating EHM, including therapies like intravenous fluids and anti-inflammatory medications
- administer anti-inflammatories as prescribed
- consider intravenous fluids for dehydrated or weak horses to maintain hydration and aid recovery
- evaluate antiviral medications early under veterinary guidance; while these are commonly considered for EHM treatment, there is limited strong evidence for benefit, and they may be most effective when used in the early stages of the disease, according to Texas A&M AgriLife Extension
Vaccination and Prevention Strategies Against Equine Herpes
- discuss vaccination benefits for reducing viral shedding
- set vaccination schedule per veterinarian recommendation
- implement cleaning and disinfection protocols routinely, including the use of a bleach dilution of one part bleach to ten parts water for disinfecting equipment and premises to help stop the spread of equine herpesvirus, as described by New Mexico State University
Testing, Release, and Post-Outbreak Clearance
- require repeated negative tests before ending quarantine
- document test results and clinical recovery milestones
- communicate clearance decisions with animal health officials
Conclusion and Further Resources for Event Organizers
Equine herpesvirus (EHV) remains a significant threat to horse populations worldwide, with the potential to cause severe respiratory disease and, in some cases, progress to the neurologic form known as equine herpes myeloencephalopathy (EHM). For event organizers, the stakes are high: an outbreak can disrupt events, impact the health of horses, and cause emotional and financial strain for horse owners and equine practitioners alike.
Vigilance is essential. Recognizing clinical signs—such as nasal discharge, fever, and neurologic symptoms like incoordination or urine dribbling—can make the difference in early detection and containment. Monitoring rectal temperatures and watching for any horse showing signs of illness should be standard practice at all gatherings.
To reduce the risk of viral spread, event organizers must enforce robust biosecurity practices. This includes preventing nose-to-nose contact between horses, avoiding the communal use of grooming supplies and feed buckets, and maintaining strict hygiene protocols throughout the event. Prompt isolation of sick horses and immediate veterinary evaluation are critical steps in protecting both healthy horses and the broader horse community.
If a horse tests positive for equine herpesvirus, it is vital to act quickly in coordination with state animal health officials. Implementing quarantine, restricting movement, and following official guidance can help contain the outbreak and minimize its impact. Having a clear plan for managing sick horses—including access to veterinary care and designated isolation areas—ensures a rapid and effective response, as supported by the Colorado Department of Agriculture’s equine outbreak guidance, Texas A&M AgriLife Extension, and New Mexico State University.
