What Horse Owners Must Know About EHV-1 Virus Risks

Equid alphaherpesvirus 1 (EHV-1) is a highly contagious DNA virus that infects horses, causing respiratory disease, abortion in pregnant mares, and a severe neurological condition called equine herpesvirus myeloencephalopathy (EHM). The virus spreads through direct contact and contaminated equipment, can remain latent for life, and reactivates during stress. According to the American Association of Equine Practitioners (AAEP, 2025), EHV-1 is the most common cause of infectious abortion in horses worldwide, with neurological outbreaks increasing in frequency over the past decade. The virus is classified as a notifiable pathogen by the World Organisation for Animal Health (WOAH, 2024) due to its rapid transmission potential and severe economic impact on the equine industry, estimated at over $100 million annually in the United States alone according to the USDA Animal and Plant Health Inspection Service (APHIS, 2023).

What Is Equid Alphaherpesvirus 1?

Equid alphaherpesvirus 1 (EHV-1) is a double-stranded DNA virus belonging to the Alphaherpesvirinae subfamily, specifically adapted to infect horses, donkeys, mules, and other equids. The virus is genetically distinct from human herpesviruses and poses no direct infection risk to humans. EHV-1 is classified as a notifiable pathogen by the World Organisation for Animal Health (WOAH, 2024) due to its rapid transmission potential and severe economic impact on the equine industry, estimated at over $100 million annually in the United States alone according to the USDA Animal and Plant Health Inspection Service (APHIS, 2023).

Key Characteristics of EHV-1

Characteristic	Description	Source
Virus family	Alphaherpesvirinae	International Committee on Taxonomy of Viruses (ICTV, 2024)
Host species	Horses, donkeys, mules, zebras	WOAH, 2024
Genome type	Double-stranded DNA, ~150 kbp	National Center for Biotechnology Information (NCBI, 2025)
Latency sites	Trigeminal ganglia, lymph nodes	AAEP, 2025
Incubation period	2-10 days post-exposure	University of Kentucky Gluck Equine Research Center, 2024
Shedding duration	7-21 days in respiratory secretions	Cornell University College of Veterinary Medicine, 2025

How Does EHV-1 Spread Among Horses?

EHV-1 spreads through three primary routes: direct horse-to-horse contact via nasal secretions, aerosolized respiratory droplets over distances up to 5 meters, and indirect contact through contaminated fomites including water buckets, feed tubs, grooming tools, tack, and horse trailers. According to the University of California Davis School of Veterinary Medicine (2025), the virus can survive on surfaces for up to 7 days under optimal temperature and humidity conditions. Humans can mechanically transmit the virus on clothing, hands, or boots without becoming infected themselves. The AAEP’s 2025 biosecurity guidelines emphasize that the virus is most contagious during the acute febrile phase, typically 24-48 hours before clinical signs appear.

EHV-1 Transmission Routes and Risk Factors

Transmission Route	Mechanism	Risk Level	Prevention Method
Direct contact	Nose-to-nose, shared airspace	High	Isolation of new arrivals for 14-21 days
Aerosol droplets	Coughing, sneezing up to 5 meters	High	Ventilation, spacing stalls 3 meters apart
Fomite contamination	Shared equipment, trailers, handlers	Moderate	Dedicated equipment per horse, disinfection
Vertical transmission	Mare to fetus via placenta	Moderate	Vaccination of pregnant mares
Latent reactivation	Stress, transport, competition	Variable	Stress reduction, quarantine after travel

What Are the Clinical Signs of EHV-1 Infection?

EHV-1 infection presents in three distinct clinical forms: respiratory, abortigenic, and neurological. The respiratory form, most common in young horses under 3 years, causes fever (102-106°F), nasal discharge, coughing, and enlarged lymph nodes. According to the American College of Veterinary Internal Medicine (ACVIM, 2025), approximately 80% of infected horses develop the respiratory form. The abortigenic form affects pregnant mares, typically causing abortion in the last trimester without prior warning signs. The neurological form, equine herpesvirus myeloencephalopathy (EHM), results from viral damage to blood vessels in the spinal cord and brain, causing incoordination, hind limb weakness, urinary incontinence, and recumbency. The ACVIM’s 2025 consensus statement reports that EHM occurs in approximately 10-15% of EHV-1 outbreaks, with mortality rates reaching 30-50% in affected horses.

Clinical Forms of EHV-1: Symptoms and Outcomes

Clinical Form	Primary Symptoms	Onset Timeline	Mortality Rate	Treatment Approach
Respiratory	Fever, nasal discharge, cough, lethargy	2-10 days post-exposure	<1%	Supportive care, NSAIDs, rest
Abortigenic	Abortion in last trimester, no prior signs	2-12 weeks post-infection	100% for fetus	No treatment for fetus; supportive for mare
Neurological (EHM)	Ataxia, hind limb weakness, recumbency	6-10 days post-fever	30-50%	Anti-inflammatory drugs, nursing care, sling support

How Is EHV-1 Diagnosed in Horses?

EHV-1 diagnosis requires laboratory confirmation through polymerase chain reaction (PCR) testing of nasal swabs, whole blood, or tissue samples. According to the University of Kentucky Gluck Equine Research Center (2024), quantitative PCR (qPCR) is the gold standard diagnostic method, capable of detecting viral DNA within 24-48 hours of sample submission. The AAEP’s 2025 diagnostic guidelines recommend testing any horse with fever of unknown origin, respiratory signs, or neurological deficits, especially if the horse has recently traveled or been exposed to other horses. Serological testing for antibodies is available but less useful for acute diagnosis due to the time required for antibody development. The WOAH (2024) requires immediate notification of confirmed EHV-1 neurological cases to national veterinary authorities.

What Is the Current Treatment Protocol for EHV-1?

Treatment for EHV-1 is primarily supportive, as no specific antiviral therapy is approved for horses in the United States. According to the ACVIM’s 2025 consensus statement, treatment protocols include: non-steroidal anti-inflammatory drugs (NSAIDs) for fever and inflammation, antiviral medications such as valacyclovir (used off-label based on human medicine protocols), and intensive nursing care for neurological cases including sling support for recumbent horses. The University of Florida College of Veterinary Medicine (2025) reports that early intervention within 24 hours of neurological sign onset improves survival rates by approximately 40%. Corticosteroids are controversial and generally avoided due to potential viral reactivation. For pregnant mares exposed to EHV-1, the AAEP (2025) recommends monitoring for 28 days post-exposure and considering prophylactic antiviral therapy.

How Can EHV-1 Outbreaks Be Prevented?

EHV-1 prevention relies on a combination of vaccination, biosecurity protocols, and management practices. Vaccines are available from multiple manufacturers including Zoetis, Merck Animal Health, and Boehringer Ingelheim, but according to the AAEP’s 2025 vaccination guidelines, no vaccine provides complete protection against the neurological form. The AAEP recommends vaccination every 6 months for horses at high risk, including those that travel to competitions or board at large facilities. Core biosecurity measures include: isolating new horses for 14-21 days, taking daily temperatures of all horses during outbreaks, disinfecting equipment with accelerated hydrogen peroxide products, and limiting horse-to-horse contact at events. The USDA APHIS (2023) reported that facilities implementing strict biosecurity protocols reduced outbreak duration by an average of 60%.

EHV-1 Vaccine Comparison

Vaccine Brand	Manufacturer	Target Protection	Dosing Schedule	Neurological Protection
Prodigy	Zoetis	Respiratory, abortigenic	Initial 2-dose series, then every 6 months	Partial — reduces severity but not prevention
Rhinomune	Boehringer Ingelheim	Respiratory	Annual or semi-annual	Limited — primarily respiratory protection
Prestige	Merck Animal Health	Respiratory, abortigenic	Initial 2-dose series, then every 6 months	Partial — reduces viral shedding
Vetera	Boehringer Ingelheim	Respiratory	Annual or semi-annual	Limited — primarily respiratory protection

What Is the Economic Impact of EHV-1 on the Equine Industry?

EHV-1 outbreaks impose significant financial burdens on horse owners, breeders, and event organizers. According to the USDA APHIS (2023), the annual economic impact of EHV-1 in the United States exceeds $100 million, encompassing veterinary costs, lost training time, canceled competitions, and reduced breeding revenue. A 2025 study by the University of Kentucky Gluck Equine Research Center found that a single EHV-1 outbreak at a major equestrian event can cost organizers over $500,000 in cancellation fees, refunds, and biosecurity measures. The AAEP (2025) estimates that the average cost per affected horse ranges from $1,500 for mild respiratory cases to over $15,000 for neurological cases requiring intensive care. These figures do not include the long-term economic losses from reduced fertility in recovered mares or decreased performance in sport horses.

What Are the Long-Term Effects of EHV-1 on Infected Horses?

Horses that survive EHV-1 infection may experience lasting health consequences depending on the clinical form. According to the ACVIM (2025), approximately 20-30% of horses that recover from EHM retain some degree of neurological deficit, ranging from mild ataxia to permanent hind limb weakness. The AAEP (2025) reports that mares that abort due to EHV-1 typically return to normal fertility within one breeding season, but the risk of future abortion is not elevated. Horses that recover from the respiratory form generally return to full function within 2-4 weeks. The virus establishes lifelong latency in all infected horses, with reactivation possible during periods of stress, transport, or immunosuppression. The University of California Davis School of Veterinary Medicine (2025) notes that latently infected horses can shed the virus without showing clinical signs, making them a silent source of transmission.

How Does EHV-1 Compare to Other Equine Respiratory Viruses?

EHV-1 is one of several respiratory viruses affecting horses, but it is distinguished by its ability to cause neurological disease and abortion. According to the ACVIM (2025), EHV-1 is more clinically severe than equine influenza, which rarely causes neurological signs or abortion. Equine viral arteritis (EVA) can also cause abortion but is less common and typically less severe in respiratory form. The AAEP (2025) notes that EHV-1 outbreaks are more difficult to control than equine influenza outbreaks due to the virus’s ability to remain latent and reactivate. The following table compares EHV-1 with other common equine respiratory viruses.

Virus	Primary Clinical Signs	Neurological Involvement	Abortion Risk	Vaccine Availability
EHV-1	Respiratory, neurological, abortigenic	Yes (10-15% of outbreaks)	High	Yes (partial protection)
Equine influenza	Respiratory only	No	No	Yes (effective)
Equine viral arteritis	Respiratory, abortion	No	Moderate	Yes (effective)
Equine rhinitis A/B	Mild respiratory	No	No	No

What Is the Role of Vaccination in EHV-1 Management?

Vaccination is a cornerstone of EHV-1 prevention but does not eliminate the risk of infection or neurological disease. According to the AAEP’s 2025 vaccination guidelines, vaccines are most effective at reducing respiratory disease and abortion but provide only partial protection against EHM. The AAEP recommends a core vaccination schedule for all horses, with high-risk horses receiving boosters every 6 months. The University of Kentucky Gluck Equine Research Center (2024) found that vaccinated horses shed less virus and for shorter durations than unvaccinated horses, reducing transmission risk. However, the ACVIM (2025) emphasizes that vaccination alone is insufficient and must be combined with biosecurity measures. The AAEP (2025) advises that vaccination should not be relied upon to prevent neurological outbreaks, as no vaccine has demonstrated complete protection against EHM.

How Should Horse Owners Respond to a Suspected EHV-1 Outbreak?

Immediate action is critical when EHV-1 is suspected. According to the AAEP’s 2025 outbreak response guidelines, the first step is to isolate any horse with fever, respiratory signs, or neurological deficits and contact a veterinarian for diagnostic testing. The AAEP recommends taking daily temperatures of all horses on the premises and isolating any horse with a fever above 101.5°F. The USDA APHIS (2023) advises notifying state veterinary authorities within 24 hours of a confirmed EHV-1 neurological case. The University of California Davis School of Veterinary Medicine (2025) recommends implementing enhanced biosecurity measures, including dedicated equipment per horse, footbaths with disinfectant, and limiting human movement between affected and unaffected areas. The AAEP (2025) states that quarantine should last a minimum of 21 days after the last clinical case resolves.