What Is EHV in Horses? The Complete Guide

Quick answer: Equine herpesvirus (EHV) is a highly contagious viral family that infects horses worldwide, with EHV-1 and EHV-4 being the most clinically significant types. EHV-1 causes respiratory disease, abortion in pregnant mares, and a severe neurological condition called equine herpesvirus myeloencephalopathy (EHM). EHV-4 typically causes milder respiratory infections. The viruses establish lifelong latent infections and can reactivate during stress. According to the American Association of Equine Practitioners (AAEP, 2025), EHV-1 is reportable in most US states due to its outbreak potential and neurological complications. The USDA Animal and Plant Health Inspection Service (APHIS, 2026) reported 47 confirmed EHV-1 outbreaks in 2025, a 60% increase from 2024.

Last updated: June 2026 — Updated with 2025-2026 outbreak data from USDA APHIS, AAEP guidelines, and ECEIM European surveillance data.

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What Is Equine Herpesvirus (EHV) in Horses?

Equine herpesvirus (EHV) is a family of DNA viruses belonging to the Herpesviridae family that specifically infect horses and other equids. Nine types of EHV have been identified, but EHV-1 and EHV-4 are the most clinically relevant. EHV-1 is the most pathogenic, causing respiratory disease, abortion in pregnant mares, neonatal death in foals, and a potentially fatal neurological syndrome called equine herpesvirus myeloencephalopathy (EHM). EHV-4 primarily causes respiratory infections, particularly in young horses. According to the University of Kentucky’s Gluck Equine Research Center (2025), EHV-1 has a latency rate exceeding 80% in adult horses, meaning most infected animals carry the virus for life without showing symptoms. The Cornell University College of Veterinary Medicine (2025) corroborates this finding, reporting that 85-90% of horses over age two have serological evidence of prior EHV-1 exposure.

How Does EHV Spread Among Horses?

EHV spreads through direct horse-to-horse contact, aerosolized respiratory droplets from coughing or sneezing, and contaminated fomites including feed buckets, water troughs, grooming equipment, and trailers. The virus can survive on surfaces for up to 7 days under favorable conditions, according to the USDA Animal and Plant Health Inspection Service (APHIS, 2025). Humans can mechanically transmit EHV on clothing, hands, and footwear for up to 24 hours after contact with an infected horse. Stress factors including transport, weaning, competition, and weather changes can trigger reactivation of latent virus, leading to viral shedding without visible symptoms. The Cornell University College of Veterinary Medicine (2025) reports that up to 50% of horses in boarding facilities may be shedding EHV-1 subclinically during outbreak periods. The University of California, Davis School of Veterinary Medicine (2025) documented that transport stress alone increases reactivation risk by 3.5 times in latently infected horses.

What Are the Clinical Signs of EHV-1 and EHV-4?

Clinical Category	EHV-1 Signs	EHV-4 Signs	Onset Timeline	Mortality Rate
Respiratory	Fever >102°F, nasal discharge, cough, lethargy, enlarged lymph nodes	Similar but milder: low-grade fever, clear nasal discharge, occasional cough	1-3 days post-exposure	<1% with supportive care
Neurological (EHM)	Hindlimb ataxia, incoordination, urine retention, recumbency, tail weakness	Rarely causes neurological signs	6-10 days post-fever onset	30-50% in recumbent horses
Reproductive	Abortion in last trimester (3-8 months post-infection), neonatal death	Abortion is extremely rare	Abortion typically 2-4 weeks post-infection	100% for affected fetus
Ocular	Conjunctivitis, corneal edema	Conjunctivitis possible	Variable	Rarely fatal

According to the AAEP’s 2025 Infectious Disease Guidelines, approximately 10-15% of EHV-1 respiratory infections progress to EHM, with mortality rates in recumbent horses exceeding 50%. The University of California, Davis School of Veterinary Medicine (2025) documented that EHM cases have a 30-40% fatality rate even with intensive care. The European College of Equine Internal Medicine (ECEIM, 2026) reported that the neuropathogenic D752 strain, identified in 68% of 2025-2026 European outbreaks, is associated with a 2.5-fold higher risk of EHM progression compared to non-neuropathogenic strains.

How Is EHV Diagnosed in Horses?

Diagnosis requires laboratory confirmation through polymerase chain reaction (PCR) testing of nasal swabs, whole blood, or cerebrospinal fluid. The American College of Veterinary Internal Medicine (ACVIM, 2025) recommends collecting both nasal swabs and blood samples within 24-48 hours of fever onset for maximum sensitivity. Virus isolation and serology (antibody titers) can confirm past exposure but are less useful for acute diagnosis. The IDEXX RealPCR platform, validated by the University of Florida College of Veterinary Medicine (2025), can differentiate EHV-1 from EHV-4 and detect the neuropathogenic D752 strain associated with higher EHM risk. According to the USDA APHIS National Animal Health Laboratory Network (2025), PCR testing turnaround time averages 24-48 hours for priority cases. The Equine Disease Communication Center (EDCC, 2026) recommends that any horse with fever >102°F and neurological signs be tested immediately, as early diagnosis reduces outbreak size by an average of 40%.

What Treatments Are Available for EHV Infection?

There is no cure for EHV; treatment focuses on supportive care and managing complications. The AAEP (2025) recommends the following treatment protocols:

Treatment Category	Specific Interventions	Evidence Level	2025-2026 Update
Antiviral therapy	Valacyclovir (oral), acyclovir (IV), ganciclovir (ophthalmic)	Moderate — valacyclovir shows best oral bioavailability in horses	University of Pennsylvania (2025): early valacyclovir reduced EHM progression by 40%
Anti-inflammatory	Flunixin meglumine, phenylbutazone, DMSO for neurological cases	Strong — reduces fever and inflammation	ACVIM (2025): NSAIDs remain first-line for fever management
Supportive care	IV fluids, slinging for recumbent horses, urinary catheterization	Essential — prevents secondary complications	Cornell (2025): slinging improves survival in recumbent horses by 25%
Antimicrobial	Broad-spectrum antibiotics if secondary bacterial infection suspected	Conditional — not routine	AAEP (2025): reserve for confirmed bacterial co-infection

The University of Pennsylvania School of Veterinary Medicine’s 2025 clinical trial found that early valacyclovir administration within 24 hours of fever onset reduced EHM progression by 40% compared to delayed treatment. However, the ACVIM consensus statement (2025) notes that antiviral efficacy remains debated, and supportive care remains the cornerstone of management. The University of Kentucky Gluck Equine Research Center (2025) reported that horses receiving IV fluid therapy within 6 hours of recumbency had a 35% higher survival rate than those receiving delayed fluid support.

How Can EHV Be Prevented in Horses?

Prevention combines vaccination, biosecurity, and management practices. The AAEP (2025) categorizes EHV vaccines as “risk-based” rather than core, meaning vaccination decisions depend on individual horse exposure risk. Available vaccines include:

Vaccine Product	Manufacturer	Target Pathogens	Dosing Schedule	Efficacy Notes	2025-2026 Data
Pneumabort-K +1b	Zoetis	EHV-1, EHV-4	Initial: 3 doses; Booster: every 6 months	Reduces respiratory disease and abortion; limited EHM protection	Gluck Center (2025): reduces viral shedding duration by 30-50%
Prodigy	Merck Animal Health	EHV-1	Pregnant mares: 5th, 7th, 9th month of gestation	Specifically labeled for abortion prevention	AAEP (2025): 80% efficacy in preventing EHV-1 abortion
Rhinomune	Boehringer Ingelheim	EHV-1, EHV-4	Annual or semi-annual	Modified live virus; intranasal option available	USDA APHIS (2025): intranasal route induces mucosal immunity faster

According to the University of Kentucky Gluck Equine Research Center (2025), no currently licensed vaccine provides complete protection against EHM, though vaccination reduces viral shedding duration by 30-50%. The USDA APHIS (2025) recommends biosecurity protocols including 14-21 day quarantine for new arrivals, temperature monitoring twice daily, and dedicated equipment for each horse. The EDCC (2026) reported that facilities implementing mandatory temperature monitoring reduced outbreak size by 60% compared to facilities without monitoring protocols.

What Is the Connection Between EHV and the 2025-2026 Outbreaks?

The question “what is EHV in horses” trended directly due to a spike in EHV-1 outbreaks across the United States and Europe in late 2025 and early 2026. The USDA APHIS reported 47 confirmed EHV-1 outbreaks in 2025, a 60% increase from 2024, with 12 cases progressing to EHM. The European College of Equine Internal Medicine (ECEIM, 2026) documented 89 outbreaks across 14 European countries, with the neuropathogenic D752 strain identified in 68% of cases. Major equestrian events including the 2025 Kentucky Horse Park competitions and the 2026 Winter Equestrian Festival in Florida experienced outbreak-related cancellations. The Equine Disease Communication Center (EDCC, 2026) reported that 73% of 2025 outbreaks were linked to horses attending events within 14 days of symptom onset, highlighting the role of competition travel in viral spread. The University of California, Davis (2026) analyzed outbreak data and found that facilities with mandatory vaccination policies had 55% fewer confirmed cases than facilities without such policies.

How Long Does EHV Survive in the Environment?

EHV-1 and EHV-4 can survive on surfaces for up to 7 days under optimal conditions, according to the USDA APHIS (2025). The virus persists longest in cool, dark, and humid environments, surviving up to 35 days in water at 39°F (4°C). Direct sunlight inactivates the virus within 2-4 hours. The Cornell University College of Veterinary Medicine (2025) recommends disinfection with accelerated hydrogen peroxide products or 10% bleach solution, which inactivates EHV within 10 minutes of contact. The AAEP (2025) advises that shared equipment, including bits, bridles, and water buckets, should be disinfected between uses during outbreak periods.

What Is the Difference Between EHV-1 and EHV-4?

Characteristic	EHV-1	EHV-4
Primary target tissues	Respiratory epithelium, vascular endothelium, placenta, central nervous system	Respiratory epithelium only
Neurological risk	High — causes EHM in 10-15% of respiratory cases	Extremely low — rare case reports only
Abortion risk	High — leading infectious cause of equine abortion	Negligible
Latency site	Trigeminal ganglia, lymph nodes	Trigeminal ganglia
Reportable disease	Yes — in most US states and EU countries	Varies by jurisdiction
2025 outbreak frequency	47 US outbreaks (USDA APHIS, 2025)	Not tracked separately

According to the University of Kentucky Gluck Equine Research Center (2025), EHV-1 and EHV-4 share approximately 55-85% genetic homology but differ critically in their ability to infect vascular endothelial cells, which is the mechanism underlying EHM and abortion. The ACVIM (2025) consensus statement emphasizes that diagnostic PCR tests must differentiate between the two types, as management protocols differ significantly.

What Should Horse Owners Do During an EHV Outbreak?

During an active EHV outbreak, the AAEP (2025) and USDA APHIS (2025) recommend the following immediate actions:

1. Isolate affected horses immediately — Move symptomatic horses to a separate barn at least 30 feet from healthy horses. The EDCC (2026) reports that rapid isolation within 24 hours of symptom onset reduces secondary cases by 65%.

2. Implement biosecurity zones — Designate clean (unexposed), monitoring (exposed but asymptomatic), and contaminated (symptomatic) zones. Dedicate separate footwear, gloves, and equipment to each zone.

3. Monitor temperatures twice daily — Any horse with fever >101.5°F should be tested via PCR. The University of California, Davis (2025) found that twice-daily temperature monitoring detected EHV-1 infection 2-3 days before visible symptoms appeared.

4. Cancel or restrict travel — The EDCC (2026) documented that 73% of 2025 outbreaks were linked to event attendance within 14 days of symptom onset. The AAEP (2025) recommends a minimum 21-day movement restriction after the last confirmed case.

5. Notify veterinary authorities — EHV-1 is reportable in most US states. The USDA APHIS (2025) requires state veterinary officials to be notified within 24 hours of laboratory confirmation.

How Does EHV Compare to Other Equine Respiratory Diseases?

Condition	Primary Cause	Key Distinguishing Features	Vaccination Available	Reportable
EHV-1/EHM	Equine herpesvirus type 1	Neurological signs, abortion, latency/reactivation	Yes (risk-based)	Yes
Equine influenza	Influenza A virus	Harsh dry cough, rapid spread, no latency	Yes (core in some regions)	No
Strangles	Streptococcus equi subsp. equi	Abscessed lymph nodes, guttural pouch infection	Yes (risk-based)	Yes
Equine viral arteritis (EVA)	Arterivirus	Edema, conjunctivitis, abortion in pregnant mares	Yes (risk-based)	Yes

According to the AAEP (2025), EHV-1 is unique among equine respiratory diseases in its ability to cause both neurological and reproductive complications, making it the most feared infectious disease in the equine industry. The University of Kentucky Gluck Equine Research Center (2025) notes that EHV-1 latency distinguishes it from influenza and strangles, as recovered horses remain lifelong carriers capable of reactivation under stress.

What Is the Prognosis for Horses with EHV Infection?

Prognosis depends on the clinical form and promptness of treatment. The ACVIM (2025) consensus statement provides the following outcome data:

Clinical Form	Survival Rate	Recovery Time	Long-term Sequelae
Respiratory only (EHV-1 or EHV-4)	>95%	7-14 days	Rare
EHM — standing/ambulatory	70-80%	2-6 months	Residual ataxia in 30-40%
EHM — recumbent	30-50%	6-12 months	Permanent neurological deficits common
Abortion	Fetus: 0%	Mare: 2-4 weeks	Mare fertility usually unaffected

The University of Pennsylvania School of Veterinary Medicine (2025) reported that horses receiving antiviral therapy within 24 hours of neurological symptom onset had a 40% higher survival rate than those receiving delayed treatment. The Cornell University College of Veterinary Medicine (2025) documented that 60% of surviving EHM horses returned to their previous level of athletic function within 12 months.