December 2025 USEF Webinar on EHV-1 and Biosecurity
Comprehensive Guide for Horse Owners, Trainers, and Facilities
Overview
Equine Herpes Viruses (EHV) are a family of nine known herpesviruses affecting horses. Among them, EHV-1 and EHV-4 are the most clinically significant, primarily causing respiratory disease (rhinopneumonitis). Nearly all horses are exposed to EHV-1 by two years of age. After initial infection, the virus may become latent, meaning a horse can carry it without showing signs until it is reactivated by stress, travel, or strenuous exercise. An estimated 60% of horses harbor latent EHV-1.
EHV-1 vs. EHV-4
EHV-1
- Causes respiratory illness, abortion, foal death, and sometimes neurologic disease.
- Two strains exist: D752 (“neurotropic”) and N752 (“wild type”) – both are capable of causing neurologic disease
- Vaccination reduces respiratory and reproductive disease but cannot prevent the neurologic form.
EHV-4
- Primarily associated with upper respiratory tract infection (fever, nasal discharge, mild cough).
What is EHV-1 Myeloencephalopathy (EHM) or Neurologic EHV-1?
EHM results from inflammation and small blood clots in the vessels surrounding the brain and spinal cord.
- Begins with fever (1–3 days after infection; up to 10 days)
- Neurologic signs appear around 7 days post-infection, progressing rapidly
- Clinical signs may include:
- Incoordination and weakness
- Loss of tail tone
- Urine dribbling
- Head tilt
- Difficulty rising or inability to stand
Only about 10% of infected horses develop neurologic signs, and outbreaks may involve a single case or multiple horses.
How EHV Spreads
EHV-1 and EHV-4 spread easily through:
- Airborne particles from an infected horse
- Direct nose-to-nose contact
- Contaminated hands, clothing, equipment, feed buckets, trailers, and tack
- Shared water sources
The virus can survive up to 7 days in the environment but is easily killed with disinfectants.
Diagnosis
Horses showing both fever and neurologic signs should be examined and tested by a veterinarian.
- Recommended diagnostic testing includes:
- Nasal swab for PCR testing
- Blood sample for PCR testing
PCR (polymerase chain reaction) testing detects the DNA of the virus and using both nasal and blood samples improves diagnostic accuracy.
Any horse that dies or is euthanized due to neurologic disease should be submitted for a necropsy examination.
Vaccination Recommendations
Vaccination plays an important role in reducing disease risk and limiting viral spread, but:
- No current vaccine prevents EHM.
- Recommended groups include:
- Foals, weanlings, yearlings
- Show and performance horses
- Horses in frequent contact with new animals
- Pregnant mares should receive vaccines specifically licensed to prevent EHV-1 abortion.
- For high-risk populations, vaccinate every 6 months.
Your veterinarian is an excellent resource in determining whether your horses need EHV vaccines and how frequently to vaccinate.
Risk Assessment
Horses should be considered at increased risk if they have experienced:
- Transport, especially with horses from multiple venues
- Presence at horse shows, farms, training centers or other venues where horses mingle
- Contact with horses showing fever, respiratory illness, or undiagnosed illness
- Exposure to horses with neurologic signs once non-infectious causes are ruled out
Immediate Precautions for At-Risk Horses
* Adapted from American Association of Equine Practitioners Infectious Disease Guidelines: Equine Herpesvirus (EHV-1 & EHV-4)
1. Stop Movement
- Confine the horse to its stall or an isolation area
- Any horse with any clinical sign should be isolated
2. Disease Surveillance
- Take rectal temperature twice daily
- Owners must contact a veterinarian for any fever
- Veterinarians should examine suspect horses and perform diagnostic testing
3. Restrict Human Access
- Only essential personnel may enter the area
- All must follow strict biosecurity measures
- Security staff may be needed to monitor entry/exit
Quarantine Procedures for Confirmed Cases
- A state or voluntary quarantine begins once EHV-1 is confirmed
- Quarantine extends 21 days after the last clinical sign (typically fever)
- Total duration typically 28–35 days
Outbreak Management
EHM is a state-reportable disease in Florida. Once a neurologic EHV-1 case is confirmed:
- The attending veterinarian must notify state animal health officials
- State veterinarians determine quarantine measures and movement restrictions
Treatment
There is no cure for EHV or EHM. Management is supportive:
- Anti-inflammatory medications
- IV fluids for horses unable to drink
- Intensive nursing care for down horses
- Antibiotics only if secondary bacterial infections are present
- Antiviral drugs (e.g., valacyclovir) may help prevent infection in exposed horses
