11/21/2025
An update and overview from Dr. Emilie Setlakwe for our clients on the recent EHV-1 outbreak:
1. EHV Overview: Equine Herpes Virus (EHV) is a family of DNA viruses (EHV-1, -2, -3, -4, -5) that occurs in horse populations worldwide. The most common strains are EHV-1 and EHV-4.
2. Forms of Disease: EHV-1 and EHV-4 typically cause a respiratory disease that is usually mild and self-limiting. These strains can also cause abortion. More rarely, EHV-1 (and very rarely EHV-4) can cause the neurological form of the disease, known as Equine Herpesvirus Myeloencephalopathy (EHM), which damages the spinal cord and can lead to permanent neurological deficits or be fatal.
3. Current Outbreak: An outbreak of a particularly aggressive strain of EHV-1 began in early November 2025. It was linked to the World Championship barrel racing finals held in Waco, Texas, from November 5th to 9th.
4. Outbreak Spread: Following the initial cases in Texas, positive horses with the neurological manifestation (EHM) have been identified in Oklahoma and Louisiana. As a result, many equestrian events have been canceled as a precautionary measure.
5. Transmission: EHV is spread through direct nose-to-nose contact, coughing, and contact with contaminated surfaces such as shared equipment, and the clothing and boots of handlers etc.
6. Clinical Signs:
- Respiratory Form: Fever, nasal discharge, depression, lethargy, and enlarged lymph nodes.
- Neurological Form (EHM): Fever, swelling of the legs, ataxia (wobbliness), weakness of the hind end, urine dribbling and recumbency (down horse that cannot get up).
- Other associated signs: Abortion and neonatal foal death.
1. Diagnosis: A veterinarian can diagnose EHV by taking samples from a nasal swab and from the blood for PCR testing. Antibody testing is also an option.
Actionable Takeaways
1. Quarantine and Monitoring: Any horse that was exposed to a positive horse or a horse exhibiting symptoms should be quarantined and monitored for the development of clinical signs.
2. Treatment: Treatment is generally supportive care. For the neurological form (EHM), specific medications such as anti-thrombotics and antivirals may be administered.
3. Vaccination: Vaccination is recommended to help prevent the respiratory form of the disease. While the current vaccines do not prevent the neurological form, having horses well-vaccinated improves herd immunity and decreases the chances of large-scale outbreaks. Vaccination strategies will differ based on a horse's exposure risk and should be discussed directly with a veterinarian.
4. Biosecurity: Proper biosecurity is critical since the disease is spread via aerosol particles from nasal discharge and contaminated surfaces. This includes the disinfection of surfaces and equipment that come into contact with horses.
5. Management Practices: Recommended practices include maintaining up-to-date vaccinations on all horses on a property, practicing good biosecurity when traveling and showing, and quarantining any new horses or horses returning to a farm after travel for at least 21 days (3 weeks).
6. Veterinary Consultation: Owners with direct questions pertaining to their horse's individual risks should contact their veterinarian to decide on the appropriate course of action.
Additional Notes
- The incubation period for EHV is typically from 2 to 10 days.
- Horses can shed the virus and be contagious before they develop clinical signs during the incubation period.
- Horses with clinical signs should be considered contagious until cleared by their veterinarian through testing, quarantine, or both.
- Most horses with the respiratory form of EHV make a full clinical recovery in a couple of weeks.
- Horses that develop the neurological form (EHM) can take much longer to recover and may not ever fully recover from the neurological damage. In some cases, this form of the disease is fatal.
Resources and References
- For direct questions pertaining to your horse's risks, please feel free to contact your veterinarian at Tryon Equine Hospital. We are happy to help and answer questions.
Sincerely ,
Dr Emilie Setlakwe, DVM, MSc, Dipl. ACVIM (LA)
Equine herpesvirus (EHV) is a family of equine viruses named by numbers including EHV-1, 2, 3, 4 and 5 with EHV-1,3,4 posing the most risk for domestic horses. EHV is a common DNA virus that occurs in horse populations worldwide. The two most common species are EHV-1 and EHV-4.