Equine herpesvirus – What to know
Monday, December 1, 2025
Media Contact: Kinsey Reed | Communications Specialist | 405-744-6740 | kinsey.reed@okstate.edu
Equine herpesvirus is a family of nine viruses named by numbers.
EHV-1, the virus associated with the ongoing outbreak, is most commonly associated
with respiratory tract infections. EHV-1 is also associated with neurologic disease
and abortion. Upwards of 70% of horses are infected with EHV-1 as a youngster and
carry it with them for the remainder of their lives.
Respiratory illness caused by EHV-1 often presents with fever, clear nasal discharge
and coughing. Horses will spread the virus before they actually show outward signs
that they are sick. Pregnant mares may experience sudden late-term abortion of the
fetus. In some horses, the virus progresses to equine herpes myeloencephalopathy (EHM),
causing incoordination, hind-limb weakness, reduced tail strength, dribbling urine
or sudden inability to stand. Commercial vaccines are available for EHV-1 and -4.
While vaccination for EHV is known to be beneficial in reducing respiratory and reproductive
disease, it cannot protect a horse from developing neurological symptoms.
Equine herpesvirus is spread from horse to horse through nasal discharge, aerosolized
droplets from coughing and contact with contaminated surfaces. People can also inadvertently
spread the virus between horses on their hands, clothing or equipment, though humans
themselves cannot contract the disease.
Veterinarians advise that any horse showing signs of illness should be isolated from
all other horses immediately, and owners should contact their local veterinarian immediately,
as swift assessment of possible exposure(s) and strict quarantine procedures are essential
in stopping further spread of the virus.
Such health and safety methods might include the following measures:
- Monitor rectal temperatures twice daily for all horses that may have been exposed, including those in Texas and any horses housed with animals from the original outbreak locations. A normal temperature ranges from 99.0*F to 101.5*F. Any elevation in temperature may indicate early infection. Note: The use of non-steroidal anti-inflammatory drugs, such as phenylbutazone (“Bute”), flunixin meglumine (“Banamine”) or firocoxib (“Equioxx”) can mask a fever. Recheck your horse’s temperature 12-24 hours after the last dose of any of these medications.
- Restrict movement on and off their property.
- Follow strict biosecurity protocols.
- Maintain thorough notes on symptoms and changes in behavior.
Close collaboration with a veterinarian is critical for determining appropriate testing, treatment and monitoring. Proper EHV testing plays a significant role in confirming cases. When collecting samples, you must obtain a deep nasal swab using an appropriate synthetic or flocked swab and a minimum of 2–5 milliliters of blood in an EDTA (purple-top) tube. Samples must be handled with care, kept cold but not frozen, and shipped overnight for timely processing. The Oklahoma Animal Disease Diagnostic Laboratory also accepts hand-delivered samples, including after-hours drop-off.
State officials and veterinarians continue to monitor the situation closely. Horses showing symptoms consistent with EHM must be reported to animal health officials to assist with assessment of the outbreak. While EHV remains a serious concern, early detection, proper isolation, and strong biosecurity measures can significantly reduce the risk of widespread transmission. Horse owners are encouraged to stay informed through official state updates and to contact their veterinarian with any questions or concerns regarding symptoms, exposure or testing.
Written by Dr. Kaitlin Mielnicki
Veterinary Viewpoints is provided by the faculty of the OSU Veterinary Medical Teaching Hospital. Certified by the American Animal Hospital Association, the hospital is open to the public providing routine and specialized care for all species, as well as emergency care. Call 405-744-7000 for an appointment or see more information at vetmed.okstate.edu.