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Contact: Deidra Ashley, CVMMarCom@missouro.edu
As new cases of Equine Herpesvirus (EHV-1 and EHV-4) and Equine Herpes Myeloencephalopathy (EHM) are confirmed in Louisiana, Oklahoma and Texas, horse owners across the region are seeking clear, practical guidance. The University of Missouri’s College of Veterinary Medicine has not identified any cases of EHV at this time, and the Veterinary Health Center continues its standard biosecurity protocol, including temperature checks on every horse prior to entry.
To help owners and the public understand the evolving situation and reduce spread, Mizzou’s equine internal medicine veterinarians break down the essential facts and the most effective steps horse owners can take right now.
What do horse owners need to know about the current EHV situation?
A horse virus outbreak has been confirmed to have begun at a Waco, Texas, barrel racing event. As of Thursday, Nov. 20, multiple states have reported EHV-1/EHM cases including Louisiana, Oklahoma and Texas.
Because exposed horses might not show clinical signs immediately, more cases are likely over the next two weeks as testing continues nationwide.
“One of the most important things owners can do right now is stay alert to even subtle changes in their horses. Early signs like a mild fever or nasal discharge might not seem alarming on their own, but they can be the first indicators of EHV exposure. Catching those signs early gives your veterinarian the best chance to intervene and limit the risk to other horses on the property.” — Lynn M. Martin, DVM, MPH, DACVIM (LAIM)
What symptoms should owners monitor?
EHV-1 and EHV-4 commonly cause fever and upper respiratory signs, but in some horses EHV-1 progresses to EHM, a neurological form of the disease.
Watch for:
- Fever > 101.5 degrees Fahrenheit.
- Nasal discharge.
- Hindlimb weakness or stumbling.
- Difficulty urinating or defecating.
- Loss of tail tone (“dead tail”).
- Dog-sitting posture or ataxia (drunken or wobbly gait).
Are these diseases transmissible to humans?
No — current veterinary and public-health guidance indicate that these diseases do not infect humans, although people can carry the virus mechanically (on clothing, hands or equipment) and inadvertently spread it between horses.
What should exposed horses do right now?
Any horse that attended the Waco event or any venue with known EHV-1/EHM cases should enter strict quarantine.
Isolation guidelines:
- Keep returning horses at least 30 feet from all others.
- Groups that traveled together can be quarantined as a cluster.
- No nose-to-nose contact.
- Do not share buckets, tack, pitchforks, halters, lead ropes or equipment.
- Check rectal temperatures twice daily.
- Caretakers should change clothes between groups or wear disposable PPE (gloves, shoe covers).
“For horses returning from affected events, twice-daily temperature checks are one of the most reliable tools we have. Fever often appears before any neurological signs, so identifying that change early allows owners and veterinarians to act quickly and safely. With EHV, time matters. The sooner a fever or behavior change is recognized, the sooner we can intervene and help protect the rest of the herd.” — Hannah R. Leventhal, DVM, MS, DACVIM (LAIM)
How should equipment and trailers be disinfected?
Remove all visible organic material first (bedding, manure, debris), then apply a 1:10 bleach solution. Allow surfaces to fully dry before rinsing.
Other effective disinfectants include:
- Quaternary ammonium compounds.
- Accelerated peroxide products.
- Peroxygen compounds.
- Iodophors.
Always follow label contact times for proper disinfection.
How does testing work, and where can Missouri veterinarians submit samples?
Because timing matters, most veterinarians use a combination of nasal swabs and whole blood to catch the virus at different stages. Testing is available in-state through Mizzou’s Veterinary Medical Diagnostic Laboratory (VMDL).
Key points for owners and veterinarians:
- Best samples: A nasal swab often detects virus early, while blood can remain positive longer into infection.
- Testing after known exposure: Some veterinarians recommend testing around days 2 and 10, or between days 10–14, to give owners a clearer sense of whether infection is developing.
- Testing is not a substitute for quarantine. Horses should continue to follow state veterinary guidance on isolation even if early results are negative.
“Because EHV can be detected at different points in the infection process, using both a nasal swab and whole blood offers the best chance of identifying the virus. Nasal shedding tends to happen early, while the virus can remain detectable in blood for a longer period. Our goal at VMDL is to provide fast, reliable testing so veterinarians can make informed decisions for their patients.” — Shuping Zhang, PhD, DACVM
What happens if a horse tests positive?
Most EHV-1/EHM–positive horses can be safely managed on-farm as long as they remain standing, ambulatory and able to eat, drink, urinate and defecate normally. In these cases, veterinarians can provide supportive care and monitor for changes. Mizzou’s equine internal medicine clinicians are available to support referring veterinarians with treatment guidance, biosecurity recommendations and testing through the VMDL.
If a horse becomes recumbent (unable to stand or rise), it requires referral to a facility with specialized isolation and intensive care capabilities. At this time, the Veterinary Health Center is not receiving suspect EHV-positive horses, but our clinicians are available to consult with referring veterinarians and help identify appropriate care options.
EHM can be severe, and while the mortality rate can be high (up to 75%), some horses do recover with prolonged, intensive care. Rehabilitation may take weeks to months.
EHV-1/EHM is a reportable disease, meaning veterinarians must notify the state veterinarian’s office when a case is confirmed, and positive horses must remain in strict isolation following state guidelines.
When can horses be released from quarantine?
Release depends on testing and exposure timeline:
- 14 days after last known exposure + negative test.
- 21 days of isolation if no testing is performed.
Owners should stay in close contact with their veterinarian, as state guidance might evolve as the outbreak develops.
Should owners vaccinate their horses?
- It is not recommended to vaccinate exposed horses during an outbreak.
- Non-exposed horses should receive an EHV-1 booster if they have not been vaccinated within the past three months.
“If a horse was potentially exposed, vaccinating now can cause more confusion than clarity because it can interfere with how the immune system responds during the early stages of infection. For healthy horses with no known exposure, a booster is still a smart step in lowering risk.” — Kile S. Townsend, DVM, MS, FHEA, DACVIM (LAIM)
Mizzou’s biosecurity status
The Mizzou Veterinary Health Center has had zero EHV cases to date. Our standard biosecurity practices remain in place, including mandatory temperature checks for all horses entering the facility.
Owners with questions can call the Equine Hospital at 573-882-3513.