There have been increasing numbers of outbreaks of equine herpes virus (EHV-1) and its associated equine herpes myeloencephalopathy (EHM) infection over the past several years. The latest event as of this writing was an extensive outbreak in November 2025 that stemmed from a Woman’s Professional Rodeo Association event in Waco, Texas, with horses disseminating to more events and others returning home. Fortunately, practices put into place after the 2011 Ogden, Utah herpes outbreak helped stem the exposure of highly infectious EHV-1 throughout the country. Following a 21-day quarantine for all exposed horses and their properties, the outbreak seemed to quiet down fairly quickly.
Not all horses infected with EHV-1 developed EHM; about 10 – 15% of infected horses developed EHM. Of EHM cases, 80 – 85% survived and 65% returned to athletic activities once recovered. So, it is an infectious disease to be taken seriously, with a need for diligent surveillance and rapid response in the event of an outbreak.
There are valuable lessons to be learned from these infectious disease situations. Let’s look at useful information on how you can be proactive in taking effective measures to stem the possibility of an outbreak.
Although many equine practitioners routinely vaccinate their equine patients against equine rhinopneumonitis (EHV-1), those vaccines do NOT protect against the neurologic form of equine herpes myeloencephalopathy (EHM). What vaccines may accomplish is to limit the degree of viral shedding and thereby decrease the viral dose in the environment.
Equine herpes virus isn’t going anywhere and continues to circulate within the horse population worldwide. Two excellent presentations by expert panels of internal medicine specialists have discussed EHV-1 and EHM recognition and prevention.
A 2022 podcast sponsored by Platinum Performance included Steve Reed, DVM, DACVIM; Lori Bidwell, DVM, DACVAA, CVA; and Josie Traub-Dargatz, DVM, MS, DACVIM.
At the 2025 American Association of Equine Practitioner’s Convention (AAEP), Ben Buchanan, DVM of Brazos Valley Equine Hospital in Texas and Katie Flynn, DVM, Senior Staff Veterinarian for the United States Equestrian Federation (USEF) discussed details about the current outbreak.
EHV-1 is spread through nose-to-nose contact, by grooms, braiders, community water buckets, and shared equipment. It is notable that EHV-1 can survive for 48 hours on clothing, buckets, and equipment. The virus can survive 21 days in water, and up to 30 days in ideal environmental conditions. The virus likes cool, damp and moist places.
A horse infected with EHV-1 develops an initial fever and then breaks with respiratory disease. The disease can take a rapid turn with a horse identified with a fever in the morning yet becomes neurologic in the afternoon, and is on death’s door by midnight. Even a mild fever is concerning (102 – 1030 F) and by checking temperatures twice daily, a febrile horse can be isolated quickly to protect others at a facility. That said, the recent 2025 outbreak showed an unusual departure from the norm: Some infected horses did not spike a fever despite being positive for the virus on nasal swabs – this makes it difficult to monitor for and to detect an active infection. In addition, even some of the EHM horses did not show a fever.
With a herpesvirus outbreak, it appears that the first (index) case is the most seriously affected – the horse goes down with EHM and is paralyzed. The next exposed horses experience a similar magnitude of infection but as time goes on, cases become less severe. Neurologic cases of EHV-1 usually happen in the first half of an outbreak. Outbreaks usually last 4 – 6 weeks. In the second half is when fevers become more prevalent with less severe neurologic signs.
If a horse leaves a premises at the initial time of outbreak, it is shedding virus. On the newly infected property, the same results occur as before, i.e. the first sick horses develop full blown EHM and then there is an eventual decrease in neurologic cases over time on the new premises.
EHV is a latent herpesvirus that is acquired by at least 80% of the horse population in their early years. It can reactivate, especially with stress such as is incurred with travel, competition, and intense exercise like racing and speed events. An affected horse can shed virus in nasal secretions and in the tear film despite no overt signs of disease. Testing at events has demonstrated that 80% of air samples and 20% of surfaces test positive for EHV-1 at events.
To kill virus on surfaces and garments…


