Equine Herpesvirus 1 (EHV-1)

EHV is spread from horse to horse through contact with nasal discharge or spread as aerosol droplets in shared spaces (aisles, arenas, trailers). Horses can also contract the virus by coming in contact with contaminated surfaces such as shared stalls, water, feed, tack, and transport vehicles. Humans can spread the virus from horse to horse through contaminated hands, clothing and equipment. The virus can survive from minutes to hours in sunlight and up to 21 days in cool, damp, shaded areas and water sources.

• Fever >101.5°F (often the first sign)
• Lethargy
• Reduced appetite
• Nasal discharge
• Cough
• Stocking-up (swollen legs)

Call your veterinarian immediately if any of these signs appear.

Equine Herpesvirus Myeloencephalitis (EHM), the neurologic form of EHV-1, is comparatively rare however thirty-three cases in six states have been associated with the November 2025 outbreak as of December 3, 2025.

• Wobbly
• Weak hind end
• Toe dragging/scuffed shavings
• Difficulty rising/“dog-sitting”
• Urine dribbling
• Behavior changes
• Head tilt
• Seizures

EHV-1 is diagnosed by PCR testing of nasal swab and whole blood samples. EHV-1 is a reportable disease because it can not only affect your horse but the health of horses around you. If your horse is experiencing neurological signs of EHM use the large animal neurologic disease reporting tool to submit information to the Arizona State Veterinarian’s office to help protect Arizona equids.

Supportive care and rest are the primary treatments. Non-steroidal anti-inflammatory medications are used to control fever, pain, and inflammation. In some cases, antivirals are used. Severe neurologic cases may require hospitalization and intensive care.

Ranges from 2 to 10 days. Horses can shed the virus during the incubation period, before they develop clinical signs. Horses with clinical signs (nasal discharge, neurologic signs) should be considered contagious until cleared by their veterinarian through testing, quarantine, or both.

Infected equids become lifelong carriers and can intermittently shed the virus even when showing no clinical signs. This is particularly true with donkeys and mules who tend to be asymptomatic carriers. It is thought that most horses become infected with EHV early in life. Reactivation and viral shedding can occur periodically, especially during stressful events such as travel, illness, etc.

Although infection with EHV-1 is lifelong, most horses make a full clinical recovery from the respiratory form in 1-2 weeks. Horses with EHM may take longer to recover and neurological signs may never fully resolve. Mortality rates for horses with EHM can range from 15-30%.

The most effective prevention is implementing strong biosecurity practices both at home and while traveling. When at home take temperatures twice daily for any exposed horses as well as for all horses returning from travel. Isolate new arrivals for 14 days (including both new horses and horses returning to a farm after travel). Do NOT share tack, buckets, rags, or grooming tools. Disinfect stalls, walls, gates, doorknobs, etc. Limit visitors and prevent horse-to-horse contact. Take care of healthy horses first, and after working with sick animals change clothes/disinfect prior to returning to healthy ones. When traveling, only haul healthy, afebrile horses. Bring your own labeled buckets. Do NOT submerge hoses. Keep horses separated in alleys, arenas, and tie areas. Avoid communal water troughs entirely. Disinfect tack and grooming tools daily. Monitor temperatures before, during, and after travel.

There is a vaccination available to help prevent the respiratory form of EHV-1 however there is currently no vaccine labeled for the prevention of the neurologic form of EHV-1 (EHM). The American Association of Equine Practitioners (AAEP) recommends horses that travel or have high movement receive the vaccine with booster vaccinations every six months. The currently available vaccine can decrease viremia and nasal shedding, which helps reduce spread and lowers herd-level risk. Consult your veterinarian to determine the appropriate vaccinations and schedule for your horse(s).

Compliance with federal interstate movement and state import requirements is important from a population health standpoint in encouraging responsible participation in organized equine events. Valid Interstate Certificates of Veterinary Inspection (ICVI) are required for virtually all movement of equids across state lines and typically also a state import requirement must be adhered to. This is important because it documents that a veterinarian has examined those specific horse(s) and verified on that day that there were no obvious signs of infectious or contagious disease. By everyone doing their part, including event organizers verifying health documentation on entry to a show for out-of-state participants, or even requiring in-state participants to produce an ICVI, we can promote responsible participation of healthy horses in organized events. If you will be traveling across state lines, notify your veterinarian well ahead of your travel dates so that they can schedule your horse(s) for their ICVI exam and ensure you will have the necessary documentation.

• News and ALERTS from the Arizona Department of Agriculture
• Biosecurity guidance and instruction for taking horse vital signs available from the Informed Arizona Equestrian