EHV-1 in Horses: Respiratory vs Neurologic Signs and Show Biosecurity

EHV-1 in horses: California's quarantine is released

Update (2026-11-03): California has released the quarantine set after a neurologic case of EHV-1 in horses in Los Angeles County, because monitoring found no additional cases. EHV-1 is equine herpesvirus type 1, a common horse virus that usually causes a mild respiratory illness but can trigger a serious neurologic disease called EHM in some horses.

The Equine Disease Communication Center (EDCC) first posted the California Department of Food and Agriculture (CDFA) report on Oct. 23, 2026 (Alert 5368). The CDFA alerts page lists the quarantine as released on Nov. 2, and the EDCC posted the release on Nov. 3 (Alert 5408, outbreak 7834). Here's the final picture:

• Horse: 18-year-old Thoroughbred gelding, Los Angeles County
• Sign reported: ataxia (wobbly, uncoordinated movement)
• Diagnosis: EHM secondary to EHV-1, confirmed Oct. 23
• Status at the first report: alive, isolated at his home premises and recovering
• Exposed horses: 37 on the same premises, quarantined with enhanced biosecurity; none became a confirmed case
• Final count: 1 confirmed case, 37 exposed horses
• Quarantine: released; CDFA says the monitoring period was fully completed with no additional cases
• Not reported: vaccination status, onset date, length of the monitoring period

It stayed one confirmed case on one property, with no further confirmed cases. But the risk season is just starting: the ACVIM consensus says most EHM clusters happen from late autumn through spring, while Southern California's fall show and racing calendar is busy. Keep the thermometer habit and your barn's no-sharing rules going.

How a quarantine monitoring period ends

A quarantine isn't lifted on a set date. It ends when the exposed horses get through a monitoring period with no new cases, and state animal health officials decide how long that is and what testing it takes. CDFA didn't publish the length of this one; its notice dates the confirmation to Oct. 23 and the release to Nov. 2, and says no additional cases were confirmed and the quarantine period was fully completed.

The AAEP's EHV-1 control guidelines give owners a useful benchmark:

• The usual clock: keep isolation on a premises with a confirmed case for 28 days after the last suspected new infection. A new case starts the count again.
• A shorter option: about 21 days may be enough if no horse has had a fever, abortion or neurologic signs in that time and all exposed horses test negative on nasal swabs.
• What fills those days: temperatures twice a day, logged, with your vet testing any horse that runs a fever.
• The final word: state officials' rules on quarantine length and testing always come first.

A release also doesn't mean "no risk." The AAEP notes there is never an all-clear when horses mix, because EHV-1 is common and can hide in healthy-looking carriers. The quarantine ends; everyday biosecurity doesn't.

The earlier Maryland case

A month before the California case, on Sept. 24, the EDCC and The Equiery reported a confirmed respiratory EHV-1 case in a 4-year-old sport horse mare in Baltimore County, Maryland. Her signs began Sept. 20 and she was alive; her vaccination status was not reported. Together the two cases show both faces of the same virus.

What is EHV-1 in horses?

EHV-1 is a very common virus that most horses meet early in life. Usually it causes a mild respiratory illness, but in a small share of horses it can also cause abortion in pregnant mares, illness in newborn foals or a serious neurologic disease called EHM.

Like other herpesviruses, EHV-1 can hide. A horse that caught it years ago may carry the virus silently and start shedding it again when stressed, for example by hauling, a show or a change of barn. The 2024 ACVIM consensus statement cites recent studies estimating that 15% to 27% of horses carry the virus in this hidden form.

That is why a healthy-looking horse can bring EHV-1 home from a show. It's also why good habits matter at every event, not only when there's news.

Respiratory vs neurologic EHV-1 (EHM)

The same virus can look very different from horse to horse.

According to the AAEP, EHV-1 often causes a two-phase fever: one spike around day 1 or 2 after infection and a second around day 6 or 7. Respiratory cases usually don't cough much.

Signs of EHM, the neurologic form

With EHM there may be few respiratory signs, so fever above 101.5°F (38.6°C) is often the only warning. The AAEP lists these neurologic signs:

• Stumbling or poor coordination, especially behind (ataxia, the sign reported in the California gelding)
• Hind limb weakness or swaying
• Loss of tail tone (the tail feels limp when you lift it)
• Dribbling urine
• Dullness or lethargy
• Head tilt
• Leaning on a wall or fence for balance
• Being unable to get up

The ACVIM consensus notes that most EHM clusters happen in late autumn, winter and spring, and lists older age and being female as risk factors.

How EHV-1 spreads

EHV-1 spreads mainly through nasal secretions. The AAEP lists these routes:

• Nose-to-nose contact over stall walls, at the in-gate or on the trailer
• Shared water and feed buckets
• Tack, grooming tools and wipe rags used on more than one horse
• People's hands and clothes, the route most often forgotten
• Trailers that carried a shedding horse
• The air near a horse that is shedding the virus

The virus usually survives up to about 7 days in the environment, and up to a month in ideal conditions. The good news: most disinfectants kill it easily, as long as you clean off dirt and manure first.

Incubation is typically 4 to 6 days, but it can be as short as 24 hours. A horse can come home from a weekend show looking perfect and run a fever midweek.

Why fall show and racing season matters

Fall packs a lot of risk into a few weeks: championship shows, race meets, clinics, sales, and long hauls between states. Horses from many barns share aisles, wash racks and water sources, and travel stress can reactivate hidden virus in carrier horses.

The California case is a reminder that EHM can show up in an older horse at home, not only at an event. The ACVIM lists older age as a risk factor, so give your senior horses the same fever checks as your show string.

The AAEP says the classic way EHV-1 spreads is horses coming home from events, even events where no disease was reported. Check your show's health rules and entry requirements before you go; our Coggins test guide covers the paperwork side.

Horse show biosecurity checklist

Print this, tape it in the tack room and share it with your barn. For more packing ideas, see show prep.

Before you leave

• Take and record every horse's temperature for a few days so you know their normal
• Talk to your vet about whether your horse's vaccines are current for your plans
• Pack your own buckets, hay net, sponges, towels and a digital thermometer
• Don't travel a horse that has a fever or nasal discharge

At the show

• No nose-to-nose greetings with other horses, including friends' horses
• Never use communal water troughs; fill your own bucket from the hose without dunking the nozzle
• Don't share bits, sponges, rags or grooming tools
• Wash or sanitize your hands between your horse and anyone else's
• Take temperatures twice a day, morning and evening, and write them down
• Check the EDCC board and the show or track office for health notices before you unload, at every event this fall and winter

When you get home

• Isolate returning horses and new arrivals away from the herd, with their own buckets, tools and turnout
• Keep taking temperatures twice a day; the AAEP suggests at least 21 days of separation, and your vet can tailor that
• Handle and feed the isolated horses last, or wear separate boots and outer clothes
• Clean, then disinfect, trailers, buckets and tack
• Wash show clothes in hot water with detergent

What vaccination does and doesn't do

Vaccines for EHV-1 exist, and many vets recommend them for show and breeding horses. According to the AAEP, some can reduce nasal shedding and help protect against respiratory disease and abortion.

What they don't do: no current vaccine carries a label claim for preventing EHM, and the ACVIM consensus found no evidence that vaccination prevents it. A vaccinated horse can still get infected, so vaccinate as part of the plan, not as the whole plan.

Your vet will pick the product and schedule for your horse's age, job, travel and whether a mare is pregnant.

When to call the vet right away

Call your veterinarian immediately, and don't move horses on or off the property, if you see:

• A temperature above 101.5°F (38.6°C), especially after a show or a new arrival
• Wobbliness, stumbling or a weak, swaying hind end
• Dribbling urine or trouble urinating
• A limp tail or a horse leaning on the wall
• Nasal discharge with fever in a horse that recently traveled
• A pregnant mare that has been exposed or runs a fever

EHV-1 can only be confirmed with lab tests your vet runs, such as a nasal swab and a blood sample. Don't guess from signs alone, and let the vet decide on care. Keep a thermometer in your kit; our first aid kit checklist shows how to take vital signs.

What to watch next

• After the Los Angeles County release. That quarantine is closed with one confirmed case. Any new case in California would appear as a new alert from CDFA and the EDCC, not as part of this one.
• The EDCC alert board. Check equinediseasecc.org/alerts weekly this fall for new cases near you. One confirmed case is not a cluster.
• Your show's or track's announcements. Organizers and state animal health officials post any new entry rules.
• Other fall diseases. On the day of its EHV-1 report in September, Maryland also reported a fatal West Nile case in Cecil County; see our West Nile virus guide. Strangles was also reported at boarding barns in September, so read our strangles explainer.

The habit that protects against all of these is the same: a thermometer, a notebook and a no-sharing rule. For more everyday basics, visit health basics.