As if COVID-19 wasn’t enough, now the equestrian community is dealing with EHV-1 breakouts in multiple US states and several European countries. So, what is EHV-1 and why is it so serious?

Equine Herpesvirus-1 (EHV-1)

Equine herpesviruses are DNA viruses that are found all over the world. Almost all horses have been infected with the viruses before the age of two, and have most of the time show no serious side effects. Once a horse is infected, the virus can become latent — or inactive — resulting in a carrier state with no external signs of disease. The latent virus can be reactivated during times of stress, such as with long-distance travel or strenuous exercise. Current estimates are that at least 60% of horses have a latent EHV-1 infection. EHV-1 can cause respiratory disease, abortion, and neonatal death. But it also can cause neurological disease, and that’s the variety that everyone is worried about.

The neurological form of EHV- is called equine herpesvirus myeloencephalopathy (EHM). EHV can cause inflammation of the blood vessels supplying the spinal cord and brain. The neurologic signs are a direct result of that inflammation and small blood clots that affect the nervous tissue. During an outbreak of EHM, usually only 10% of infected horses develop neurologic signs, but that percentage can be higher.

EHM Symptoms

Clinical signs typically begin with a fever greater than 101.5 F, which usually appears 4-9 days after infection, although some horses don’t show experience symptoms for as long as 14 days. Neurologic signs appear around 8-12 days post-infection, often after the fever breaks, and progress rapidly over 24 to 48 hours. According to the UC Davis School of Veterinary Medicine, horses with neurological disease caused by EHV-1 infection can quickly become uncoordinated and weak and have trouble standing. Difficulty urinating and defecating may also occur. Often the rear limbs are affected more severely than the front, which results in dog-sitting and toe-dragging. Signs of brain dysfunction may occur as well, including extreme lethargy and a coma-like state or head tilt. About 50% of horses with fevers develop neurologic signs and older horses are at a higher risk of developing EHM. Some horses may develop EHM even without any preceding fever and/or respiratory signs.

How is EHV-1 Spread?

All EHV-1 manifestations are highly contagious and spread by direct horse-to-horse contact via the respiratory tract through nasal secretions. Even scarier, the virus can also be spread indirectly through contact with physical objects that are contaminated with the virus including human contaminated hands or clothing, equipment and tack, trailers, brushes and other grooming equipment, and contaminated feed and water buckets.

Horses can shed EHV-1 (from the nose) for up to 3 weeks after infection. The air around the horse that is shedding the virus can also be contaminated with the infectious virus, and while it can be airborne, it is difficult to establish the distance the virus can spread in this manner under typical horse management and environmental conditions. According to Tuft’s Cumming School of Veterinary Medicine, EHV-1 does not survive well outside of the horse. Experimentally, the virus survived for up to a week at ambient temperatures when dried onto paper, wood or rope, and up to 35 days on horsehair or burlap.

How to Kill EHV-1

The virus is easily killed in the environment by most disinfectants, such as a a 1:10 dilution of bleach to water. To avoid spreading pathogens when handling horses that have been exposed to the virus, practice good hand hygiene. You should wash hands with soap and dry thoroughly or use alcohol-based hand sanitizer (sound familiar?). Clothing should be washed in hot water and dried in a dryer. Don’t forget about your shoes, too. Do not wear the same shoes used to care with an EHM horse to care for other horses.

What to do if your horse has been exposed

Infections other than EHV-1 can also spread by horse-to-horse contact, so keeping a horse with a fever isolated is always a good practice. If the horse is horse returning from any event and there have been other horses diagnosed with EHV-1, the horse should be isolated whether or not there is a fever and you should use different, dedicated equipment to feed, clean and work with them. Wear separate clothing and practice rigorous hygiene procedures.

Of course, you should work closely with your vet to determine the best approach to keeping your horses healthy, but typically, they recommend taking their temperature twice a day (you can miss fever spikes if taken only once per day). Elevation in body temperature is often the first and most common sign of an EHV-1 infection. If your horse has a fever of 101.5 degrees F or greater, consider having nasal swabs and blood drawn by your vet. Review the AAEP EHV Control Guidelines with your vet to develop a more detailed response plan.

What treatments are available?

Although there are vaccines against EHV-1, none are effective against EHM and there are no direct treatments, just supportive care. Anti-inflammatory drugs and intravenous fluids may be used, Banamine can help bring down the fever, and antibiotics may be used to treat secondary bacterial infections. Anti-viral and anti-coagulant drugs may help horses exposed to the virus to decrease the risk of developing EHM.

Where are the current Outbreaks?

• In Europe, the equine herpesvirus-1 outbreak began at an FEI jumping competition in Valencia, Spain, with cases spreading to Belgium, France, and Germany. The FEI calls it “probably the most serious EHV-1 outbreak in Europe for decades.” So far 11 horses have died and nearly 100 are sick.
• Maryland: On March 9, one horse at Laurel park has displayed neurological signs. Four barns are now under quarantine, which will last a for a minimum of 14 days pending further positive tests.
• Florida: Ocala had two cases of confirmed EHM in early. A third horse tested negative. The World Equestrian Center has resumed activities but implemented stricter requirements including health certification for horses competing.
• Utah: As of March 12, one horse, confirmed with EHM was competing at the Weber County Complex 8 days prior to becoming sick and later euthanized. Events at the complex have been canceled for the next two weeks and two more horses, previously at the complex have fevers and are being tested.
• Kentucky: On March 11, a thoroughbred filly has shown signs of EHM and is currently being tested; 27 exposed horses are being swabbed for PCR testing.
• New York: On March 11 two horses in Brewster, NY were confirmed as having EHM and two others tested positive for EHV-1. One of the four horses had competed at Old Salem Farm on March 5 and started showing symptoms on March 7th.
• California: On March 10 a gelding in Los Angeles has been confirmed positive for EHM

Of course there are always risks when you take your horse to competitions. Your best defense is to keep your horse vaccinated, practice good biosecurity and