Sleeping sickness in horses is a common name for equine encephalomyelitis, a group of viral infections that cause severe inflammation of the brain and spinal cord. The name comes from the drowsiness and depression that infected horses display as the virus attacks their nervous system. Three main types exist: Eastern equine encephalomyelitis (EEE), Western equine encephalomyelitis (WEE), and Venezuelan equine encephalomyelitis (VEE). Of these, EEE is by far the most dangerous, killing the majority of horses it infects.
The Three Types and Where They Occur
EEE is the most common and lethal form. It occurs primarily in the eastern United States and Canada, though cases have appeared further west in recent years. Horses infected with EEE often do not survive.
WEE is far milder, with survival rates of 70 to 80 percent in infected horses. It has not been detected in the United States since the late 1990s, making it a minimal concern today.
VEE is found in Central and South America, with occasional cases in Mexico. Death rates from VEE are variable but can reach as high as 90 percent in some outbreaks, putting it on par with EEE in severity.
How Horses Get Infected
These viruses spread through mosquito bites, not direct contact between animals. The virus cycles naturally between mosquitoes and wild birds in freshwater swamp environments. A specific mosquito genus feeds almost exclusively on birds, keeping the virus circulating in that population. Horses (and people) only get infected when a different mosquito species acts as a “bridge,” biting an infected bird and then biting a mammal.
Horses are what scientists call dead-end hosts. Although they become seriously ill, the amount of virus in their blood is not high enough to pass the infection on to new mosquitoes. This means a sick horse cannot spread the disease to other horses, other animals, or people. Every case requires its own mosquito bite from the bird-mosquito cycle.
Symptoms and How Quickly They Progress
After a mosquito bite delivers the virus, there is an incubation period of roughly 4 to 10 days before any signs appear. The first symptoms are often a high fever, loss of appetite, and general depression. At this early stage, a horse may simply look “off” or lethargic, which is where the name sleeping sickness originates.
Within one to two days of the initial fever, neurological signs can develop rapidly. These include:
- Head pressing: the horse pushes its head against a wall or fence, a hallmark sign of brain inflammation
- Circling or aimless wandering
- Difficulty swallowing
- Loss of coordination and stumbling
- Seizures
- Paralysis, often starting in the hind legs
- Inability to stand, leading to recumbency (lying down and unable to rise)
Once a horse goes down and cannot stand, the prognosis is extremely poor. With EEE, death typically occurs 2 to 10 days after symptoms begin. Horses that survive any form of equine encephalomyelitis frequently have lasting neurological damage, including personality changes, ongoing seizures, or permanent coordination problems.
How It Is Diagnosed
A veterinarian will suspect sleeping sickness based on the combination of fever and rapidly worsening neurological signs, especially during mosquito season in an area where the virus is known to circulate. Blood testing is the primary method of confirmation. The lab looks for specific antibodies the horse’s immune system produces in response to the virus. In some cases, cerebrospinal fluid (the liquid surrounding the brain and spinal cord) is also tested.
The virus itself is very difficult to isolate from a living animal. Most confirmed viral samples come from brain tissue, which generally means diagnosis in a living horse relies on the antibody tests rather than detecting the virus directly.
Treatment Options
There is no antiviral medication that kills equine encephalomyelitis viruses. Treatment is entirely supportive, meaning the veterinary team tries to keep the horse alive and comfortable while its immune system fights the infection. This typically involves intravenous fluids to prevent dehydration, medications to reduce inflammation and fever, and anti-seizure drugs if needed.
Horses that cannot stand may need sling support to keep them upright, since large animals that stay down for extended periods develop serious complications from their own body weight compressing muscles and restricting blood flow. Even with aggressive supportive care, survival rates for EEE remain very low. The horses that do recover often require months of rehabilitation and may never return to their previous level of function.
Vaccination and Prevention
Vaccination is the single most effective defense against sleeping sickness and is widely available. Combination vaccines that protect against EEE and WEE (and sometimes VEE and West Nile virus) are part of the core vaccination schedule recommended for all horses in North America. Most veterinarians administer these annually, with horses in high-risk areas or warm climates sometimes receiving a booster every six months to maintain protection through a longer mosquito season.
Foals typically receive their first vaccinations starting at 4 to 6 months of age, with a booster series to build full immunity. Previously unvaccinated adult horses also need an initial two-dose series before annual boosters are effective.
Beyond vaccination, reducing mosquito exposure helps lower risk. Eliminating standing water around barns, water troughs, and pastures removes mosquito breeding sites. Fans in stalls disrupt mosquitoes’ ability to land on horses. Stabling horses at dawn and dusk, when mosquitoes are most active, also reduces exposure. Mosquito repellents approved for use on horses provide an additional layer of protection, though they should not be relied on as the sole preventive measure.
Risk to Humans
The same mosquito-borne viruses that cause sleeping sickness in horses can also infect people. You cannot catch the virus from a sick horse, since it requires a mosquito bite from the bird-mosquito cycle. In humans, EEE is rare but extremely serious when it does cause brain inflammation. The same mosquito control and personal protective measures that reduce risk for horses, like avoiding outdoor activity at peak mosquito hours and using insect repellent, apply to people living in the same areas.