An Overview of Eastern Equine Encephalitis

Eastern equine encephalitis (EEE) is a rare but potentially life-threatening illness caused by the eastern equine encephalitis virus (EEEV). As the name implies, the virus is known for infecting horses. However, it can also infect other mammals—including humans.

EEEV is an arbovirus (as is West Nile virus) and is spread by mosquitos. The first cases of EEEV in the United States were identified in horses in 1831. The first cases in humans were not diagnosed until 1938.  

According to the Centers for Disease Control (CDC), most cases in humans have been found in Florida, Massachusetts, New York and North Carolina. Between 2009 and 2018, there have been 72 cases with at least one in a total of 21 states.

EEE is uncommon and fatal cases are rare, but if you live in an area where the virus is more likely to be found (endemic) it’s important to understand how it spreads and causes illness. There are steps you can take to reduce your risk of exposure to EEEV.  

You should also be aware of the signs and symptoms of EEE. While some cases of the infection are mild, the more severe form can be fatal.  

Symptoms

The incubation period for EEEV is usually four to 10 days. Many people who become infected with EEEV don’t have symptoms (are asymptotic). If they do begin to feel sick, the severity of the illness will depend on individual factors, such as their age and their overall state of health.

EEEV can cause two different forms of illness: systemic and encephalitic. The systemic form of the illness is generally less severe. When EEE reaches the brain (the encephalitic form) the resulting illness can be fatal.

The systemic form of EEE looks and feels like a mild case of the flu. Fever, chills, and body aches may appear as soon as four days after being bitten by a mosquito infected with the virus, though it may be up to 10 days before someone starts to feel sick. Symptoms usually don’t last longer than a week or two.

Systemic EEE Symptoms

• Fever 
• Chills 
• Fatigue 
• Body aches and joint pain 
• Headaches
• Nausea

The more severe form of EEE (encephalitic) also starts out with flu-like symptoms. However, in these rare cases, the virus migrates to the brain. When brain tissue becomes inflamed and swells, a person develops a high fever, headache, and vomiting. These symptoms tend to come on quickly and progress rapidly.  

Once the brain has been infected by EEE—usually within four days from when a person first started to feel sick—a person can become confused and disoriented, have seizures, or fall into a coma. 

Encephalitic EEE Symptoms

• Flu-like symptoms (usually before neurological symptoms)
• Sudden high fever 
• Headache 
• Stiff neck 
• Vomiting 
• Confusion 
• Sensitivity to light (photophobia) 
• Disorientation 
• Seizures  

As is usually true of most infectious illnesses, infants, seniors, and anyone with a compromised immune system are more likely to experience severe illness if they are infected with EEEV. People in these groups are also at greater risk of developing complications from EEE, including death.  

One out of every 3 people with the severe form of EEE will die. As the virus infiltrates the brain, the illness becomes rapidly and progressively worse. Once symptoms develop, a person may die within 10 days.

If someone survives a major EEE infection, they often have permanent symptoms and may become disabled. Neurological damage from the virus has been shown to cause physical and mental illness for those who survive the infection. 

The longterm health of these individuals is often compromised; many will die within a few years of getting sick with EEE.

EEE in Animals 

Unlike humans, horses can be vaccinated against EEE. Pet owners and people working or living with animals in places where EEEV is found should know the signs of EEEV in animals.

Humans cannot get sick with EEE from contact with an infected horse—the virus can only be spread to humans through the bite of an infected mosquito. However, an awareness of signs of EEE in animals can alert humans to the potential risk of contracting the virus if they are bitten by a mosquito.  

Symptoms of EEE in horses may include those seen in humans, such as fever and listlessness. Sickened animals can also develop more severe neurological symptoms including: 

• Blindness 
• Twitching 
• Trouble walking  
• Loss of bowel/bladder control 
• Paralysis, coma, and eventually death 

These symptoms may also be due to infections with other viruses that can cause brain swelling in animals. Any horse showing symptoms should be evaluated by a veterinarian and tested for EEEV if they are in a region where the virus has been identified.

EEEV can also affect other mammals, though it’s much less common. Rarely, EEE has been seen in domestic dogs bitten by infected mosquitoes. 

The symptoms of EEE in dogs are often the same as those in horses and humans, including fever and neurological symptoms such as seizures. Dogs may also refuse to eat, have uncoordinated movements, and display unusually aggressive behavior.

Causes

EEE is caused by a virus. In nature, the virus is typically found in freshwater swamps. When it first starts to spread to hosts, it relies on specific species of mosquito: Culiseta melanura and Culiseta mortisans.

From there, the virus passes from these mosquitos to the birds they feed on, most of which live in swampy areas. C. melanura, C. mortisans, and birds don’t directly infect humans with EEEV. 

The virus can only continue to spread if another species of mosquito that feeds on birds and mammals (such as Aedes) becomes a “bridge vector.” The transmission cycle can continue when one of these mosquitos bites an infected bird, then bites a horse or human.  

Once the virus finds a horse or human host, it doesn’t continue to spread. The levels of the virus in the body of a human or horse with EEEV aren't enough to infect more mosquitos, which would be necessary to continue the transmission cycle.

A person infected with EEEV can't spread it to another human and humans can't catch the virus from contact with a sick animal—with one possible exception.

In rare cases of EEE in emus, there may be an increased risk of transmission. Emus often experience bloody diarrhea from the infection which may be capable of spreading the virus. Animals or human caretakers who touch an emu’s infected blood or feces may be at risk for contracting EEEV. 

Cases of EEE are very rare in the United States. From 2009 to 2018 there was an average of seven cases reported each year (a range from three to 15). These cases were concentrated in states where EEEV is endemic, including:

• Massachusetts 
• North Carolina 
• New York 
• Michigan 
• Florida 

EEE in humans has also appeared sporadically throughout New England and the Gulf Coast. Maine, New Hampshire, and Georgia have also seen multiple cases in the last decade.

The exact incidence of EEE in the U.S. may be higher than the statistics imply. EEE is a notifiable disease and states are encouraged to inform the CDC of confirmed human cases. However, because most people who are infected with EEEV have no or only mild symptoms, they are unlikely to seek medical care and be tested for the virus.

Statistics indicate that EEEV is most active in the summer months (July, August, and September).

Cases of EEE affecting the brain are more frequent in males than females. According to the CDC, severe illness is most likely to occur in people younger than 15 or older than 50, as well as people who have compromised immune systems.

Outside of the U.S., cases of EEE have been found in animals throughout Central and South America as well as the Caribbean. While EEEV has been isolated in these regions, it's not uncommon for human cases to be tied back to someone who traveled to an area of the U.S. where the virus is endemic.

Diagnosis

If a person has symptoms of EEE and seeks medical attention, a doctor will start by ruling out more likely causes. For example, influenza and meningitis can cause the same symptoms as EEE, but these infections are much more common. Testing for these infections can usually be done with a simple blood test.

When more common conditions have been ruled out, doctors working in areas where EEEV is known to lurk may consider more invasive tests—especially if other cases of EEE have recently been reported.  

Inflammation and swelling in the brain may be visible on a CT scan or MRI. If a doctor has ruled out other explanations for these symptoms, the next step will be to test for EEEV.  

There may be signs of inflammation or EEEV antibodies present in a person’s blood and spinal fluid. If blood tests do not provide enough information, a doctor may need to perform a lumbar puncture (also called a “spinal tap”) to get spinal fluid for testing.  

During a spinal tap, a needle is inserted into the lower back. Then, a small amount of fluid from the spinal canal is drained into a vial which can be sent to a lab and examined for EEEV.  

Compared to more common pathogens, it can be difficult to determine if EEEV is present in a sample. Therefore, the sample must be tested in a lab that has the technology to examine it for EEEV antibodies. In states where EEE is known to occur, labs are more likely to be equipped to perform the tests. Healthcare providers can also work with the state health department or send samples to the CDC for testing.  

In some cases, a diagnosis of EEE may not be confirmed until after a person has died and their brain tissue tested as part of an autopsy.  

All confirmed cases of EEE need to be reported. Public health departments in each state collect data on infectious diseases and report it to the CDC each year.

Treatment

There is no cure for EEE and no antiviral medication specific for EEEV has been developed. As with other viral illnesses, antibiotics are not effective for treating EEE.  

The amount of medical care a person with EEE needs will depend on how sick they are, as well as their individual risk factors. Infants and older adults, those with another medical condition, or people with weakened immune systems are more likely to become seriously ill. By contrast, an otherwise healthy young adult may have mild symptoms that resolve on their own and don’t even warrant at a call to their doctor.  

People with the encephalitic form of EEE almost always require hospitalization. There, they can be tested for EEEV and receive medical care to ease their symptoms. They may need to be admitted to the intensive care unit and put on a respirator or into a medically induced coma to allow their body to try to fight the virus. 

While there may not be a cure, specific course of treatment, or vaccine, there are steps you can take to reduce your risk of contracting EEEV.

Intravenous fluids may be used to rehydrate someone, while antipyretics can reduce fever. Treatment with IVIg (intravenous immunoglobin) has yielded mixed results and needs more study. Case reports have variably shown steroids to help or hurt outcome, but theoretically can reduce inflammation and cerebral edema .

As with other illnesses spread by mosquitoes, using insect repellant and wearing protective clothing are the primary ways to reduce the risk of infection. If you live, work, or participate in outdoor recreation in states where EEEV has been identified, be especially mindful of when mosquitoes are most active. For example, in the summer months of the year and at dawn or dusk.

Making sure all windows in your home have screens and that your yard is free of standing water can also reduce your exposure to mosquitoes.

A Word From Verywell

Eastern equine encephalitis (EEE) is rare in humans. People infected with the virus may have no symptoms, mild symptoms, or severe life-threatening illness. Although EEE can also infect horses, people can't catch the virus from a horse or another mammal—including humans. One in three people who contract EEE will die and those who survive often become permanently disabled. There is no vaccine, cure, or specific treatment for EEE. However, there are ways to reduce your risk of contracting the virus. EEEV is only transmitted to humans by mosquitoes. You can protect yourself from mosquito bites by using insect repellent and wearing long sleeves when you're outside, especially in areas or at times of day when mosquitos are most active where you live.