What Is Eastern Equine Encephalitis: Causes, Symptoms And Prevention

What Is Eastern Equine Encephalitis

What Is Eastern Equine Encephalitis: Causes, Symptoms And Prevention


Eastern equine encephalitis (EEE), also referred to as triple E, is a viral illness transmitted to humans and horses by the bite of an infected mosquito.

There are two main types of encephalitis: primary and secondary. Primary encephalitis occurs when a virus directly infects the brain and spinal cord. Secondary encephalitis occurs when an infection starts elsewhere in the body and then travels to your brain.

About 5% of people who become infected may develop an extremely rare and potentially life-threatening infection that can cause swelling and inflammation of the brain, known as encephalitis, according to the Centers for Disease Control and Prevention (CDC).

What Are The Symptoms Of Encephalitis?

The symptoms of encephalitis can range from mild to severe.

Mild symptoms include:

  • Fever
  • Headache
  • Vomiting
  • Stiff Neck
  • Lethargy (exhaustion)

Severe symptoms include:

  • Fever of 103°F (39.4°C) or higher
  • Confusion
  • Drowsiness
  • Hallucinations
  • Slower movements
  • Coma
  • Seizures
  • Irritability
  • Sensitivity to light
  • Unconsciousness

Infants and young children show different symptoms. Call a doctor immediately if your child is experiencing any of the following:

  • vomiting
  • bulging fontanel (soft spot in the scalp)
  • constant crying
  • body stiffness
  • poor appetite.

As its name suggests, Eastern equine encephalitis tends to occur in the eastern portion of the United States, but the illness also occurs in the south along the Gulf Coast and in the Great Lakes region.

In the U.S., most cases are seen east of the Mississippi River between July and September, said Dr. Brian Chow, an infectious disease specialist at Tufts Medical Center in Boston. States with the highest numbers of reported cases of EEE between 2009 and 2018 include Florida, Massachusetts, New York, North Carolina and Georgia. Every year, cases begin popping up when the weather gets warm and mosquitoes thrive, from late spring to early fall, according to the CDC.

What causes Eastern Equine Encephalitis? 

Many different viruses can cause encephalitis. It’s helpful to categorize the potential causes into three groups: common viruses, childhood viruses, and arboviruses.

The EEE virus is transmitted by the bite of infected female mosquitoes. These mosquitoes are commonly found in marshlands and swampy areas and often feed on birds, Chow told Live Science.

Birds act as reservoirs for the EEE virus and can become infected without showing signs of the illness. These reservoirs then pass the virus on to mosquitoes, which can spread it to horses and other mammals, including humans.

Common viruses

The most common virus that causes encephalitis in developed countries is herpes simplex. The herpes virus typically travels through a nerve to the skin, where it causes a cold sore. In rare cases, however, the virus travels to the brain.

This form of encephalitis usually affects the temporal lobe, the part of the brain that controls memory and speech. It can also affect the frontal lobe, the part that controls emotions and behavior. Encephalitis caused by herpes is dangerous and can lead to severe brain damage and death.

Other common viruses that can cause encephalitis include:

  • Mumps
  • Epstein-Barr virus
  • HIV
  • Cytomegalovirus.

Childhood viruses

Vaccines can prevent the childhood viruses that used to cause encephalitis. Therefore, these types of encephalitis are rare today. Some childhood viruses that can cause encephalitis include:


Arboviruses are viruses carried by insects. The type of arbovirus that’s transmitted depends on the insect. Below are different types of arboviruses:

  • California encephalitis (also called La Crosse encephalitis) is transmitted through mosquito bites and mainly affects children. It causes few to no symptoms.
  • St. Louis encephalitis occurs in the rural Midwest and southern states. It’s generally a mild virus and causes few symptoms.
  • West Nile virus is most often found in Africa and the Middle East. However, it can occur in the United States. It’s usually relatively mild, causing flu-like symptoms. However, it can be fatal among older adults and people with weak immune systems.
  • Colorado encephalitis (also called Colorado tick fever) is transmitted by the female wood tick. It’s typically a mild disease, and most people will recover quickly.
  • Eastern equine encephalitis is spread by mosquitoes. It affects both humans and horses. Although rare, it has a 33 percent mortality rateTrusted Source.
  • Kyasanur forest disease is transmitted through tick bites. People can also get it by drinking raw milk from goats, sheep, or cows. Hunters, campers, and farmers are most at risk for getting this disease.


Symptoms of the mild form of the infection may resemble the flu, with a sudden high fever, chills and aches in the muscles and joints, and the illness may last up to two weeks.  The severe form of EEE also starts out with flu-like symptoms. But about four days after these symptoms first appear, signs of a brain infection may show up, Chow said. These symptoms may include confusion, excessive drowsiness and a bad headache, and the complications can worsen quickly, leading to seizures, coma and sometimes death.

Risk Factors For Encephalitis?

The groups most at risk of encephalitis are:

  • older adults
  • children under the age of 1
  • people with weak immune systems

You may also have a higher risk of getting encephalitis if you live in an area where mosquitos or ticks are common. Mosquitos and ticks can carry viruses that cause encephalitis. You’re more likely to get encephalitis in the summer or fall when these insects are most active.

Diagnosis And Treatment

Your doctor will first ask you about your symptoms. They may perform the following tests if encephalitis is suspected. Doctors can use a spinal tap (lumbar puncture), in which they analyze a sample of fluid surrounding the spinal cord, to detect whether the EEE virus is present. Physicians may also use other diagnostic tests, such as a CT scan or an MRI of the brain, to reveal signs of inflammation, as well as blood tests to detect high levels of antibodies, which are proteins produced by the immune system when it’s fighting a virus.

Although there is a vaccine to prevent Eastern equine encephalitis in horses, there is no vaccine to prevent the brain infection in people.  Treatment for the severe form of EEE usually involves a stay in the intensive care unit, where the patient is placed on a ventilator for breathing problems and may be put in a medically induced coma.

Medications may be given to treat seizures or help relieve brain inflammation. But once the virus enters the nervous system, there are often lingering neurological consequences, Chow said. Most of the deaths linked with EEE are due to swelling and pressure inside the brain and spinal cord.

Complications Associated With Encephalitis?

Most people who are diagnosed with severe encephalitis will experience complications. Complications resulting from encephalitis can include:

  • Loss Of Memory
  • Behavioral/personality changes
  • Epilepsy
  • Fatigue
  • Physical weakness
  • Intellectual disability
  • Lack of muscle coordination
  • Vision problems
  • Hearing problems
  • Speaking issues
  • Coma
  • Difficulty breathing
  • Death

Complications are more likely to develop in certain groups, such as:

  • older adults
  • people who have had coma-like symptoms
  • people who didn’t get treatment right away.

How To Avoid Eastern Equine Encephalitis

Mosquitoes have limited range and usually travel only a mile or two from where they breed. States where EEE activity is known to occur typically monitor and test the mosquito populations for the virus during warm-weather months and may spray insecticides in communities identified as hotspots for infected mosquitoes.

Spraying insecticide helps to reduce the threat of EEE, but it does not eliminate the risk of the illness. People should also take the following steps to protect themselves from mosquito bites:

  • Use insect repellent. Choose repellents with DEET, picaridin or oil of lemon eucalyptus. Permethrin is an insecticide that can be sprayed on clothing to ward off mosquitoes.
  • Limit outdoor activitiesbetween dusk and dawn, the peak feeding times for mosquitoes. When mosquitoes are most active, keep skin covered outdoors by wearing shoes, socks, long-sleeved shirts and long pants.
  • Remove standing water, which can be a breeding ground for mosquitoes, Chow said. Empty standing water from kiddie pools, flower pots and buckets.
  • Repair holes in screenson windows and doors.

Additional resources:

This article on "Hkitnob: Health Columns" is for informational purposes only, and is not meant to offer medical advice.