Key Points

• Eastern equine encephalitis is caused by a virus spread to people through the bite of an infected mosquito.
• It is a rare but serious disease.
• People living in areas where eastern equine encephalitis virus circulates should protect themselves by preventing mosquito bites.

Overview

Eastern equine encephalitis virus is spread to people by the bite of an infected mosquito.

Only a few cases are reported in the United States each year. Most cases occur in eastern or Gulf Coast states.

Although rare, eastern equine encephalitis is very serious. Approximately 30% of people with eastern equine encephalitis die, and many survivors have ongoing neurologic problems. Symptoms of eastern equine encephalitis can include fever, headache, vomiting, diarrhea, seizures, behavioral changes, and drowsiness.

There are no vaccines to prevent or medicines to treat eastern equine encephalitis.

You can reduce your risk of infection with eastern equine encephalitis virus by preventing mosquito bites.

Key points

• Eastern equine encephalitis is caused by a virus usually spread to people through the bite of an infected mosquito.
• Eastern equine encephalitis virus circulates in the environment between mosquitoes and birds typically found in swamps.
• The virus can rarely be transmitted through solid organ transplantation.

Primary cause

Eastern equine encephalitis is caused by a virus that is found in North America and the Caribbean.

A closely related virus, Madariaga virus (formerly known as South American eastern equine encephalitis virus), causes disease primarily in horses in Central and South America.

How it spreads

• Eastern equine encephalitis virus circulates in the environment between mosquitoes and birds typically found in freshwater hardwood swamps.
• People and some other animals (e.g., horses and emus) become infected often by different mosquitoes (known as bridge vectors) that feed on both birds and animals.
• People and horses are considered "dead-end" hosts. This means they do not spread the virus, even though they get sick, to mosquitoes that bite them.
• Eastern equine encephalitis virus was transmitted through organ transplantation involving one organ donor and three organ recipients.

Key points

• Eastern equine encephalitis is caused by a virus in the genus Alphavirus.
• The virus is maintained in the environment by Culiseta melanura mosquitoes and avian hosts.
• Humans and other animals are typically infected by bridge vectors and are considered dead-end hosts.
• Rare transplant-associated transmission of eastern equine encephalitis virus has been documented.

Virus

Eastern equine encephalitis virus is a member of the genus Alphavirus, family Togaviridae.

Eastern equine encephalitis virus has a single-stranded, positive-sense RNA genome. The virus particles are spherical and have a diameter of 60–65 nm.

There are four lineages of the eastern equine encephalitis virus antigenic complex.

• Group I consists of only eastern equine encephalitis virus and is endemic in North America and the Caribbean and causes most human disease cases.
• The other three groups (IIA, IIB, and III) are caused by related Madariaga virus (formerly known as South American eastern equine encephalitis virus) and cause primarily equine illness in Central and South America.

Transmission

Eastern equine encephalitis virus is maintained in a cycle between Culiseta melanura mosquitoes and avian hosts in freshwater hardwood swamps. Cs. melanura is not considered to be an important vector of eastern equine encephalitis virus to humans because it feeds almost exclusively on birds. Transmission to people requires another mosquito species to create a "bridge" between infected birds and uninfected mammals, such as people or horses. Most of the bridge species are within the Aedes, Coquillettidia, and Culex genera. Eastern equine encephalitis virus has been documented to be transmitted through solid organ transplantation, with one organ donor transmitting the infection to three organ transplant recipients.

Eastern equine encephalitis virus can cause a febrile illness or neurologic disease, including meningitis or encephalitis.

Most people infected with eastern equine encephalitis virus do not develop symptoms. For those who develop symptoms, the time from an infected mosquito bite to onset of illness (incubation period) ranges from 4 to 10 days.

Eastern equine encephalitis virus infection can result in febrile illness (fever) or neurologic disease. The type of illness will depend on the age of the person and other factors.

Febrile illness is characterized by fever, chills, body aches, and joint pain. The illness lasts 1 to 2 weeks, and most people recover completely when there is no central nervous system involvement.

Neurologic disease can include meningitis (inflammation of the membranes that surround the brain and spinal cord) or encephalitis (inflammation of the brain). Signs and symptoms of neurologic disease include fever, headache, vomiting, diarrhea, seizures, behavioral changes, drowsiness, and coma. In infants, neurologic disease often occurs soon after onset; in older children and adults, encephalitis might occur after several days of systemic illness.

• Approximately a third of all people with eastern equine encephalitis die. Death usually occurs 2 to 10 days after onset of symptoms but can occur much later.
• Many people who recover are left with long-term physical or mental problems. These can range from mild to severe intellectual disability, personality disorders, seizures, paralysis, and cranial nerve dysfunction.
• People with severe disease and ongoing disabilities often require long-term care and die within a few years.

If you think you or a family member might have eastern equine encephalitis, talk with your healthcare provider.

If you think you or a family member might have eastern equine encephalitis, talk with your healthcare provider.

Healthcare providers diagnose eastern equine encephalitis based on:

• Signs and symptoms
• History of living in or traveling to an area where EEE virus is known to circulate
• History of possible exposure to the mosquitoes that can carry eastern equine encephalitis virus
• Laboratory testing of blood or spinal fluid

Your healthcare provider can order tests to look for eastern equine encephalitis virus infection or other infections that can cause similar symptoms.

Key points

• Serologic testing is the primary method for diagnosing eastern equine encephalitis.
• A positive eastern equine encephalitis virus-specific immunoglobulin (Ig) M test result should be confirmed by neutralizing antibody testing at a state public health laboratory or CDC.
• Contact your state or local health department for assistance with diagnostic testing.

Considerations

Serologic testing remains the primary method for diagnosing eastern equine encephalitis. However, molecular testing (e.g., reverse transcription-polymerase chain reaction, RT-PCR) might be appropriate for patients who are immunocompromised.

Eastern equine encephalitis virus is difficult to isolate from clinical samples; almost all isolates (and RT-PCR positive samples) have come from brain tissue or cerebral spinal fluid (CSF).

Eastern equine encephalitis is a nationally notifiable condition. All cases should be reported to state or local public health authorities. Reporting can assist local, state, and national authorities in recognizing outbreaks of this rare disease and instituting control measures to prevent further infections.

Recommended tests

Combined with a consistent clinical presentation in an endemic area, a rapid and accurate diagnosis of eastern equine encephalitis can be made by the detection of eastern equine encephalitis virus-specific IgM antibodies in serum or CSF.

• Eastern equine encephalitis virus IgM testing is available commercially, at some state health departments, and at CDC.
• A positive eastern equine encephalitis virus IgM test result should be confirmed by neutralizing antibody testing at a state public health laboratory or CDC.
• Some patients who are severely immunocompromised might require molecular (e.g., RT-PCR) testing for diagnosis.

To submit specimens for testing, please contact your state or local health department. They can assist you with determining if samples should be sent to the CDC Arbovirus Diagnostic Laboratory for further testing. Specimens should be submitted to CDC through state health departments. All results will be sent from CDC to the appropriate state health department.

Key points

• There is no specific treatment for eastern equine encephalitis; clinical management is supportive.
• Monitor patients closely for severe complications.
• Counsel patients about using personal protective measures to prevent mosquito bites.

Treatment

There is no specific treatment for eastern equine encephalitis; clinical management is supportive. Patients with severe meningeal symptoms often require pain control for headaches and antiemetic therapy and rehydration for associated nausea and vomiting. Patients with encephalitis require close monitoring for the development of elevated intracranial pressure, seizures, and inability to protect their airway.

No eastern equine encephalitis virus vaccines are available for use in humans. In the absence of a vaccine, prevention of eastern equine encephalitis depends on personal protective measures to decrease exposure to infected mosquitoes. This includes using insect repellent, wearing long-sleeved shirts and pants, treating clothing and gear with 0.5% permethrin, and taking steps to control mosquitoes indoors and outdoors.

Eastern equine encephalitis virus has been documented to be transmitted through solid organ transplantation and likely can be transmitted through blood transfusions. People recently diagnosed with eastern equine encephalitis should not donate blood for 120 days (4 months) after their illness.

Eastern equine encephalitis virus infections temporally associated with organ transplantation or blood transfusion should be reported promptly to the appropriate state or local health department.