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Eastern Equine Encephalitis Virus

Click here for an update on the status of EEE virus in Indiana.

Eastern equine encephalitis (EEE) virus is the most dangerous mosquito-borne virus that is naturally present in the state of Indiana. EEE is primarily transmitted in Indiana by mosquitoes in the genus Coquillettidia. People infected with EEE virus can develop severe inflammation in the brain or in the membranes surrounding the brain and spinal cord. Only a few cases of EEE virus disease are reported in the United States each year. Most occur in Eastern or Gulf Coast states. There were regional outbreaks of EEE virus disease in the US in 2019 and 2020.

Eastern equine encephalitis virus. Centers for Disease Control and Prevention.

Eastern equine encephalitis virus. Centers for Disease Control and Prevention.

  • Transmission

    EEE virus is maintained in nature via transmission among songbirds by Culiseta melanura mosquitoes, which breed in sphagnum bogs. Culiseta melanura is not considered to be an important vector of EEE virus to people because it feeds almost exclusively on birds.

    Transmission to people requires a mosquito species with a wider variety of feeding preferences to create a bridge between infected birds and uninfected mammals, such as people or horses. In Indiana, the bridge vector that is primarily responsible for transmission of EEE virus to people is the Coquillettidia perturbans mosquito, but some mosquitoes in the genus Aedes may also play a role. Coquillettidia perturbans breeds in cattail marshes and has a flight range of up to 5 miles.


    The Indiana Department of Health  (IDOH) considers 13 Northern Indiana counties to be at risk for EEE virus transmission due to the presence of suitable habitat for the vector mosquitoes: Adams, Allen, DeKalb, Elkhart, Kosciusko, LaGrange, Lake, LaPorte, Marshall, Noble, Porter, Steuben, and St. Joseph. IDOH and the Indiana State Board of Animal Health conduct enhanced surveillance for EEE virus activity in mosquitoes, horses, and people in these counties.

  • Signs and Symptoms

    Signs and symptoms of EEE virus disease usually appear within 4 to 10 days after a bite from an infected mosquito. EEE virus infection can result in a systemic (flu-like) illness, or it can attack the nervous system, causing inflammation in the brain or the membranes that surround the brain and spinal cord. Some people who become infected with EEE virus may not develop any symptoms.

    Symptoms of systemic EEE virus infection appear abruptly and include fever, chills, body aches, joint pain, and fatigue. People with systemic EEE virus infection are usually sick for 1 to 2 weeks and can recover completely. In some older children and adults, systemic EEE virus infection can progress to infection of the nervous system; in infants, nervous system infection can occur without any signs of systemic illness first. Patients with nervous system infection can have fever, headache, vomiting, diarrhea, seizures, behavioral changes, drowsiness, and coma. People who are younger than 15 years and older than 50 years are at the greatest risk of severe disease if infected with EEE virus.

    Approximately one in three cases of EEE virus infection of the nervous system are fatal. Death usually occurs 2 to 10 days after onset of symptoms but can occur much later. Many people who recover will experience severe, permanent complications. These can include seizures, paralysis, mild to severe brain dysfunction, severe intellectual impairment, and personality disorders. Many patients with complications require long-term care and die within a few years.

  • Diagnosis

    Diagnosis of EEE virus disease is based on the patient’s signs and symptoms and appropriate laboratory testing. If you think you have EEEV disease, contact your healthcare provider.

  • Treatment

    No specific medication is available to treat EEE virus disease. People with severe illness usually require hospitalization, supportive care, and/or rehabilitation.

  • Prevention

    The best way to prevent EEE virus disease is to avoid mosquito bites. See our mosquito prevention page for more information.

    If evidence of widespread EEE virus activity is detected in Northern Indiana, IDOH may coordinate aerial application of pesticide as an emergency measure to reduce the risk of EEE virus infection. IDOH will not initiate aerial pesticide application in any jurisdiction without the explicit invitation and consent of county commissioners and Tribal representatives in affected areas.

  • Resources
    Information for Providers

    For more information about Eastern equine encephalitis virus, visit the CDC EEE virus webpage.

    For diagnosis, treatment, and testing information, click here.

    In accordance with changes to Indiana’s Communicable Disease (CD) Rule (410 IAC), as of April 1st EEE virus is reportable immediately upon suspicion. Visit the Communicable Disease Reporting page for more information.

    Health Advisory on Arthropod-Borne Viral (Arboviral) Disease

Page Last Updated: August 30, 2023