Breaking Down the 2019 Eastern Equine Encephalitis Outbreak in the US


According to CDC data, 34 cases of EEE were recorded in humans in 2019. In total, 94% of patients were diagnosed with encephalitis and 6% were diagnosed with meningitis.

Eastern equine encephalitis (EEE) virus made a comeback in the United States in 2019. The mosquito-borne illness landed on Contagion®’s list of top infectious disease outbreaks for the year.

EEE has been existence for centuries and 12 US-based epidemics were recorded between 1831 and 1959. Between 2003-18, an average of 8 EEE cases were reported annually in humans, but annual case counts rose dramatically in 2019.

The US Centers for Disease Control and Prevention (CDC) refers to the virus as the cause of “one of the most severe arboviral diseases in North America.” The latest issue of the CDC’s Morbidity and Mortality Weekly Report features more details on the 2019 outbreak in the United States.

As of October 15, 2019, 34 cases of EEE were reported to the CDC from 7 states. The highest number of cases were reported from Massachusetts.

CDC authors note that the virus usually presents as a systemic febrile illness but often leads to neurologic disease. The neuroinvasive disease has an estimated case fatality rate of 30% and half of survivors typically experience neurologic sequelae.

Among the 34 patients, 32 (94%) were diagnosed with encephalitis and 2 (6%) were diagnosed with meningitis. All patients were hospitalized and 12 (35%) died as a result.

According to the report, the onset of illnesses ranged from June 18—September 20, 2019. The majority of illness symptoms (62%) were detected in August.

Available information on the affected indicates that 26 (76) of patients were male. Patients ranged in age from 5-78 years with a median age of 64 years. Among the fatal cases, 10 (83%) of patients were male and the median age was 72 years. Deaths occurred a median of 12 days after onset of illness.

The authors report that the case-fatality ratio was 64% among individuals aged >70 years and was 22% among younger individuals.

“The risk for human infection in a given year depends on multiple factors, including weather, abundance of birds and mosquitoes that can transmit the virus, human behavior, and clinical awareness and diagnostic testing practices,” the authors write, indicating that it is not clear why a high number cases were recorded in 2019.

Currently, there are no available antiviral drugs or vaccines that are safe and effective against EEE. At this point in time, patients with EEE are treated with supportive care including intensive care and ventilator assistance. Social support and counseling are recommended for the patient and their family members due to the serious, and sometimes long-term, effects of the infection.

The report recommends that providers operating in areas at risk for EEE transmission should consider the mosquito-borne infection when aseptic meningitis and encephalitis are observed and proceed by obtaining appropriate serum or cerebrospinal fluid specimens for testing.

If a case is suspected, providers are instructed to report it to their state or local health department and send the specimen to confirm diagnosis. In the event of a suspected case, state and local officials may implement vector control to reduce the risk of further transmission.

“Because human vaccines against EEEV are not available, prevention depends on community and household efforts to reduce vector populations (eg, applying insecticides and reducing breeding sites) and personal protective measures to decrease exposure to mosquitoes (eg, use of repellents and wearing protective clothing),” the authors conclude.

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