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What is Eastern Equine Encephalitis Virus and How Does it Spread?

Eastern equine encephalitis virus (EEEV) is a zoonotic alphavirus and arbovirus, and was first recognized in horses in 1831 in Massachusetts. The first confirmed human cases were identified in New England in 1938. EEEV is present today in North, Central and South America, and the Caribbean. In rare cases, those that contract the virus will develop the serious neuroinvasive disease, Eastern equine encephalitis (EEE). EEE may also be commonly referred to as Triple E or sleeping sickness. EEEV is a vector-borne disease that is transmitted to humans through the bite of an infected mosquito. Culiseta melanura is the primary vector among birds, but this mosquito species does not typically feed on humans. It is believed that EEEV is mainly transmitted to humans and horses by bridge vectors that have contracted the virus by feeding on infected birds. Bridge vector species of mosquitoes may include Coquillettidia pertubans, Aedes sollicitans, and Ochlerotatus canadensis. The risk of contracting the EEE virus is highest during the summer months, and those who live and work near wetland and swamp areas are at higher risk of infection. EEEV is only spread to humans via mosquito bite, and cannot be transmitted directly by other humans or horses. There is an EEEV vaccine available for horses, and owners are encouraged to discuss vaccination with their veterinarian.

icon-thermometer Symptoms of EEE Infection
  • Symptoms typically occur four to ten days after a bite from an infected mosquito and include fever, headache, vomiting, muscle aches, joint pain, and fatigue.
  • In rare cases, infection occurs in the brain and spinal cord leading to sudden high fever, stiff neck, disorientation, seizures, and coma.
  • The mortality rate of those that develop EEE is about 33%, the highest among arboviruses transmitted in the U.S.
icon-treatment Treatment of EEE Infection
  • There is no human vaccine for EEE or anti-viral drugs for treatment of EEE.
  • For severe illnesses, supportive treatment includes hospitalization, IV fluids, and respiratory support.

Eastern Equine Encephalitis: A Rare but Serious Neuroinvasive Disease

ICON-risk_easter_equine_encephalitis_EEE.pngEEE is a rare but very serious disease that involves inflammation and swelling of the brain. Fortunately, only 5% of human EEEV infections result in EEE. However, one out of three people who develop EEE will die, and many survivors have mild to severe brain damage.

Of those who contract the EEE virus, the elderly (ages 50 and older) and young (ages 15 and younger) are at the greatest risk of developing encephalitis.

Eastern Equine Encephalitis and the United States

Between four (4) and thirty-eight (38) cases of EEE were reported every year in the U.S. from 2011 to 2020.

Massachusetts (26), Michigan (18), Florida (9), Georgia (7), and North Carolina (7) reported the highest number of cases during this time period.

Source: ArboNET, Arboviral Diseases Branch, Centers for Disease Control and Prevention

A Global View of Eastern Equine Encephalitis

EEE affects areas throughout North and South America, with outbreaks occurring mainly in the eastern coastal areas of the United States and Canada, the Caribbean, and Argentina.

Know Your Mosquitoes

In the U.S., Culiseta melanura is the mosquito responsible for the spread of EEEV in the mosquito-bird-mosquito cycle. Known as the black-tailed mosquito, Cs. melanura can be found in swamps from the Great Lakes and Maine to southern Florida and southeastern Texas. It is distinguished by its unusually long, curved dark-scaled proboscis. This mosquito is also unique because it overwinters as larvae, as opposed to most mosquito species that overwinter as adults or eggs.

EEEV is mainly transmitted to humans by bridge vectors that contract the virus by feeding on infected birds. Bridge vectors may include Aedes, Coquillettidia, and Culex species.

Aedes mosquitoes have distinct black and white markings on their body and legs. They bite during the daytime only, with the highest levels of activity occur in the early morning and evening hours. Members of the Aedes genus are known vectors of EEE, Zika virusdengueyellow feverWest Nile virus, and chikungunya.

Coquillettidia mosquitoes have slender bodies and long legs. They are commonly found in humid, low-lying areas that have warm summer and lots of vegetation. In addition to acting as vectors for EEE, Coquillettidia mosquitoes are also known to transmit West Nile virus to humans.

Culex mosquitoes are brown with whitish markings on the abdomen. They typically bite at dusk and at night, and are known to vector several diseases including EEE, West Nile virus, Japanese encephalitis, St. Louis encephalitis, and avian malaria.

Controlling Mosquitoes and Eastern Equine Encephalitis

An Integrated Mosquito Management (IMM) program is essential to helping prevent mosquito bites and transmission of serious vector diseases in the United States.  As part of an effective IMM program, VDCI recommends a 4-pronged approach to target all phases of the mosquito’s life cycle.

1:  Public Education

Mosquito control professionals can only do so much, and this is why we rely on a well-educated public in order to have a successful mosquito control program. Educating the public empowers people to take control of the mosquitoes breeding in their back yard and gives them the tools needed to reduce mosquito annoyance.

2:  Surveillance 

Surveillance allows us to detect mosquito species in a given area as well as any changes in populations. With this valuable data, we are able to more effectively time larvicide applications and more accurately target adulticide activities.

3:  Larval Mosquito Control

Our trained field technicians inspect both known sources of standing water and any newly discovered sites for the presence of mosquito larvae. Eliminating mosquitoes prior to their becoming adults is an important element of controlling EEE and other mosquito-borne diseases because it stops mosquitoes before they acquire the virus and have the opportunity to transmit it to people.

4:  Adult Mosquito Control

When necessary, adulticide applications are conducted with EPA-approved pesticides that are used in the safest and most environmentally sound way possible. Additionally, VDCI regularly tests adult mosquitoes and takes all appropriate measures to prevent them from developing resistance; thereby, minimizing the number of applications needed to control the population.


VDCI is a company built on the foundations of public health, ethics, professionalism, and technical expertise. We establish vector management programs that are based on an understanding of the underlying vector’s ecology and rooted in the current science of environmentally sound control measures.