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). From 2009 to 2018, between three (3) and fifteen (15) cases of EEE were reported annually in the U.S. 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.

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 virus, dengue, yellow fever, West 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.