Eastern Equine Encephalitis
encephalitis virus is a mosquito-borne alphavirus, first recognized in humans in
1938. It cycles between Culiseta melanura mosquitoes and birds in freshwater
swampy areas. The virus is capable of infecting mammals, birds, amphibians and
reptiles. The virus causes severe illness in humans and horses, although both
are considered "dead-end" hosts for the virus, as the viral load is insufficient
to be transmitted to mosquitoes, furthering the spread of disease. Aedes spp. or
Coquillettidia spp. are the most likely mosquito vectors that pass the virus
from birds to humans. The virus can be found in the eastern, Gulf and
north-central areas of the United States. It is also in regions of Central and
South America and the Caribbean. Most activity occurs between May and August but
it can be seen throughout the year in Florida.
Symptoms develop 3-10 days after the bite of an infected mosquito and begin with
a sudden onset of fever, general muscle pains, and a headache of increasing
severity. Symptoms can become more severe over 1-2 weeks and infected
individuals will either recover or show onset of encephalitis characterized by
seizures, vomiting and focal neurological deficits. Severe encephalitic cases
often suffer from coma or death. Approximately one-third of people with
encephalitis caused by EEEV will die from the disease, making it one of the most
serious mosquito-borne diseases in the United States. Of those who recover, many
will suffer lasting effects.
There is currently no therapeutic treatment for EEE. Current methods consist
primarily of symptom treatment and supportive care. A vaccine has been developed
and is in use for horses, as the case-fatality in equines is 80-90%. A human
vaccine is currently under research.
People under the age of 15 or over 50 seem to be at greatest risk for severe
disease. Typically one or two human cases are reported each year in Florida
(range 0-5). The state averages over 70 reported cases of equine EEE each year.
In years when conditions favor the spread of the EEE, the number of reported
equine cases can exceed 200. EEEV is not believed to have the potential to cause
a human epidemic in Florida.
Human EEE Cases by Month in Florida, 1957-2011
For mosquito-borne disease prevention
For more information about EEE in horses
see FDACS Animal Industry:
Also see the Centers for Disease Control EEE page: