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Eastern equine encephalitis virus (EEE), commonly called Triple E or, sleeping sickness (not to be confused with Trypanosomiasis) is a zoonotic alphavirus and arbovirus present in North, Central and South America and the
Caribbean. EEE was first recognized in Massachusetts, United States in 1831 when 75 horses died mysteriously of viral encephalitis. Epizootics in horses have continued to occur regularly in
the United States. It can also be identified in asses and zebras. Due to the
rarity of the disease its occurrence can cause economic impact in relation to
the loss of horses and poultry. EEE
is found today in the eastern part of the country and is often associated with
coastal plains. It can most commonly be found in east and gulf coast states. In Florida about one to two human cases are
reported a year although over sixty cases of equine encephalitis are reported.
Some years in which there are favorable conditions for the disease there number
of equine cases are over two-hundred. Diagnosing equine encephalitis is challenging
because many of the symptoms are shared with other illnesses and patients can
be asymptomatic. Confirmations may require a sample of cerebral spinal fluid or
brain tissue although CT scans and MRI scans are used to detect encephalitis.
This could be an indication that the need to test for Eastern Equine
Encephalitis is necessary. If a biopsy of the cerebral spinal fluid is taken it
is sent to a specialized laboratory for testing.
· The causative agent, later identified as a togavirus was
first isolated from infected horse brains in 1933. In 1938, the first confirmed
human cases were identified when thirty children died of encephalitis in the
northeastern United States. These cases coincided with outbreaks in horses in
the same regions. The fatality rate in humans is 33% and there is currently no
cure for human infections. This virus has four variations in the types in
lineage. The most common to the human disease is Group 1 which is considered to
be endemic in North American and the Caribbean while the other three lineages,
Group IIA, IIB, and III are typically found in Central and Southern America
causing equine illness.
· These two clades may actually be distinct viruses. The NA strains appear to be monotypic
with a mutation rate of 2.7 × 10−4 substitutions/site/year (s/s/y). It appears to have diverged
from the SA strains 922 to 4,856 years ago. The SA strains are divided into two
main clades and a third smaller one. The two main clades diverged between 577
and 2,927 years ago. The mutation rate in the genome has been estimated to be
1.2 × 10−4 s/s/y.
· EEE is capable of infecting a wide range of animals including mammals, birds, reptiles and amphibians. The virus is maintained in nature
through a bird—mosquito cycle.
There are two mosquito species primarily involved in this portion of the cycle,
they are Culiseta
melanura and Cs.
morsitans. These mosquitoes feed on the blood of birds. The
amount of virus found in nature increases throughout the summer as more birds
and more mosquitoes become infected.
· Transmission of EEEV to mammals (including humans) occurs via
other mosquitoes, species that feed upon the blood of both birds and mammals.
These other mosquitoes are called bridge vectors because they carry the virus
from the avian hosts to other types of hosts, particularly mammals. The bridge
vectors include Aedes vexans, Coquillettidia
perturbans, Ochlerotatus
canadensis and Ochlerotatus
sollicitans. Ochlerotatus
canadensis also
frequently bites turtles.
· Humans, horses and most other infected mammals do not circulate
enough virus in their blood to infect additional mosquitoes. There have been
some cases where EEEV has been contracted through lab exposures or from
exposure of the eyes, lungs or skin wounds to brain or spinal cord matter from
infected animals.
· In humans, there are two ways in which
the virus can progress, systematic and encephalitic, depending on the person's
age. Encephalitic involves swelling of the brain and can be asymptomatic while
the systemic illness occurs very abruptly. Those with the systemic illness
usually recover within one to two weeks. While the encephalitis is more common
among infants in adults and children it usually manifests after experiencing
the systemic illness.Symptoms
include high fever, muscle pain, altered mental status, headache, meningeal irritation, photophobia, and seizures, which occur three to 10 days after the bite of an infected mosquito. Due to the virus's effect on the brain, patients who
survive can be left with mental and physical impairments such as personality
disorders, paralysis, seizures, and intellectual impairment.