Alphaviruses are
RNA-containing viruses that cause a wide variety of
mosquito-transmitted disease.
Three
arthropod-borne alphaviruses, western equine encephalitis viruses,
eastern equine encephalitis viruses and Venezuelan equine
encephalitis viruses are the aetiological agents of a sometimes severe
encephalomyelitis in equines and humans in the New World.
Eastern Equine Encephalitis
Eastern equine
encephalitis (EEE) is a mosquito-borne viral disease.
Eastern equine
encephalitis virus ( EEEV ) is a member of the family Togaviridae,
genus Alphavirus.
Eastern equine encephalitis (EEE) is
encountered primarily in the eastern and north central United States
and adjoining provinces of Canada.
The reservoir is birds. Horses and
humans have “dead-end” infections.
Mosquitoes are vectors.
It takes from 3
to 10 days to develop symptoms of EEE after being bitten by an
infected mosquito.
Epizootics among horses usually precede
epidemics in humans.
EEE occurs primarily during summer in
temperate zones.
Symptoms
range from mild flu-like illness to
Eastern equine
encephalitis (inflammation of the brain), coma and death.
The diagnosis of
EEEV infection is serology, especially IgM testing of serum and
cerebrospinal fluid.PCR,
histopathology with immunohistochemistry, and virus culture of autopsy
tissues are useful in fatal cases.
Western equine
encephalitis
Western equine encephalitis (WEE) is
encountered primarily in the western and central United States and
Canada (it is also found, less frequently, in Brazil).
The reservoir is birds, but horses and
humans have “dead-end” infections.
Vectors are mosquitoes, WEE occurs
primarily in May to September.
Symptoms range from
mild
flu-like illness to frank encephalitis, coma and death.
Venezuelan Equine Encephalitis
Venezuelan equine encephalitis (VEE) is
encountered principally in Venezuela, Colombia, Central America,
Mexico, and the southwestern United States.
The reservoir is rodents, many other
mammals, and birds. Horses and humans have “dead-end” infections.
Vectors are mosquitoes.
VEE occurs throughout the year.
Clinical
presentations:
Influenza-like manifestations ; abrupt onset of severe headache,
chills, fever, myalgia, retro-orbital pain, nausea and vomiting ;
conjunctival and pharyngeal injection ;
Most cases are
mild with symptoms lasting for 3 - 5 days ; Some cases
have diphasic fever.
CNS involvement -
encephalitis with disorientation, convulsions, paralysis, coma and
death
In many cases
convulsions are often seen in children. Abortions and fetal deaths
occur in pregnant women infected with VEE virus.
Further reading:
Venezuelan
equine encephalomyelitis. In: Monath TP, ed. The arboviruses:
epidemiology and ecology. Boca Raton, Florida: CRC Press, 1989;203-32.
; Emergence of a new epidemic/epizootic Venezuelan equine encephalitis
virus in South America. Proc Natl Acad Sci U S A 1995;92:5278-81 ;
Virological and serological studies of Venezuelan equine
encephalomyelitis in humans. J Clin Microbiol 1976;4:22-7.
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