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Japanese Encephalitis is a viral disease transmitted by mosquitoes.
The Japanese
encephalitis virus is a virus from the family
Flaviviridae
.
This virus occurs along the Orient,
from Korea and Japan in the north to India and Malaysia in the south.
The disease has been recognized
in Japan since 1871 and was named Japanese ‘B’ encephalitis to
distinguish it from ‘encephalitis A’ (encephalitis lethargica , von
Economo’s disease) which was then prevalent.
The virus was first isolated in Japan
during an epidemic in 1935.
Several large epidemics have occurred
since then. Epidemics show a seasonal incidence (summer-autumn) in
temperate regions, though this is not evident in tropical areas.
Culex tritaeniorhynchus, a rural
mosquito that breeds in rice fields is the principal vector. Birds and
pigs act as reservoir hosts.
Japanese encephalitis virus causes the
most serious clinical disease among the five viruses of this group.
The disease typically has an abrupt
onset with fever, headache and vomiting , rapidly progressing to
encephalitis.
Case fatality rate in some epidemics
has been as high as 50 per cent.
Recovery is sometimes associated with serious
neurological sequelae, especially in infants and
children.
The large majority of infections are, however,
asymptomic and it has been estimated that 500 to 1000
inapparent infections occur for every case of clinical
disease.
Japanese encephalitis was
first recognized in India in 1955 when the virus was isolated from
Culex vishnui mosquitoes during an outbreak of encephalitis in
Tamil Nadu.
Epidemic with
high case fatality have occurred in West Bengal and Assam since 1973.
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