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Tick-borne encephalitis
is caused by
tick-borne encephalitis virus (TBEV),
is a member of
the virus genus
Flavivirus
, of the family Flaviviridae.
A closely
related virus Russian spring-summer encephalitis virus (RSSEV), is
responsible for a similar disease with a more severe clinical course.
A number of viruses
belonging to the Russian Spring Summer Encephalitis (RSSE) complex
cause encephalitis along a wide area of the northern land mass from
Scotland to Siberia.
The names given to the disease vary
from one area to another depending on the variation in the prominent
clinical features.
Thus in Scotland it is called ‘louping
ill’ as the disease occurs primarily in sheep in which it causes a
curious ‘leaping’ gait.
Human cases that result from contact with sheep are
mild and present as aseptic meningitis.
It is called Central European
Encephalitis, biphasic meningoencephalitis and RSSE, in Central
Europe, Eastern Europe and USSR respectively.
RSSE is the most
serious form, with high rates of fatality and permanent paralytic
sequelae in some survivors.
Infection is transmitted by the bite of Ixodid
ticks.
The virus is transmitted transovarially in ticks so
that they can act as the vector as well as the reservoir host.
Wild
rodents and migrating birds are other reservoirs.
Biphasic meningoencephalitis may be transmitted to man by drinking milk of
infected goats.
Vertical
transmission from an infected mother to fetus has occurred.
The disease is
most often manifest as meningitis (inflammation of the membrane that
surrounds the brain and spinal cord), encephalitis (inflammation of
the brain), or meningoencephalitis (inflammation of both the brain and
meninges).
The incubation
period is usually between 7 and 14 days and is asymptomatic.
A characteristic
biphasic febrile illness follows (lasts 2 to 4 days). Other symptoms
include fever, malaise, anorexia, muscle aches, headache and nausea or
vomiting.
Involvement of
central nervous system include symptoms of meningitis (Eg:
fever, headache, and a stiff neck) or encephalitis (Eg: drowsiness,
confusion, sensory disturbances,or motor abnormalities such as
paralysis) or meningoencephalitis.
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