Encephalitis (inflammation of the brain) due
to a flavivirus, a virus transmitted by a mosquito or tick. Flavivirus encephalitis
includes West Nile fever, Japanese encephalitis, St. Louis encephalitis, and Murray
Valley encephalitis.
These diseases typically develop after an incubation period of 5 to 15 days.
The neurologic manifestations depend on which part of the nervous system is infected
-- the meninges (to cause meningitis), the body of the brain (encephalitis), or
the spinal cord (myelitis). The more important features include a reduced level
of consciousness, which may be associated with seizures, a flaccid (floppy) paralysis
resembling that of poliomyelitis, and parkinsonian movement disorders.
Seizures are especially common in children with flavivirus encephalitis. They
occur in about 85% of children with Japanese encephalitis and Murray Valley encephalitis
and in up to 10% of adults with West Nile encephalitis.
Motor weakness is also common in flavivirus encephalitis and may be associated
with respiratory paralysis, a condition closely resembling poliomyelitis.
Parkinsonian movement disorders are common in Japanese encephalitis, in the acute
stages of infection and as one of the sequelae. In one series of cases, a quarter
of patients had a characteristic "parkinsonian syndrome" including mask-like facies,
tremors, and cogwheel rigidity, all of which are typical of Parkinson's disease.
Other movement disorders include generalized rigidity, jaw dystonia, opisthotonos,
choreoathetosis, and myoclonus.
The flaviviruses are transmitted among birds by bird-biting mosquitoes. Humans
become infected accidentally but they are "dead-end" hosts because they do not have
sufficiently high or prolonged levels of viremia (virus in the blood) to transmit
the virus further. West Nile virus can be transmitted among humans through transplanted
organs and blood products. Placental transmission of the virus from mother to baby
occurs with Japanese encephalitis virus and West Nile virus. Japanese encephalitis
is chiefly a disease of children, whereas West Nile encephalitis and St. Louis encephalitis
tend to affect adults.
Common Misspellings: flavivirus ensephilitis, flavivirus insephilitis
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