Flavivirus encephalitis: 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.