Question
Japanese encephalitis
Answer
Japanese encephalitis (JE) is a mosquito-borne viral encephalitis caused by Japanese Encephalitis virus, a Flavivirus, and the leading cause of viral encephalitis in many parts of Asia, including India.
Transmission: transmitted by Culex mosquitoes (notably Culex tritaeniorhynchus), which breed prolifically in flooded rice paddy fields; the natural transmission cycle involves pigs (an amplifying host, in which the virus replicates to high titres) and wading birds (a reservoir host), with humans as an incidental, dead-end host (human-to-human transmission does not occur, and infected humans do not develop sufficient viraemia to infect a feeding mosquito) — the strong epidemiological association with pig-rearing and rice cultivation in endemic rural areas reflects this ecological transmission cycle.
Clinical features: most infections are asymptomatic or produce only a mild non-specific febrile illness; a small proportion progress to encephalitis — high fever, headache, altered sensorium, seizures (particularly common in children), and focal neurological deficits, including a characteristic parkinsonian-like movement disorder (tremor, rigidity, mask-like facies) in some survivors; case-fatality rate is significant (~20-30%), and a substantial proportion of survivors are left with permanent neurological/neuropsychiatric sequelae (a major public health burden in endemic regions).
Laboratory diagnosis:
- IgM ELISA (MAC-ELISA) on CSF or serum — the mainstay of diagnosis, detecting JE-specific IgM antibody, which appears early in the illness.
- RT-PCR — for viral RNA, most useful very early in illness (CSF/serum), though sensitivity falls as the illness progresses (correlating with declining viraemia as the immune response develops).
- CSF examination typically shows a lymphocytic pleocytosis with mildly raised protein and normal glucose (a viral encephalitis pattern).
- Serum/CSF paired sample IgG seroconversion can also support diagnosis retrospectively.
Prevention:
- JE vaccination — live attenuated (e.g., SA 14-14-2 strain) and inactivated vaccines are available and used in national immunization programmes in endemic countries, including India.
- Vector control — mosquito control measures (source reduction, insecticide spraying) targeting Culex breeding sites, particularly around rice paddies and pig-rearing areas.
- Personal protective measures — mosquito nets, repellents, protective clothing, particularly important given the predominantly rural, agricultural exposure setting.
- Pig management — vaccination of pigs or physical separation of pig-rearing from human dwellings has been explored as an additional control measure in some settings, given the pig’s central role as the amplifying host.
Treatment: no specific antiviral therapy is available; management is entirely supportive (seizure control, management of raised intracranial pressure, and supportive/rehabilitative care for neurological sequelae).