Japanese Encephalitis
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Direct answer
Japanese encephalitis is a mosquito-borne flavivirus infection transmitted by Culex species, with pigs and wading birds as amplifying hosts. It causes encephalitis with fever, altered sensorium, seizures and extrapyramidal features. Most infections are asymptomatic, and diagnosis rests on IgM antibody capture ELISA in cerebrospinal fluid, since there is no proven specific antiviral and treatment is supportive.
What you must remember
- Vector: Culex tritaeniorhynchus, breeding in rice paddies and flooded fields, with pigs as amplifying hosts and wading birds as natural reservoirs.
- Incubation and attack rate: about five to fifteen days; most infected people are asymptomatic and only a small minority develop encephalitis.
- Clinical triad: fever, headache and altered sensorium, progressing to seizures, with extrapyramidal features such as tremor, rigidity and akinetic mutism.
- Parkinsonian features: basal ganglia and thalamic involvement produces a Parkinsonian picture that can persist as a sequel.
- Flaccid paralysis: anterior horn cell involvement causes acute flaccid paralysis, which must be distinguished from poliomyelitis.
- Diagnosis: IgM antibody capture ELISA in CSF is the usual test, with serum IgM and RT-PCR adding value; antibodies become detectable after the first few days of illness.
- Imaging: MRI typically shows bilateral thalamic lesions, a useful clue alongside the clinical picture.
- Prevention: inactivated and live attenuated vaccines; JE vaccination is part of the national schedule in endemic districts, alongside vector control in paddy areas.
Worked example: a nine-year-old from a paddy belt in the monsoon
A nine-year-old boy from a paddy-growing district presents in the monsoon with three days of fever, vomiting and drowsiness, then a generalised seizure. Examination shows neck stiffness, tremor and rigidity.
Lumbar puncture shows raised protein, mild pleocytosis and normal glucose, and MRI reveals bilateral thalamic hyperintensities.
- Differential: Herpes simplex encephalitis is the dangerous alternative, so acyclovir is started empirically until it is excluded, while cerebral malaria, scrub typhus and bacterial meningitis are considered on epidemiology and blood tests.
- Samples: CSF and serum are sent for IgM antibody capture ELISA, with RT-PCR where available, because an early sample may be negative.
- Supportive care: Airway, fluids, seizure control and monitoring for raised intracranial pressure are the core, with no proven antiviral for JE.
- Steroids: Evidence for steroids is weak, so they are not routinely used.
- Rehabilitation: Prognosis is uncertain, and many survivors have deficits such as Parkinsonian features or cognitive impairment, which makes early rehabilitation important.
- Family counselling: Siblings are counselled about vaccination and mosquito protection, since the infection is not transmitted from person to person.
Where the exam tests the vector and the diagnosis
Four traps recur on this topic.
- Trap: choosing Aedes as the vector. Correct: the question writer favours Culex tritaeniorhynchus, remembered together with the pig amplifying host.
- Trap: relying on serum IgM. Correct: CSF IgM is more informative, because serum IgM can persist after vaccination or past infection.
- Trap: calling every paralysis Japanese encephalitis. Correct: Parkinsonian features with encephalitis in the endemic season point to JE, while acute flaccid paralysis in children prompts the polio differential.
- Trap: expecting a named antiviral. Correct: JE has no specific antiviral, so the management answer is supportive care.
Candidates also forget that JE is not spread person to person, which is a simple discriminator in stems about contacts.
Frequently asked questions
Which mosquito transmits Japanese encephalitis in India?
Culex tritaeniorhynchus is the principal vector of Japanese encephalitis, breeding in rice paddies and flooded fields. Pigs act as amplifying hosts, so the mosquito that feeds on pigs and humans is the one that transmits the virus.
What is the best test to diagnose Japanese encephalitis?
IgM antibody capture ELISA in cerebrospinal fluid is the best test to diagnose Japanese encephalitis, as CSF IgM is more specific for recent infection than serum IgM. RT-PCR and serum IgM add value, and a single early negative result does not exclude infection.
Which neurological features suggest Japanese encephalitis?
Fever with altered sensorium, seizures and extrapyramidal features such as tremor, rigidity and akinetic mutism suggest Japanese encephalitis in an endemic setting. Parkinsonian features may persist as sequelae, and bilateral thalamic lesions on MRI support the diagnosis.
Is there a specific antiviral for Japanese encephalitis?
No specific antiviral has been proven for Japanese encephalitis, so treatment is supportive, covering airway, fluids, seizure control and monitoring of intracranial pressure. Acyclovir is started empirically only until herpes simplex encephalitis is excluded.
How is Japanese encephalitis prevented?
Japanese encephalitis is prevented by vaccination, with inactivated and live attenuated vaccines available, and by vector control in paddy and pig-rearing areas. Vaccination is part of the national schedule in endemic districts, and personal mosquito protection reduces exposure.