Japanese encephalitis is a mosquito-borne flaviviral infection and the most common vaccine-preventable cause of encephalitis in Asia, affecting over 50,000 patients annually with approximately 15,000 deaths. [1-2] Less than 1% of infections result in neurologic disease, but when encephalitis develops, the case-fatality rate is 20–30% and 30–50% of survivors have serious neurologic sequelae. [3-4]
The following figure illustrates the transmission cycle, clinical features, and prevention strategies for JE:
1. History
- Key HPI questions: Recent travel to or residence in endemic Asia/western Pacific; rural vs. urban setting; rice paddies, pig farms, outdoor/nighttime exposure [3]
- Incubation period: 5–15 days [3]
- Symptom progression: Abrupt onset of fever, headache, vomiting → mental status changes, generalized weakness, focal neurologic deficits, and movement disorders over the next few days [3-4]
- Seizure history: Seizures in 85% of children and 10% of adults [6]
- Movement disorders: Parkinsonian features (mask-like facies, tremor, cogwheel rigidity), choreoathetoid movements, dystonia in 20–60% [1][4]
- Important negatives: No prior JE vaccination, no history of prior flavivirus infection (dengue, West Nile), no alternative CNS infection source
2. Alarm Features
- Rapidly declining GCS / coma — 63% of deaths occur within 3 days of hospitalization [7]
- Status epilepticus [4]
- Acute flaccid paralysis (poliomyelitis-like anterior horn cell involvement) [1][3]
- Abnormal breathing patterns (45%), pulmonary edema (33%), upper GI hemorrhage (16%) — all associated with poor prognosis [7]
- Signs of raised intracranial pressure — brainstem herniation is a leading cause of death [4]
- Decerebrate rigidity [7]
3. Medications
- No licensed antiviral therapy exists for JE [3-4]
- Controlled trials showed no benefit from corticosteroids, interferon alfa-2a, ribavirin, minocycline, or IVIG [4][8]
- One retrospective study suggested a ganciclovir + methylprednisolone + IVIG combination (TAGMIC) may reduce mortality in severe JE, but this is not established standard of care [9]
- A retrospective pediatric study suggested possible benefit of high-dose IVIG in severe JE, but evidence remains limited [10]
- Empiric acyclovir should be started pending workup to cover HSV encephalitis, which is treatable and must be excluded [11]
- Anticonvulsants as needed for seizure management
- Mannitol/hypertonic saline for elevated ICP
4. Diet
- NPO if altered mental status or risk of aspiration
- Enteral nutrition via NG/OG tube for prolonged unconsciousness
- Monitor and correct hyponatremia (common finding) — fluid restriction or isotonic fluids as appropriate [3-4]
- Long-term: Nutritional rehabilitation during recovery, especially in pediatric patients with prolonged hospitalization
5. Review of Systems
- Neurologic: Headache, altered consciousness, seizures, focal weakness, tremor, dystonia, gaze palsies, opsoclonus [7]
- Respiratory: Abnormal breathing patterns, aspiration pneumonia risk [4][7]
- GI: Vomiting, diarrhea (early nonspecific phase), upper GI bleeding [4][7]
- Constitutional: High fever, rigors, malaise
- Psychiatric: Behavioral abnormalities (less common than in HSE) [11]
6. Collateral History and Family History
- Travel history is critical: Specific countries, rural vs. urban, duration, season, outdoor activities, accommodations (air conditioning, bed nets) [3]
- Vaccination history: Prior JE vaccine (IXIARO), yellow fever vaccine, or other flavivirus vaccines (affects serologic interpretation) [3]
- Occupational exposure: Rice farming, proximity to pig farms, military deployment [3]
- Family history is generally not contributory (JE is not hereditary), but household contacts with similar symptoms may suggest a cluster/outbreak
- Humans are dead-end hosts — no person-to-person transmission [1]
7. Risk Factors
- Geographic: Residence in or travel to endemic Asia/western Pacific (South/Southeast Asia, China, Japan, Korea, eastern Russia, northern Australia) [3][6]
- Environmental: Rural agricultural areas, rice paddies, flood irrigation, proximity to pig farms [3]
- Seasonal: Summer/fall in temperate regions; year-round with monsoon peak in tropical regions [1][3]
- Behavioral: Outdoor/nighttime exposure, accommodations without screens or air conditioning [3]
- Age: Primarily children in endemic areas (adults lack immunity in newly invaded areas) [1][6]
- Immunologic: Absence of prior JE immunity (unvaccinated, no prior natural infection) [3]
8. Differential Diagnosis
- Herpes simplex encephalitis (HSE): Temporal lobe involvement on MRI (vs. thalamic in JE); behavioral abnormalities and seizures more prominent; treatable with acyclovir — must be excluded early [11]
- Other arboviral encephalitides: West Nile, dengue, chikungunya, Eastern/Western equine encephalitis — geographic and serologic overlap [6][12]
- Bacterial meningitis: More acute, purulent CSF, positive Gram stain/culture [13]
- Cerebral malaria: Thick/thin smear, rapid diagnostic test; overlapping geography [12]
- Scrub typhus / rickettsial infection: Eschar, rash; common co-endemic pathogen in Asia [12][14]
- Autoimmune encephalitis (anti-NMDA receptor): Prominent psychiatric features, younger women, ovarian teratoma; CSF antibody testing [15-16]
- Neurocysticercosis: Co-infection with JE is described due to shared pig reservoir; ring-enhancing lesions on imaging [1][17]
- Tuberculous meningitis: Subacute course, basilar meningeal enhancement, low CSF glucose
9. Past Medical History
- Prior JE or flavivirus infection (confers cross-reactive immunity, complicates serology) [3]
- Prior JE vaccination status and dates [3]
- History of seizure disorder (baseline vs. new-onset)
- Immunocompromising conditions (may alter viral clearance and diagnostic approach) [3]
- Prior neurocysticercosis (co-infection described) [1]
10. Physical Exam
- Vital signs: High fever, tachycardia; abnormal respiratory patterns (Cheyne-Stokes, apneustic) [7]
- Neurologic:
- GCS assessment — altered sensorium in 96% [7]
- Parkinsonian features: Mask-like facies, coarse tremor, cogwheel rigidity [3-4]
- Movement disorders: Choreoathetosis, dystonia (limb, axial, orofacial) [1]
- Opsoclonus (20%), gaze palsies (16%), pupillary changes (48%) with waxing/waning character [7]
- Acute flaccid paralysis with areflexia (anterior horn cell involvement) [1]
- Meningeal signs: Neck stiffness, Kernig/Brudzinski signs
- Cerebellar signs are characteristically absent [7]
- Fundoscopy: Papilledema if raised ICP
- Respiratory: Signs of aspiration pneumonia, pulmonary edema [7]
11. Lab Studies
- CBC: Moderate leukocytosis, mild anemia [3-4]
- BMP: Hyponatremia (common and clinically significant) [3-4]
- LFTs: Thrombocytopenia and elevated hepatic enzymes may be seen [4]
- CSF analysis:
- Mild-to-moderate lymphocytic pleocytosis
- Slightly elevated protein
- Normal glucose ratio (CSF:plasma) — helps distinguish from bacterial/TB meningitis [3]
- JE virus-specific IgM capture ELISA (MAC ELISA):
- CSF IgM detectable in most patients ≥4 days after symptom onset [3-4]
- Serum IgM detectable ≥7 days after onset [3-4]
- CSF IgM is the most diagnostically useful test [4]
- Plaque reduction neutralization test (PRNT): Confirms infection and discriminates from cross-reacting flavivirus antibodies; ≥4-fold rise between acute and convalescent sera [3-4]
- Virus isolation and PCR are insensitive and should not be used to rule out JE [3-4]
- Blood cultures, malaria smear, scrub typhus serology to exclude mimics [12]
12. Imaging
- MRI brain (preferred modality):
- Thalamic lesions are the hallmark — bilateral, often hemorrhagic, mixed-intensity or hypodense on T2/FLAIR [4][6][18-19]
- Additional involvement of basal ganglia, midbrain, pons, medulla, and spinal cord [4]
- Hippocampal and cortical involvement may also occur (atypical) [20]
- DWI may show cytotoxic edema pattern progressing from unilateral to bilateral thalamic involvement [21]
- CT brain: Less sensitive; may show thalamic hypodensities; useful to rule out mass lesion or hemorrhage before LP
- Key distinguishing feature: Thalamic lesions suggest JE; temporal lobe involvement suggests HSE [11]
- Imaging is not pathognomonic — laboratory confirmation remains essential [18]
13. Special Tests
- Glasgow Coma Scale (GCS): Serial monitoring; GCS is a predictor of outcome [20]
- EEG:
- Diffuse delta wave activity is the most common finding, likely reflecting thalamic involvement [11][19]
- Alpha coma pattern may be seen [19]
- Delta slowing is more frequent in JE than HSE [11]
- Useful to detect subclinical seizures and guide anticonvulsant therapy
- Lumbar puncture: Essential for CSF analysis and IgM testing (after ruling out raised ICP) [6]
- Metagenomic next-generation sequencing (mNGS): Emerging tool for rapid identification when standard testing is inconclusive [22]
- Contact CDC Arboviral Diseases Branch (970-221-6400) for diagnostic assistance if needed [3]
14. ECG
- ECG is not a primary diagnostic tool for JE
- Obtain ECG to evaluate for myocarditis (rare complication of systemic viral infection)
- Monitor for arrhythmias in critically ill patients, especially those with electrolyte derangements (hyponatremia) or on QT-prolonging medications
- Continuous telemetry in ICU setting
15. Assessment
- JE is a neurologic emergency with high mortality (20–30%) and devastating morbidity (30–50% of survivors with permanent sequelae) [3-4]
- Severity stratification: GCS, presence of status epilepticus, abnormal breathing patterns, pulmonary edema, and decerebrate rigidity predict poor outcome [7][20]
- Typical presentation: Fever → headache/vomiting → altered sensorium → seizures + parkinsonian features + thalamic lesions on MRI [3][6]
- Atypical presentations: Isolated acute flaccid paralysis (polio-like), aseptic meningitis, undifferentiated febrile illness [1][3]
- Complications: Status epilepticus, aspiration pneumonia, raised ICP with herniation, pulmonary edema, upper GI hemorrhage, long-term cognitive/motor/psychiatric sequelae [4][7]
16. Treatment Plan
Initial stabilization
- Airway protection (intubation if GCS ≤8 or loss of airway reflexes)
- Seizure management with benzodiazepines → levetiracetam or phenytoin for ongoing seizures
- ICP management: Head elevation, osmotic therapy (mannitol, hypertonic saline), avoid hyperthermia
- IV fluids with attention to hyponatremia correction
Empiric therapy (pending workup)
- Start IV acyclovir empirically to cover HSV encephalitis until excluded [11]
- Broad-spectrum antibiotics if bacterial meningitis cannot be excluded
- Discontinue acyclovir once HSV is ruled out
JE-specific therapy
- No proven antiviral treatment — management is entirely supportive [3-4][8]
- Controlled trials have failed to show benefit for interferon alfa, ribavirin, corticosteroids, minocycline, or IVIG [4][8]
Supportive care
- ICU admission for severe cases
- Ventilatory support as needed
- Nutritional support (enteral feeding)
- DVT prophylaxis
- Physical/occupational therapy initiated early
- Seizure prophylaxis if recurrent
17. Disposition
- Admit to ICU: All patients with encephalitis, altered consciousness, seizures, respiratory compromise, or hemodynamic instability [4][7]
- Admit to ward: Milder presentations (aseptic meningitis with stable mental status)
- Discharge criteria: Stable neurologic exam, seizure-free, tolerating oral intake, adequate home support, rehabilitation plan in place
- Specialist consultation triggers:
- Neurology (all cases)
- Infectious disease
- Critical care / pulmonology if ventilatory support needed
- Neurosurgery if signs of herniation or need for ICP monitoring
- Rehabilitation medicine for survivors with sequelae
- Public health notification: JE is a reportable disease — notify state/local health department [3]
18. Follow Up / Return Precautions
- Follow-up timing: Neurology follow-up at 1, 3, 6, and 12 months post-discharge for assessment of sequelae [7][20]
- Expected sequelae in survivors: Motor weakness, cognitive impairment, seizure disorders, behavioral/psychiatric disturbances, movement disorders [1][6]
- Rehabilitation: Prolonged physical, occupational, speech, and cognitive rehabilitation often required [13]
- Return precautions: Return immediately for new seizures, worsening mental status, new focal deficits, recurrent fever, respiratory distress
- Vaccination counseling: Infection with one JE virus genotype confers lifelong immunity; however, unvaccinated contacts/travelers should receive IXIARO vaccine (2-dose primary series, completed ≥1 week before travel; booster at ≥1 year if ongoing risk) [3-4]
- Prevention counseling: Mosquito bite prevention (DEET-containing repellents, permethrin-treated clothing, bed nets, avoiding outdoor exposure at dusk/dawn) for future travel to endemic areas [3]
References
1. Overview: Japanese Encephalitis. — Misra UK, Kalita J. Progress in Neurobiology. 2010.
2. National Burden of and Optimal Vaccine Policy for Japanese Encephalitis Virus in Bangladesh: A Seroprevalence and Modelling Study. — Duque MP, Paul KK, Sultana R, et al. The Lancet. Infectious Diseases. 2025.
3. Japanese Encephalitis. — Susan L. Hills and Nicole P. Lindsey CDC Yellow Book. 2026.
4. Japanese Encephalitis Vaccine: Recommendations of the Advisory Committee on Immunization Practices. — Hills SL, Walter EB, Atmar RL, Fischer M. MMWR. Recommendations and Reports : Morbidity and Mortality Weekly Report. Recommendations and Reports. 2019.
5. Recent advances to overcome the burden of Japanese encephalitis : A zoonotic infection with problematic early detection. — Pinapati KK, Tandon R, Tripathi P, Srivastava N. Reviews in Medical Virology. 2023.
6. Viral Encephalitis: Familiar Infections and Emerging Pathogens. — Whitley RJ, Gnann JW. Lancet. 2002.
7. Japanese Encephalitis (JE). Part I: Clinical Profile of 1,282 Adult Acute Cases of Four Epidemics. — Sarkari NB, Thacker AK, Barthwal SP, et al. Journal of Neurology. 2012.
8. Flavivirus Encephalitis. — Solomon T. The New England Journal of Medicine. 2004.
9. The Effect of a Combined Ganciclovir, Methylprednisolone, and Immunoglobulin Regimen on Survival and Functional Outcomes in Patients With Japanese Encephalitis. — Miao W, Guo J, Zhang S, et al. Frontiers in Neurology. 2021.
10. Efficacy and Safety of Intravenous High-Dose Immunoglobulin in Treatment of the Severe Form of Japanese Encephalitis. — Chen D, Peng X, Zhan Y, et al. Neurological Sciences : Official Journal of the Italian Neurological Society and of the Italian Society of Clinical Neurophysiology. 2022.
11. Can We Differentiate Between Herpes Simplex Encephalitis and Japanese Encephalitis?. — Kalita J, Misra UK, Mani VE, Bhoi SK. Journal of the Neurological Sciences. 2016.
12. Aetiological Profile of Acute Encephalitis Syndrome in Assam, India, During a 4-Year Period From 2019 to 2022. — Sonowal D, Sharma A, Sarmah K, et al. APMIS : Acta Pathologica, Microbiologica, Et Immunologica Scandinavica. 2024.
13. Vaccines for Preventing Japanese Encephalitis. — Schiøler KL, Samuel M, Wai KL. The Cochrane Database of Systematic Reviews. 2007.
14. Infectious Causes of Acute Encephalitis Syndrome Hospitalizations in Central India, 2018-20. — Tandale BV, Tomar SJ, Bondre VP, et al. Journal of Clinical Virology : The Official Publication of the Pan American Society for Clinical Virology. 2022.
15. Understanding and Managing Acute Encephalitis. — Kumar R. F1000Research. 2020.
16. Acute Encephalitis in Immunocompetent Adults. — Venkatesan A, Michael BD, Probasco JC, Geocadin RG, Solomon T. Lancet. 2019.
17. Japanese Encephalitis in Two Children--United States, 2010. — MMWR. Morbidity and Mortality Weekly Report. 2011.
18. A Systematic Review of Brain Imaging Findings in Neurological Infection With Japanese Encephalitis Virus Compared With Dengue Virus. — Pichl T, Wedderburn CJ, Hoskote C, Turtle L, Bharucha T. International Journal of Infectious Diseases : IJID : Official Publication of the International Society for Infectious Diseases. 2022.
19. Radiological and Neurophysiological Changes in Japanese Encephalitis. — Misra UK, Kalita J, Jain SK, Mathur A. Journal of Neurology, Neurosurgery, and Psychiatry. 1994.
20. Clinical and Radiological Spectrum of Japanese Encephalitis. — Basumatary LJ, Raja D, Bhuyan D, et al. Journal of the Neurological Sciences. 2013.
21. Longitudinal Magnetic Resonance Imaging Changes in Japanese Encephalitis. — Arahata Y, Fujii K, Nishimura T, et al. Brain & Development. 2019.
22. Detection of Japanese Encephalitis by Metagenomic Next-Generation Sequencing of Cerebrospinal Fluid: A Case Report and Literature Review. — Li X, Li J, Wu G, Wang M, Jing Z. Frontiers in Cellular Neuroscience. 2022.