Japanese Encephalitis

Viral Entry and CNS Invasion

• JEV enters through Culex mosquito bite and undergoes primary replication in subcutaneous tissues, regional lymph nodes, and the reticuloendothelial system
• Viral dissemination occurs 2-14 days post-infection, crossing the blood-brain barrier (BBB) through direct endothelial infection and/or exploitation of infected leukocytes ("Trojan horse" mechanism)
• Envelope glycoproteins E bind to cellular receptors (P-glycoprotein, DC-SIGN, heparan sulfate) facilitating viral entry into neurons and glial cells

Inflammatory Cascade and Neuronal Damage

• CNS infection triggers robust innate immune activation: toll-like receptors (TLR3, TLR7/8) recognize viral PAMPs, activating NF-κB and MAPK pathways
• Infected neurons and glia produce excessive pro-inflammatory cytokines (IL-1β, IL-6, TNF-α, IL-12) and chemokines (MCP-1, RANTES), recruiting CD8+ T cells and macrophages
• Microglia become activated, releasing reactive oxygen species (ROS), nitric oxide (NO), and matrix metalloproteinases (MMP-2, MMP-9) that degrade tight junction proteins (claudins, occludin, ZO-1), exacerbating BBB disruption

Neuronal Injury Mechanisms

• Direct viral cytopathic effects: viral proteins inhibit host protein synthesis; NS proteins antagonize interferon responses
• Excitotoxicity: elevated extracellular glutamate from infected neurons and damaged astrocytes overstimulates NMDA and AMPA receptors, causing calcium influx and neuronal apoptosis
• Antibody-dependent enhancement (ADE) in secondary infection may worsen disease through cross-reactive non-neutralizing antibodies
• Selective neuronal necrosis occurs in substantia nigra, basal ganglia, brainstem, and spinal anterior horn cells—accounting for movement disorders and motor deficits

Causative Agent

• Japanese encephalitis virus (JEV): negative-sense, single-stranded RNA flavivirus; five genotypes (I-V) with genotype I currently predominant in endemic regions
• Transmission by Culex mosquitoes (primarily Culex tritaeniorhynchus) with pigs and wading birds as amplifying hosts; humans are dead-end hosts

Epidemiological Risk Factors

• Geographic exposure: residence or travel in endemic areas (East Asia, Southeast Asia, South Asia, Oceania—particularly Vietnam, Thailand, Cambodia, Philippines, Japan, Korea, China, India)
• Seasonal exposure: JE occurs during warm months (May-October in temperate regions; year-round in tropics), correlating with Culex breeding season and increased mosquito biting rates
• Occupational/behavioral risk: farmers, rural residents, outdoor workers during dusk-dawn (peak mosquito feeding times)
• Lack of immunization: JE vaccine-naive populations in endemic areas; waning immunity with time since vaccination

Host Factors Affecting Severity

• Age extremes: children <5 years and adults >65 years have worse outcomes; children <2 years have highest mortality
• Immunocompromise: advanced HIV/AIDS, immunosuppressive therapy, hematologic malignancies increase severe disease risk
• Genetic factors: polymorphisms in TLR3, STING, and interferon regulatory factor genes associate with disease severity
• Comorbidities: diabetes mellitus, chronic kidney disease, malnutrition increase mortality risk

Prodromal Phase (Days 1-3)

• Fever: high-grade, sustained fever (39-40°C), often accompanied by chills
• Constitutional symptoms: malaise, myalgias, arthralgias, headache, nausea, vomiting
• Biphasic fever pattern may occur ("two-step fever")

Acute Encephalitic Phase (Days 4-14)

• Fever persisting with altered mental status: confusion, disorientation, agitation, delirium
• Seizures: occur in 40-70% of patients, often refractory to single-agent therapy; status epilepticus signals poor prognosis
• Focal neurological deficits: asymmetric weakness, paresis, flaccid paralysis (lower motor neuron pattern reflecting anterior horn cell involvement)
• Extrapyramidal signs: tremor (parkinsonian), choreoathetosis, rigidity, dystonia from substantia nigra/basal ganglia involvement
• Respiratory dysfunction: decreased level of consciousness, loss of gag reflex, aspiration risk; respiratory muscle weakness from spinal cord involvement
• Brainstem signs: cranial nerve palsies (CN III-VII), vertical gaze palsy, nystagmus, ataxia

Physical Examination Findings

• Altered mental status: varies from confusion to deep coma
• Meningeal signs: neck stiffness, Kernig sign, Brudzinski sign (present in ~50% despite meningitis)
• Movement abnormalities: coarse tremor (most distinctive finding), rigidity, dystonia, pyramidal signs (hyperreflexia, Babinski sign)
• Upper motor neuron findings: increased tone, hyperreflexia, Babinski sign (indicating corticospinal involvement)
• Lower motor neuron findings: flaccid weakness, hyporeflexia, fasciculations (anterior horn cell pattern affecting legs >arms)
• Autonomic instability: tachycardia, hypertension, hyperthermia, excessive salivation, profuse diaphoresis

Atypical/Fulminant Presentations

• Rapid coma development with herniation signs within hours
• Hemorrhagic encephalitis: rare but associated with worst outcomes
• Mild aseptic meningitis: occurs in ~10% without encephalitis symptoms

Clinical Diagnosis Criteria

• Acute illness with fever AND encephalitis (altered mental status ± seizures ± focal signs) in endemic region during JE season
• Cerebrospinal fluid (CSF) pleocytosis with clinical context

Laboratory Diagnosis

• Cell count: pleocytosis (50-500 cells/μL, range 10-26,000); lymphocytic predominance (~80%) but early PMN predominance possible
• Protein: elevated (mild-moderate elevation, 40-200 mg/dL); may be higher in fulminant cases
• Glucose: normal to slightly low (CSF:serum glucose ratio typically >0.4, unlike bacterial meningitis)
• Gram stain/Culture: negative (distinguishes from bacterial meningitis)

• IgM ELISA (MAC-ELISA): positive in CSF is diagnostic; serum IgM positivity strongly suggests JE if paired with negative IgM at acute phase
• IgG antibodies: indicate past infection or vaccination; paired sera showing 4-fold rise confirms diagnosis but delayed diagnosis
• Virus neutralization test (VNT): gold standard for confirmation; requires paired acute and convalescent sera (2-4 week interval)
• Sensitivity: ~90% for CSF IgM ELISA; serum IgM sensitivity ~80%

• RT-PCR: detects viral RNA from CSF (highest sensitivity early in disease, days 1-7); specificity approaching 100%
• Viral culture: low yield, requires specialized facilities; rarely performed clinically
• Antigen detection: immunofluorescence on CSF cells (less sensitive than PCR)

• Complete blood count: leukocytosis common; mild thrombocytopenia in some patients
• Liver function tests: mild elevation of transaminases; hyperbilirubinemia in severe cases
• Renal function: elevated creatinine in severe disease reflecting systemic involvement
• Blood cultures: negative (distinguishes from bacterial sepsis)
• Electrolytes: hyponatremia (SIADH) in 30% of patients; check serum osmolality

Neuroimaging

• Often normal early in disease
• Later findings: hypodensities in basal ganglia, thalamus, brainstem, spinal cord
• Edema, hydrocephalus in severe cases
• Used primarily to exclude alternative diagnoses and assess complications

• T2/FLAIR hyperintensities: characteristic pattern in substantia nigra, putamen, external globus pallidus, thalamus, midbrain
• Spinal cord involvement: central cord hyperintensity (T2) in anterior horn; correlates with motor deficits
• Gray matter predominance: distinguishes JE from demyelinating diseases
• Enhancement patterns: variable; some lesions enhance post-gadolinium
• Brainstem involvement: hyperintensities in tegmentum and substantia nigra
• Repeated imaging may show evolution and help prognosticate

• Bacterial meningitis: CSF with PMN predominance, markedly elevated protein (>200), low glucose (<50% serum)
• Viral meningitis/HSV encephalitis: normal CSF glucose, CSF IgM negative; HSV PCR positive
• Tuberculous meningitis: chronic course, low glucose, pellicle formation; acid-fast bacilli on smear/culture
• Neurocysticercosis/Toxoplasmosis: imaging findings, serology specific
• Acute flaccid paralysis: anterior horn cell lesions on MRI; CSF may show upper normal values only

• ICU monitoring: continuous cardiac and oxygen saturation monitoring; frequent neurological assessments
• Airway management: intubation and mechanical ventilation for GCS ≤8, respiratory muscle weakness, aspiration risk; consider early tracheostomy if prolonged ventilation anticipated
• Seizure management:
• First-line: lorazepam IV (0.1 mg/kg bolus, then 1-4 mg IV q5-10min) or diazepam IV (5-10 mg IV q5-10min)
• Prophylaxis/long-term control: levetiracetam (loading 20-30 mg/kg, then 500 mg BID-TID) or valproic acid (loading 15-20 mg/kg, then maintenance dosed to therapeutic level 50-100 μg/mL)
• Refractory seizures: midazolam infusion, propofol, or pentobarbital infusion; consider EEG monitoring
• Fever management: acetaminophen (650-1000 mg q4-6h) or ibuprofen (400-600 mg q6-8h); cooling blankets if hyperthermic
• Fluid management: maintain adequate hydration; avoid hyponatremia (monitor sodium q6-12h given SIADH risk); cautious fluid restriction if SIADH confirmed
• Electrolyte correction: sodium replacement for symptomatic hyponatremia using 3% hypertonic saline (goal sodium increase 4-6 mEq/L per 24h)

• Investigational agents with preclinical promise but no proven clinical benefit:
• Ribavirin: no randomized controlled trials showing benefit; anecdotal use reported
• Interferon-alpha: minimal human data; immunomodulatory rationale but unproven
• Minocycline: anti-inflammatory properties; limited pilot data, no phase III trials
• Current recommendation: supportive care remains primary; antivirals not routinely recommended pending definitive trials

• Intravenous immunoglobulin (IVIG): theoretical benefit from passive antibodies and immunomodulation; case reports and small series show possible benefit, but no RCTs; may consider in selected severe cases
• Corticosteroids: controversial; generally not recommended for JE as controlled trials show no mortality benefit and potential harm from immune suppression; reserved for severe cerebral edema only
• If used: methylprednisolone 1 g IV q6h × 3-5 days, then taper
• Dexamethasone (10 mg IV q6h) not recommended as monotherapy for JE

Management of Specific Complications

• Cerebral edema/raised ICP: head elevation 30°, osmotic therapy (mannitol 0.25-1 g/kg IV q4-6h or hypertonic saline 3% or 7.5%), avoid hypercapnia, temperature control
• SIADH: fluid restriction to 800-1000 mL/day; vaptans (tolvaptan) or normal saline if symptomatic hyponatremia
• Aspiration pneumonia: broad-spectrum antibiotics covering oral anaerobes; cefotaxime 2 g IV q4-6h plus metronidazole 500 mg IV q6-8h
• Nosocomial infection: standard ICU protocols; monitor blood, urine, tracheal cultures
• Deep vein thrombosis prophylaxis: enoxaparin 40 mg SC daily (unless contraindicated); sequential compression devices

• Early goals-of-care discussions; family support and counseling
• Pain and symptom management with morphine, lorazepam

Acute Phase Complications

• Status epilepticus: occurs in 15-30% of hospitalized patients; poor prognostic indicator; requires intensive management with ICU-level care
• Aspiration pneumonia: from dysphagia and impaired airway protection; leading infectious complication; manifests as infiltrates on CXR, fever, leukocytosis
• Acute respiratory distress syndrome (ARDS): from severe pneumonia or direct pulmonary involvement; mortality approaches 50% if develops
• Cerebral edema and increased intracranial pressure (ICP): can lead to transtentorial or tonsillar herniation, brain death
• Syndrome of inappropriate antidiuretic hormone (SIADH): hyponatremia with risk of seizures and altered mental status; usually resolves with recovery

Neurological Complications

• Spinal cord involvement ("JE myelitis"): anterior horn cell destruction causing flaccid paralysis (can mimic poliomyelitis); results in permanent paralysis in many
• Brainstem involvement: tegmental and substantia nigra lesions may cause respiratory compromise, bulbar dysfunction, vertical gaze palsy
• Movement disorders: persistent tremor, parkinsonism, dystonia even after viral clearance due to substantia nigra damage
• Hydrocephalus: from inflammation; may require ventriculoperitoneal shunt

• Motor deficits: hemiparesis, quadriplegia, spasticity
• Cognitive dysfunction: memory impairment, executive dysfunction, personality changes (frontal lobe injury)
• Psychiatric sequelae: depression, psychosis, behavioral disorders
• Movement disorders: tremor (most common chronic finding), parkinsonism, dystonia
• Seizure recurrence: chronic post-encephalitic epilepsy in 10-15%
• Speech/language dysfunction: dysarthria, aphasia from brainstem/cortical involvement
• Neurorehabilitation: required for many; quality of life often severely

Buzzwords that name the diagnosis

• Bilateral thalamic T2/FLAIR hyperintensities on MRI in a febrile encephalopathic traveler or resident of rural Asia — the single most tested imaging finding; substantia nigra involvement is the corroborating clue.
• Parkinsonism plus encephalitis: mask-like facies, cogwheel rigidity, and coarse tremor appearing acutely reflect basal ganglia/substantia nigra tropism — a combination almost unique among arboviral encephalitides.
• Polio-like acute flaccid paralysis from anterior horn cell necrosis; asymmetric, legs > arms, with preserved sensation.

Transmission facts examiners reuse

• Culex tritaeniorhynchus in irrigated rice fields; pigs and wading birds are amplifying hosts, and humans are dead-end hosts — so human-to-human and blood-borne spread are not features.
• ~99% of infections are subclinical; do not assume every exposure produces encephalitis.

Single best next step

• CSF JEV IgM capture ELISA (MAC-ELISA) is the confirmatory test of choice; CSF IgM essentially confirms neuroinvasive infection because IgM does not cross an intact blood–brain barrier. Flavivirus serologies cross-react (dengue, West Nile, Zika, prior yellow fever vaccination), so plaque-reduction neutralization settles ambiguous results.
• While awaiting results, give empiric IV acyclovir until HSV PCR returns — missing HSV encephalitis is the costly error.

The association tested most

• Vaccine is an inactivated Vero cell–derived product (IXIARO), given as a two-dose IM series 28 days apart. CDC/ACIP recommends it for travelers with planned prolonged or repeated exposure to endemic rural areas during transmission season, and for laboratory workers; complete the series before departure, with a booster if exposure continues.
• Because it is inactivated, not live, it is acceptable in immunocompromised travelers — unlike the live yellow fever vaccine, a favorite contrast.

Common distractors

• No effective antiviral exists — ribavirin, interferon-α, and IVIG are unproven; care is supportive.
• Corticosteroids do not improve mortality in JE; reserve for documented raised ICP.
• Temporal-lobe hemorrhagic necrosis points to HSV-1, not JE.