Overview
Japanese encephalitis (JE) is a viral brain infection caused by the Japanese encephalitis virus (JEV), which is transmitted to humans through the bite of an infected mosquito, primarily of the Culex species. The virus mainly circulates between mosquitoes, pigs, and wading birds, but humans can become infected accidentally. While most infections cause mild or no symptoms, in severe cases the virus invades the central nervous system, leading to inflammation of the brain (encephalitis) that can cause neurological damage or death.

Why It’s Important
Japanese encephalitis is the leading cause of vaccine-preventable viral encephalitis in Asia and the Western Pacific. Although rare among travelers, it can be life-threatening, with a case fatality rate of 20–30% and long-term neurological or psychiatric complications in up to half of survivors. Understanding how it spreads and how to prevent it is critical, especially for people living in or traveling to rural agricultural areas where transmission is most common.

Quick Facts

• Cause: Japanese encephalitis virus (flavivirus family)

• Transmission: Mosquito bites (especially Culex tritaeniorhynchus)

• Incubation period: 5–15 days

• Prevalence: Endemic in over 24 countries in Asia and the Pacific

• Vaccine-preventable: Safe and effective vaccines are available

• Severity: Less than 1% of infections cause severe disease, but those that do can be fatal or disabling

What Happens in the Body
Once bitten by an infected mosquito, the virus enters the bloodstream and can cross the blood–brain barrier, triggering inflammation and swelling in brain tissue. This inflammation disrupts normal brain function, affecting coordination, speech, and consciousness. The body’s immune response can worsen the swelling, increasing intracranial pressure and leading to neurological symptoms.

Common Signs and Symptoms

• Mild cases: Fever, headache, and fatigue (often mistaken for the flu)

• Severe cases:

  • High fever

  • Stiff neck

  • Seizures

  • Disorientation or confusion

  • Coma or paralysis

  • In children: vomiting and irritability
    Symptoms of severe disease usually appear rapidly after the initial fever and can progress within hours to days.

Diagnosis

• Laboratory tests: Detection of JEV-specific antibodies (IgM) in blood or cerebrospinal fluid (CSF)

• Imaging: MRI or CT scans may show inflammation or swelling in the brain

• Lumbar puncture: To assess for infection or inflammation

• Diagnosis is confirmed through serologic testing by public health or reference laboratories.

Treatment and Management

• No specific antiviral treatment exists for JE.

• Supportive care focuses on:

  • Reducing fever and brain swelling

  • Managing seizures

  • Maintaining breathing and hydration

• Hospitalization is often required for close monitoring.

• Rehabilitation may help survivors regain lost physical or cognitive function.

• Prevention:

  • Vaccination is the best protection.

  • Use mosquito repellent, bed nets, and protective clothing.

  • Avoid mosquito exposure, especially at dusk and dawn.

Risks and Complications

• Death in 20–30% of severe cases

• Long-term complications in 30–50% of survivors, including:

  • Paralysis

  • Seizures

  • Speech difficulties

  • Cognitive impairment

  • Behavioral changes

Who Is at Risk

• People living in or visiting rural or agricultural regions of Asia or the Western Pacific

• Children in endemic areas (most cases occur under age 15)

• Travelers staying for long periods in endemic regions

• People exposed to mosquitoes near rice fields or pig farms

Related Conditions

• West Nile virus

• Tick-borne encephalitis

• Dengue and Zika virus infections (other flaviviruses)

• Meningitis (can cause similar symptoms)

When to See a Doctor
Seek immediate medical attention if you develop fever, headache, confusion, or seizures after traveling to or living in a JE-endemic region, especially within 2 weeks of possible exposure. Early care can help manage complications and improve recovery outcomes.
Anyone planning travel to an affected region should consult a healthcare provider at least 4–6 weeks before departure to discuss vaccination.

Last Updated: October 5, 2025