Japanese Encephalitis

Etiology

Japanese encephalitis, a mosquito-borne illness caused by a single-stranded RNA virus closely related to the West Nile flavivirus, is transmitted primarily by the bite of the Culex mosquito species, most commonly Culex tritaeniorhynchus. Vector competence data support the development of accurate risk assessment models and effective control strategies. Continuous mosquito surveillance programs in both endemic regions and areas at risk for incursion remain essential to disease prevention.

The virus maintains and amplifies within intermediate hosts, eg, pigs and wading birds. Humans act as dead-end hosts because viral levels typically remain too low to transmit infection to feeding mosquitoes. Transmission through blood transfusion from infected donors has also been documented. Amplifying hosts thrive in agricultural environments (eg, farms and rice paddies), where flooded irrigation systems attract wading birds, thereby increasing infection rates in rural settings. Recent reports from South Korea, China, Singapore, and Taiwan indicate rising infections in suburban areas, prompting recommendations to extend vaccination guidance for travelers visiting these regions. Although mosquitoes account for the vast majority of infections, some experts have raised concerns that direct exposure to infected pigs may enable viral transmission through close contact without mosquito involvement.[7][8][9][10]

Epidemiology

Japanese encephalitis is the leading cause of viral encephalitis in Southeast Asia. An estimated 68,000 global cases of Japanese encephalitis occur each year.[1] Severe disease is estimated to occur in about 1 in 250 infections. Transmission is seasonal in temperate climates, peaking between May and October, but the risk persists year-round in more tropical climates. The time of greatest risk for infection is during the rainy season and the preharvest period in rice-cultivating areas due to increased mosquito vector populations. Additionally, most mosquito bites occur between dawn and dusk.

Twenty-four countries in Southeast Asia and the Western Pacific have endemic Japanese encephalitis virus transmission, placing more than 3 billion people at risk for infection. Furthermore, major outbreaks occur every 2 to 15 years. Between 1965 and 1975, more than 1 million cases were reported in China alone. However, the introduction of routine childhood vaccination programs in Japan, Korea, and Taiwan has nearly eliminated the risk in vaccinated patients despite ongoing infection in endemic animals and birds. Most cases in these areas are now reported in unvaccinated visitors. A small number of cases have been reported to have been nonmosquito-borne infections, including transplacental, transfusion, laboratory exposure, and liver transplant.[11][12][13]

Pathophysiology

The Japanese encephalitis virus attaches to host cell membranes, initially propagating at the site of the bite and nearby lymph nodes. Subsequent viremia develops, but most cases are cleared before the virus enters the central nervous system, resulting in subclinical disease. If the virus is transmitted to the brain hematogenously with the invasion of the blood-brain barrier, neuroinvasive disease develops. Japanese encephalitis virus has both direct neurotoxic effects and the capacity to alter neurostem cell development.[14]

History and Physical

Clinical Features

Most infected individuals with Japanese encephalitis report a history of mosquito exposure within an endemic region. The incubation period averages 6 to 8 days, with a range of 4 to 15 days. A prodromal phase frequently precedes neurologic involvement and presents with nonspecific symptoms, including fever, headache, nausea, vomiting, diarrhea, and myalgias, lasting several days.

As the illness advances, symptoms often progress to encephalitis, the most common neurologic manifestation. Patients may experience altered mental status, agitation, confusion, and psychosis. Adults frequently report headache and meningismus, while children more commonly present with seizures. Unusual presentations can include mutism or flaccid paralysis. Disease progression may lead to dystonia and choreoathetoid movements resembling the extrapyramidal features of Parkinson disease.

Evaluation

Patients with symptoms consistent with Japanese encephalitis are initially evaluated with neuroimaging and lumbar puncture. Magnetic resonance imaging (MRI) or computed tomography (CT) may show bilateral thalamic edema, lesions, or hemorrhage. A lumbar puncture may be significant for elevated opening pressure, elevated protein, and normal glucose. Serum laboratory studies may reveal findings of leukocytosis or hyponatremia in patients with Japanese encephalitis.

Moreover, as these findings are common in many forms of encephalitis or viral meningitis, clinical history will help narrow the differential diagnoses. If clinically suspected based on travel history, Japanese encephalitis virus immunoglobulin M (IgM) may be detected by enzyme-linked immunosorbent assay (ELISA) in serum or cerebrospinal fluid. Humans are dead-end hosts with low, transient viral loads, making virus isolation difficult.[15][16]

Treatment / Management

Japanese encephalitis lacks an effective antiviral therapy. Management is limited to supportive care with intravenous (IV) fluids and antipyretics. Anticonvulsants may be required for seizure control. Survivors often have poor neurologic outcomes requiring long-term care due to neurologic devastation and ongoing psychiatric symptoms. Up to 30% will suffer permanent intellectual, behavioral, or neurologic issues ranging from paralysis, recurrent seizures, or inability to speak or perform independent activities of daily living.[17][18]

Preventative Management

Since no effective treatment has been found, preventative strategies are critical. Preventing Japanese encephalitis is best achieved by avoiding mosquito bites entirely. Even very short periods of outdoor exposure can result in bites, so proper protective clothing—including long sleeves, long pants, socks, and closed-toe shoes — should be worn. Pant legs can be tucked into socks to prevent bites to exposed ankles.

Transmission is common during the warmer months, and mosquitoes may bite through very thin clothing; therefore, treating clothing with repellents containing permethrin, DEET, or other EPA-registered insect repellants will reduce this risk. Transmission is most frequent when mosquitoes feed, between dawn and dusk, so outdoor activities during this period should be avoided. Travelers should sleep in air-conditioned spaces or use mosquito nets or screens to prevent bites during sleep.

Vaccination

A safe and effective vaccine against Japanese encephalitis exists and can be administered using a short-course regimen. Despite proven efficacy, vaccine utilization remains limited due to hesitancy related to safety concerns, lengthy production timelines, and logistical challenges in distribution.[19] Broader awareness and improved accessibility could significantly enhance prevention efforts in endemic and high-risk areas.

The Centers for Disease Control and Prevention (CDC) currently provides specific guidelines for vaccine administration as follows:

• The vaccine is recommended for travelers who plan to spend 1 month or more in endemic areas during the transmission season. This includes travelers who will be based primarily in urban areas.

• The vaccine should be considered for short-term travelers (less than 1 month) who plan to spend substantial time outdoors in rural or agricultural areas, those who plan to participate in outdoor activities, and those staying in areas without air conditioning, screens, or bed nets. Vaccination should also be considered for travelers to areas with known outbreaks and for those with uncertain destinations, activities, or travel duration.

• The vaccine is not currently recommended for travelers with short-term travel plans to urban areas only.

Differential Diagnosis

The differential diagnosis of Japanese encephalitis remains broad, making a detailed clinical history, including travel history, essential for accurate assessment.[10][20] Clinicians must consider other conditions that present with similar neurologic and systemic manifestations to ensure appropriate evaluation and management. Diagnoses with similar presentations that should be considered include:

• Murray Valley encephalitis
• West Nile virus encephalitis
• St. Louis encephalitis
• Herpes simplex encephalitis
• Western and Eastern equine encephalitis
• Venezuelan Equine encephalitis
• Ehrlichiosis
• Enterovirus meningitis
• Mycoplasma meningitis
• Cytomegalovirus infection in the immunocompromised host
• Typhoid fever
• Dengue fever
• Malaria
• Brain abscess
• Tuberculous meningitis
• Nipah virus infection
• Rocky Mountain spotted fever
• Fungal meningitis
• Leptospirosis
• Neurocysticercosis
• Amebic meningoencephalitis
• Lupus with central nervous system involvement
• Central nervous system (CNS) tumor
• Cerebrovascular accident

Pertinent Studies and Ongoing Trials

Currently, no clinically specific antiviral drug targeting the Japanese encephalitis virus has been developed; however, several studies are underway that may hold promise for managing this disease.[21] 

Prognosis

Only 1% of patients infected with the virus will progress to encephalitis. Unfortunately, mortality for patients who do develop encephalitis is 20% to 30%. While most cases will improve in 6 to 12 months, many patients who survive will have significant neurologic and psychiatric sequelae (30% to 50% of cases). The outcomes for patients with Japanese encephalitis are guarded. Recovery does occur, but is often marked by residual neuropsychiatric deficits that may persist for months or even years.[22]

Complications

Psychosis, seizures, and permanent neurologic deficits, including paresis and cranial nerve palsies and movement disorders, may complicate Japanese encephalitis. Patients may also develop flaccid paralysis with features similar to poliomyelitis and Guillain-Barré syndrome. Severe cases of encephalitis may be complicated by elevated intracranial pressure, herniation, and death.