Is Japanese encephalitis (JE) endemic in certain regions of Asia?

Japanese Encephalitis Endemic Regions in Asia

Japanese encephalitis is endemic across a vast geographic area spanning from maritime Siberia in the north to Indonesia in the south, and from eastern India westward through East Asia to the Japanese archipelago, with transmission occurring in rural agricultural areas throughout Asia and the Western Pacific. 1

Geographic Distribution

Primary Endemic Regions

• East Asia: China, Japan, Korea, and Taiwan have historically been major endemic areas, though case numbers have declined substantially in developed countries due to vaccination programs 1
• Southeast Asia: Thailand, Vietnam, Indonesia, the Philippines, Cambodia, Laos, and Myanmar maintain active transmission, with Thailand and Indonesia accounting for the highest number of travel-associated cases 1
• South Asia: India, Bangladesh, Sri Lanka, and Nepal report ongoing transmission, with the virus spreading to new areas including Tibet and mountain districts of Nepal 1
• Western Pacific: The virus has spread eastward to Saipan, Australia's Torres Strait Islands, and northern mainland Australia 1

Epidemiologic Patterns by Region

Temperate zones (northern China, Japan, Korea, northern India, Taiwan) experience seasonal transmission peaking during summer and fall months, approximately May through September 1

Subtropical and tropical zones (Southeast Asia, southern India, Indonesia) demonstrate year-round transmission with peaks during rainy seasons, though local patterns vary based on rainfall, irrigation practices, and agricultural cycles 1

High-Risk Geographic Features

Rural Agricultural Settings

• Rice cultivation areas with flood irrigation represent the highest risk environments, as these provide optimal breeding grounds for Culex tritaeniorhynchus mosquitoes in proximity to amplifying hosts (pigs and wading birds) 1, 2
• Transmission occurs primarily in rural areas, though cases occasionally occur in urban peripheries where agricultural activities and animal reservoirs exist nearby 1
• Coastal resort areas adjacent to rice fields pose unexpected risk, as documented in Phuket, Thailand and Bali, Indonesia, where tourist destinations are situated near high-density mosquito breeding sites 1

Current Disease Burden

• Approximately 50,000 cases occur annually worldwide, though this represents substantial underreporting due to limited diagnostic capacity and surveillance in many endemic countries 1, 3, 4
• A 2011 systematic review estimated 67,900 cases typically occur annually, with an overall incidence of 1.8 per 100,000 population and 5.4 per 100,000 in children under 15 years 1
• The case-fatality ratio is 20-30%, with 30-50% of survivors experiencing permanent neurologic or psychiatric sequelae 1

Traveler Risk Assessment

Risk Stratification

Highest risk travelers include those spending ≥1 month in rural endemic areas during transmission season, with risk levels approaching that of susceptible resident populations 1, 2

Moderate risk travelers include short-term visitors (<1 month) with substantial outdoor or nighttime exposure in rural/agricultural areas, particularly during peak transmission periods 1, 2

Minimal risk travelers are those restricting visits to major urban areas with limited outdoor exposure, though risk cannot be completely eliminated 1

Travel-Associated Cases

• During 1973-2017, 85 cases were reported among travelers from nonendemic countries, with case numbers doubling in the most recent decade (2008-2017: 34 cases) compared to previous decades 1
• Thailand (n=26), Indonesia (n=13), and the Philippines (n=11) accounted for the majority of travel-associated infections 1
• Among U.S. travelers specifically, 12 cases occurred during 1993-2017, yielding an estimated incidence of less than one case per million travelers annually 1
• None of the travel-associated case-patients had received JE vaccine, and outcomes included 22% full recovery, 46% permanent sequelae, and 16% death 1

Critical Clinical Considerations

Diagnostic Suspicion

JE should be suspected in any patient with neurologic infection (encephalitis, meningitis, acute flaccid paralysis) who has recently returned from endemic areas in Asia or the Western Pacific, regardless of whether cases are currently reported among local residents 1, 5

Common Pitfall

Risk for travelers cannot be inferred from low reported incidence among local populations, as residents often have protective immunity from vaccination or prior natural infection, while the enzootic transmission cycle between mosquitoes and animal reservoirs continues unabated 1