Background

Japanese encephalitis (JE) is the most important cause of viral encephalitis in Asia. It is a mosquito-borne disease. The first case of JE was documented in 1871 in Japan.

JE is transmitted to humans through bites from infected mosquitoes of the Culex species, mainly Culex tritaeniorhynchus in Taiwan. The incubation period for JE is 5-15 days. Humans, once infected, do not develop sufficient viraemia to infect feeding mosquitoes. The virus exists in a transmission cycle between mosquitoes, pigs and/or water birds. The disease is predominantly found in rural and peri-urban areas.

Most of JE virus infections are mild with fever and headache or without apparent symptoms, but approximately 1 in 250 infections results in severe disease characterized by rapid onset of high fever, headache, neck stiffness, disorientation, coma, seizures, spastic paralysis and death. There is no antiviral treatment for patients with JE. Treatment is supportive to relieve symptoms and stabilize the patient. The case-fatality rate ranges from 5%-30% among those with disease symptoms. Of those who survive, 20%-30% suffer permanent intellectual, behavioural or neurological problems such as paralysis, recurrent seizures or the inability to speak.

Signs and symptoms

Most of JE virus infections are mild with fever and headache or without apparent symptoms, with fewer than 1% of infections developing clinical symptoms, most commonly manifesting as acute encephalitis. Symptomatic patients may initially present with non-specific symptoms, such as fever, diarrhea, headache, or vomiting. Mild cases may manifest as aseptic meningitis or fever of unknown origin, whereas severe cases can develop altered consciousness, generalized weakness, high fever, focal neurological deficits (including cranial nerve dysfunction or mild paralysis), movement disorders, Parkinsonism (characterized by mask-like face, cogwheel rigidity, and choreoathetosis due to extrapyramidal dysfunction), cognitive impairment, and disorientation in time, place, and person, and may progress to coma or death. The case-fatality rate ranges from 20%-30% among those with disease symptoms. Of those who survive, 30%-50% experience neurological or psychiatric sequelae. Neurological sequelae may include abnormal muscle tone, speech impairments, motor weakness, and dysfunction of cranial nerves or the extrapyramidal system, whereas psychiatric sequelae predominantly involve irritability and abnormal personality changes. Cognitive deficits are more commonly observed in children.

Epidemiology

Japanese Encephalitis Virus (JEV) is transmitted mainly during the warm season. In the tropics and subtropics, transmission can occur year-round but often intensifies during the rainy season and pre-harvest period in rice-cultivating regions. In Taiwan, epidemics can occur from May to October, and intensify in June and July. Since the national vaccination program was implementing in 1968, there are now around 20 to 30 sporadic JE cases annually in Taiwan.

Figure: Japanese Encephalitis cases in Taiwan, 2005-2025.

Japanese Encephalitis Surveillance in Taiwan

1. Taiwan National Infectious Disease Statistics System–Japanese Encephalitis
2. Self-reporting through the toll-free 1922 hotline or local public health authority.

Prevention and Control

Safe and effective JE vaccines are available to prevent disease. In Taiwan, the routine childhood immunization schedule for JE consists of two doses. The first dose is administered at 15 months of age, followed by a second dose 12 months later.  
Adults who live or work near pig farms, poultry and livestock farms, or mosquito-breeding sites are at higher risk and are advised to receive the JE vaccine at their own expense before the transmission season (March–April each year). Travelers visiting JE-endemic areas are encouraged to consult a travel medicine clinic, where vaccination may be recommended based on a physician’s assessment.
When visiting high-risk areas, such as pig farms, pigeon houses, or locations near rice paddies, ponds, and irrigation ditches where mosquitoes commonly breed, individuals are advised to wear light-colored long-sleeved shirts and long pants. Mosquito repellents containing DEET, Picaridin, or IR3535, approved by Ministry of Environment, should be applied to exposed skin or clothing to reduce the risk of mosquito bites and infection.
The installation of screen doors and windows, as well as the use of mosquito nets, is recommended. These should be inspected regularly, and any damage should be repaired or replaced promptly.

FAQs

1.  What is Japanese encephalitis?
   • Japanese encephalitis (JE) is a mosquito-borne disease, and is transmitted to humans through bites from infected mosquitoes. The incubation period for JE is 5-15 days. Most of JE virus infections are mild with fever and headache or without apparent symptoms, with fewer than 1% of infections developing clinical symptoms, most commonly manifesting as acute encephalitis. There is no antiviral treatment for patients with JE. Treatment is supportive to relieve symptoms and stabilize the patient. The virus exists in a transmission cycle between mosquitoes, pigs and/or water birds. The disease is predominantly found in rural areas and peri-urban areas.
2. How can reduce the risk for JE?
   • (1) JE vaccine has been proven effective in preventing the disease.
   • (2) When visiting high-risk areas, such as pig farms, pigeon houses, or locations near rice paddies where mosquitoes commonly breed, individuals are advised to wear light-colored long-sleeved shirts and long pants. Mosquito repellents containing DEET, Picaridin, or IR3535, approved by Ministry of Environment, should be applied to exposed skin or clothing. The installation of screen doors and windows, as well as the use of mosquito nets, is recommended.
3. Who should receive Japanese encephalitis vaccine in Taiwan?
   • (1) Toddlers reaching 15 months of age should receive the first dose of Japanese encephalitis vaccine and the second dose should be taken after 12 months additionally.
   • (2) Japanese encephalitis vaccination is recommended for people who live or work near pig farms or rice paddies, which are considered high-risk areas for JE infection, and for travelers visiting endemic areas.

More Information

1. WHO|Japanese Encephalitis
2. USA CDC|Japanese Encephalitis