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.

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, 2005-2022.

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, a 15–month toddler should receive the vaccine timely.

To prevent infection, avoid visiting vector-breeding sites such as pigpens at dawn and dusk when mosquitoes are most active. When needing to visit mosquito-prone places, people are advised to wear long-sleeved shirts and long pants, and apply mosquito repellent to exposed body parts. To further prevent mosquito bites and lower the risk of contracting Japanese encephalitis, use mosquito net when sleeping. All travelers to JE endemic areas should take precautions to avoid mosquito bites to reduce the risk for JE.

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, but approximately 1 in 250 infections results in severe disease. 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.
2. What is the treatment for Japanese encephalitis?
   • There is no antiviral treatment for patients with JE. Treatment is supportive to relieve symptoms and stabilize the patient.
3. How can reduce the risk for JE?
   • (1) JE vaccine has been proven effective in preventing the disease.
   • (2) To prevent infection, avoid visiting vector-breeding sites such as pigpens at dawn and dusk when mosquitoes are most active. When needing to visit mosquito-prone places, people are advised to wear long-sleeved shirts and long pants, and apply mosquito repellent to exposed body parts to avoid mosquito bites.
4. 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 near or work in close proximity to pig farms or rice paddy fields which are at risk for Japanese encephalitis infection, and travelers who visit endemic areas.

More Information

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