April 3, 2025 Author : Saraiva da Cunha (Infectious diseases) Japanese encephalitis is a viral infection transmitted by mosquitoes. It is endemic in most Asian countries, Australia and other countries in the western Pacific; It is estimated that between 30-50 thousand cases occur annually, although other projections point to around 100 thousand cases and 25 thousand deaths per year; It is caused by the Japanese encephalitis virus , which has 5 distinct genotypes and multiplies mainly in pigs and aquatic birds, and is the most important cause of viral encephalitis in Asia; The virus is transmitted to humans and animals mainly through bites from mosquitoes of the species Culex tritaeniorhynchus , which are carriers after having bitten infected pigs or water fowl; Transmission is more likely in rural areas, particularly near rice-growing farms, due to culture on flooded soils, and in many countries there is a seasonal pattern of transmission related to the rainy season; Infected people are not a source of infection , that is, uninfected mosquitoes that bite these people are not infected; Infected animals do not transmit the infection to people, either directly or through the consumption of food of animal origin; In endemic countries populations, children up to 15 years of age are the most affected group and adults usually have natural immunity; Vaccination against Japanese encephalitis is recommended for travelers to high-risk areas. Symptoms, diagnosis, and treatment Japanese encephalitis virus infection may not cause any symptoms, but it can also cause severe, disabling illness and death. 1% of infected people develop encephalitis; 20%-30% of people who develop encephalitis die. After infection, the incubation period is on average 6-8 days but can vary between 4 and 15 days. Initial symptoms are not very specific, and are: Fever Headache Nausea and vomiting Muscle pain Diarrhea Neurological symptoms related to brain inflammation and infection may subsequently occur, such as: convulsions, especially in children, disorientation, drowsiness, stiff neck, tremors, movement disorders, paralysis, mental state alterations, delirium, among others. Encephalitis (inflammation and infection of the brain) is the most serious consequence of infection with the Japanese encephalitis virus, and it can lead to permanent neurological damage in 30% to 50% of cases. The diagnosis of Japanese encephalitis is based on symptoms, the epidemiological context and history of travel to endemic areas, and on tests such as laboratory tests, brain imaging tests and the study of cerebrospinal fluid obtained by lumbar puncture. There is no treatment aimed at eliminating the Japanese encephalitis virus. The treatment of this infection is aimed at relieving symptoms and is based on medication and support measures, such as rest and hydration, among others. The most serious cases require hospitalization. It may take 6-12 months for a complete recovery. Prevention of Japanese encephalitis Prevention of Japanese encephalitis is done through vaccination and avoiding mosquito bites. Vaccination: Is adopted for children in some countries where the disease is endemic, for example in Japan and Taiwan, among others; It is recommended for travelers to countries where the disease is endemic, particularly in the case of long trips, or short trips with significant exposure to the outdoors, both to urban and rural areas, or when there is a chance of spending the night outdoors without protection by mosquito nets; In Portugal, the vaccine is no longer sold and, therefore, pharmacies and hospitals that need it must initiate an exceptional use authorization process following the guidelines of national authorities. Individual measures to prevent mosquito bites mainly include: Use of repellents; Wear clothes that cover all areas of the body well, closed shoes and a hat; Placing mosquito nets on the windows of homes and protecting beds; Use of air conditioning or other ventilation that interferes with the presence of mosquitoes; Avoid proximity of water; Avoid being outdoor when mosquitoes are most active. After returning from travel to endemic areas, travelers should be alert for any symptoms that may be related to Japanese encephalitis and seek immediate medical attention if they occur. Sources European Centre for Disease Prevention and Control. Japanese encephalitis . Hills SL, Solomon T. Japanese encephalitis . UpToDate Simon LV, Sandhu DS, Goyal A, Kruse B. Japanese encephalitis. Treasure Island: StatPearls Publishing; 2025. Meet the infectious diseases clinical team from Hospital da Luz network. The content published in the Health Guide is intended for the general public and is provided for informational and educational purposes only. It does not replace individual medical assessment, diagnosis, or personalized clinical advice from a qualified healthcare professional. The content is produced or reviewed by physicians and other healthcare professionals at Hospital da Luz and is based on the knowledge available at the time of publication or the most recent update, as indicated at the beginning of each item.
