A case of Japanese Encephalitis has been reported in the province of Dak Lak in the Central Highlands of Vietnam. This is the first case for 2024 and likely signals the start of the season of risk.

Investigations are being carried out by the Vietnamese Centre for Disease Control and catch-up vaccination programs have commenced in the area. Mosquito reduction measures are also underway.

Japanese encephalitis is a viral disease transmitted by mosquitos from birds and pigs to humans. It is the leading cause of vaccine preventable encephalitis in Asia.

Japanese encephalitis is a health concern in 24 countries in Southeast Asia and the Western Pacific region. Cases have been reported in southern Australia in the last few years when La Nina weather patterns were present.

More than 3 billion people live in endemic areas.

Environmental determinants of Japanese Encephalitis include climate, rainfall, temperature, water bodies and proximity to irrigation and pig farming. It is most prevalent in rural areas.

In temperate regions of Asia, transmission is seasonal, with peak cases of infection in the summer and autumn months. In the tropics and subtropics, transmission can occur year-round, peaking during the wet season.

Dak Lak Province has a more temperate climate with an average temperature of 24C. The wet season commences in May and lasts to October. It wouldn’t be unusual to see early cases in May.  It is important to understand that most cases of Japanese Encephalitis are asymptomatic. A single case alerts public health professionals on the presence of the virus and the need to instigate prevention measures.

For most travellers to Asia and parts of the Western Pacific, the risk of Japanese Encephalitis is low, particularly for short term visitors to urban areas. This risk is estimated to be less than 1 in a million per month of stay but will change according to season, destination, duration of travel and intended activities.  The level of accommodation is important when assessing risk.

Vaccination is available to those considered more at risk.  These would include expatriate workers (cumulative risk), long term backpackers and travellers off the beaten path.