Japanese Encephalitis

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 emerged in Japan during the early 1870s, but the first described widespread epidemic occurred in 1924, involving 6,000 cases. Early studies following this initial outbreak indicated a transmission via a mosquito vector and a seasonal disease occurrence.

In endemic countries it has always been a disease of children with most adults having natural immunity. This pattern is slowly changing as more and more children are vaccinated.

This mosquito vector breeds in areas of ground water such as rice paddies. In temperate climate areas such as China and Nepal, cases occur mostly during the monsoon months. Most infected bites do not cause disease but about 1 in 250 infections will result in symptomatic disease with a high rate of death or permanent disability (50%).

Japanese Encephalitis was identified for the first time in the Murraylands of southern Australia in 2022. The northern and southern river systems became connected after 3 consecutive La Nina seasons. This allowed wetland birds (herons, egrets etc) to bring the disease south. Previously it was only considered an Australian risk in the Torres Strait and Cape York. It is unknown whether the risk will continue after La Nina finishes.

Three cases of Japanese Encephalitis have been notified in Australia in the past 12 months.

There are currently 24 countries in the South-East Asia and Western Pacific regions that have Japanese Encephalitis transmission risk. More than 3 billion people live in endemic areas. The World Health Organization (WHO) states that major outbreaks of Japanese Encephalitis occur every 2-15 years. The incidence of JEV cases in Japan, Republic of Korea and some regions in China, Nepal, Sri Lanka, Thailand and Vietnam have declined in recent years, largely as a result of immunisation.

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.

Environmental determinants of Japanese Encephalitis prevalence include climate, rainfall, temperature, wind, water bodies, and the proximity to irrigation and pig farming.

Japanese Encephalitis is most prevalent in rural or agricultural areas, often associated with rice farming, as the breeding grounds for the transmitting mosquito rely on adequate supply of water. 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.

Vaccination is available and is recommended for expatriates and long-term travellers to many countries in Asia. In the last 10 years cases have also been reported in Papua New Guinea.

Two vaccines are available in Australia, Imojev and Jespect. Imojev is a live vaccine and care must be taken in prescribing it. Jespect is inactivated.