Japanese encephalitis has been reported in the Torres Strait Islands, and two fatal cases were reported in mainland northern Australia in 1998. There were reported cases in Kachin State, Myanmar in 2013. There were 116 deaths reported in Odishas Malkangiri district of India in 2016.In 2022, the notable increase in distribution of the virus in Australia due to climate change became a concern to health officials as the population has limited immunity to the disease and the presence of large numbers of farmed and feral pigs could act as reservoirs for the virus. In February 2022, Japanese encephalitis was detected and confirmed in piggeries in Victoria, Queensland and New South Wales. On 4 March, cases were detected in South Australia. By October 2022, the outbreak in eastern mainland Australia had caused 42 symptomatic human cases of the disease, resulting in seven deaths.Human, cattle, and horses are dead-end hosts as the disease manifests as fatal encephalitis. Pigs act as an amplifying host and have a very important role in the epidemiology of the disease. Infection in swine is asymptomatic, except in pregnant sows, when abortion and fetal abnormalities are common sequelae. The most important vector is Culex tritaeniorhynchus, which feeds on cattle in preference to humans. The natural hosts of the Japanese encephalitis virus are birds, not humans, and many believe the virus will therefore never be eliminated.
Prevention
Infection with Japanese encephalitis confers lifelong immunity. There are currently three vaccines available: SA14-14-2, IXIARO (IC51, also marketed in Australia, New Zealand as JESPECT and India as JEEV) and ChimeriVax-JE (marketed as IMOJEV). All current vaccines are based on the genotype III virus.A formalin-inactivated mouse-brain-derived vaccine was first produced in Japan in the 1930s and was validated for use in Taiwan in the 1960s and in Thailand in the 1980s. The widespread use of vaccine and urbanization has led to control of the disease in Japan and Singapore. The high cost of this vaccine, which is grown in live mice, means that poorer countries have not been able to afford to give it as part of a routine immunization program.The most common adverse effects are redness and pain at the injection site. Uncommonly, an urticarial reaction can develop about four days after injection. Vaccines produced from mouse brain have a risk of autoimmune neurological complications of around 1 per million vaccinations. However where the vaccine is not produced in mouse brains but in vitro using cell culture there are few adverse effects compared to placebo, the main side effects being headache and myalgia.The neutralizing antibody persists in the circulation for at least two to three years, and perhaps longer. The total duration of protection is unknown, but because there is no firm evidence for protection beyond three years, boosters are recommended every three years for people who remain at risk. Furthermore, there are no data available regarding the interchangeability of other JE vaccines and IXIARO.