Emergence of locally acquired Japanese encephalitis virus in Australia, January 2021-June 2022: a national case series.

Background and methods: In March 2022, an outbreak of Japanese encephalitis virus (JEV) infection was identified in temperate south-eastern Australia, with detections in humans and animals. The unexpected emergence of JEV prompted a national public health response and a Communicable Disease Incident of National Significance was declared. JEV has previously only been identified in tropical north-eastern Australia in localised outbreaks. This article provides a descriptive analysis of the human case epidemiology of the national outbreak from 1 January 2021 to 30 June 2022.

Results: There were 42 confirmed and probable human cases of JEV identified as acquired in Australia between 1 January 2021 and 30 June 2022. Seven deaths occurred (case fatality rate: 17%). Cases were identified in five Australian jurisdictions (New South Wales, Victoria, Queensland, South Australia and the Northern Territory). The majority of cases were aged 60 years and over (55%; 23/42), with a median age of 61.5 years (range: 0-79 years; interquartile range: 45-70 years); cases were predominantly of non-Indigenous status (90%; 38/42). Sixty-seven percent (28/42) were male. Of the cases with geographical data available (n = 41), all were likely exposed to the virus in 31 unique local government areas across regional and remote Australia.

Conclusions: Cases were detected across Australia in five jurisdictions, requiring a national public health response, to detect and prevent further cases. There was widespread geographical distribution over 18 months. Given the risk of further cases and possible endemicity of JEV being established in Australia, expanded environmental and human surveillance programs are required.