Background: The first ever outbreak of Japanese Encephalitis virus (JEV) in New South Wales (NSW), Australia was reported in February 2022; and was subsequently declared a Communicable Disease Incident of National Significance. Given more than 99% of cases do not result in clinical disease and JEV was not routinely tested for, we undertook a cross-sectional serological survey in five towns to measure seroprevalence to JEV and identify risk factors for infection.

Methods: We used convenience sampling to recruit residents who lived or worked within 20 kilometres of the eligible five towns. We collected demographic and risk factor data using an online questionnaire, and a 5 ml blood sample. Blood samples were tested for antibodies against JEV using in-house epitope blocking enzyme immunoassay (DEB). Participants were excluded from the analysis if they had been born in a JEV endemic country of birth, self-reported prior JEV vaccination or travelled for more than one month in a JEV endemic country.

Results: We recruited 1,048 residents, 917 (87.5%) were included in the study. The median age of participants was 52 years (interquartile range: 37-62 years), and 559 (61%) were female. The proportion of JEV seropositive samples was 8.7% (95% CI 7.0-10.7). Residents from each of the five towns had evidence of JEV antibodies. Of the seropositive samples, the median age was 61 years (interquartile range: 48-70 years). Occupation, outdoor recreation activities, and time spent outdoors were not found to have elevated prevalence odds ratios.

Conclusions: In Australia, significant proportions of the community may be vulnerable to infection in regions with transmission. Enhanced surveillance to closely monitor the epidemiology of JEV is required to understand the progression of the virus over subsequent seasons. Vaccination and mosquito control and avoidance measures are essential to the JEV public health response.