Standing at the crossroads: the emergence of Japanese encephalitis in Australia

Japanese encephalitis virus (JEV) was detected in pigs (Sus scrofa) in four Australian states in 2022. Human cases, including three deaths, were also reported. Japanese encephalitis virus is maintained in nature in an infection cycle between mosquitos and waterbirds. This outbreak was the first report of ongoing JEV transmission on the Australian mainland. Other members of the JE serogroup, West Nile virus and Murray Valley encephalitis virus, are already established in endemic cycles in Australia and large outbreaks of infection with these viruses have been reported following widespread flooding events. As an arbovirus that affects both domestic and wild animals, as well as people, JEV is a classical One Health issue. The effects of climate change drive longer, warmer and wetter conditions that favour expansion of invertebrate vectors and vertebrate reservoir hosts, which in turn increases the likelihood of spillover events, which affects domestic animals and humans. Horses and humans are considered to be ‘dead-end’ hosts of viruses in the JEV serogroup, as there is insufficient viraemic titre for subsequent transmission to mosquitos. However, pigs are considered amplifying hosts of JEV infection and contribute to maintenance of ongoing infection cycles. The 2022 outbreak highlighted the importance of a One Health approach in responding to arbovirus outbreaks. A focus on human cases and a lack of sensitive and specific diagnostic tests limits the capacity of authorities to undertake effective surveillance of the distribution of an outbreak in other animal species that are more important to the ecology of JEV.