Japanese encephalitis virus: an emerging and re-emerging virus in Australia

Japanese encephalitis virus (JEV) first emerged in the Torres Strait of north-eastern Australia in 1995, with three human cases, and widespread infection of pigs on a number of islands. The virus was shown to belong to genotype II. Further cases occurred in 1998, including the first case on mainland Australia on Cape York. A second genotype of JEV, genotype Ia, was reported in mosquitoes and pigs in 2000-04, possibly displacing genotype II. JEV re-emerged in Australia with a fatal human case on the Tiwi Islands, Northern Territory, in 2021, and shown to belong to genotype IV. This case was followed about a year later by a large outbreak of JE; first detected in piggeries in four states, Queensland, New South Wales, Victoria, and South Australia, resulting in reproductive losses affecting 80 piggeries and 42 human cases, with seven fatal cases. The wide geographic spread of cases suggested that the virus had been circulating for a number of months or even years prior to detection, and has led to significant concern that the virus will become endemic to Australia, in a similar ecology to Murray Valley encephalitis virus. Known competent mosquito vectors and ardeid birds, as maintenance hosts, occur in Australia, and it is probable that feral pigs will provide an additional wildlife reservoir of virus. Little is known of the properties of genotype IV, but it is expected to have a similar ecology and pathogenesis to other JEV genotypes.