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

Abstract

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.

The Emergence of Japanese Encephalitis Virus in Australia in 2022: Existing Knowledge of Mosquito Vectors

“…Soon after JEV emerged in the mid-1990s, it was postulated that environmental conditions were suitable for the establishment of the virus on the Australian mainland [22]. However, despite at least two introductions of the virus into the region [13,20], there was no evidence to suggest this scenario eventuated [19].…”

“…A sentinel pig program detected JEV activity every year (except 1999) in the Torres Strait between 1995 and 2005 [18,19], although identification of only genotype 1 viruses from the year 2000 onward suggested that a separate incursion of the virus into the region had occurred [20]. Activity in 1998 and 2004 spread onto Cape York Peninsula, but there was no evidence that the virus had become established in natural transmission cycles on the mainland [14,21], despite ecological conditions potentially being suitable for its establishment [22]. The status of JEV in northern Australia had become less clear, as most forms of targeted surveillance were gradually phased out after 2011 [19].…”

Comparative genomics of Japanese encephalitis virus shows low rates of recombination and a small subset of codon positions under episodic diversifying selection

“…Historically, GIII was the dominantly detected strain, however it has recently been superseded by GI [25][26][27]. GII is largely confined to Southeast Asia and Northern Australia [28], and GIV and GV are generally confined to tropical Southeast Asia [29], however the 2022 outbreak of JEV in Australia was determined to be GIV, representing a range expansion of this genotype [30]. Vaccines are largely derived from GIII genotypes [29,31] and a growing body of evidence suggests that these vaccines show reduced efficacy toward GI and GV strains [32][33][34].…”

AustralianCulex annulirostrismosquitoes are competent vectors for Japanese encephalitis virus genotype IV

“…The mosquito-borne Japanese encephalitis virus (JEV; Orthoflavivirus japonicum) recently emerged in South-eastern Australia [1]. JEV is the major cause of human viral encephalitis in Southeast Asia, with an estimated 68,000 to 100,000 cases occurring annually [2,3].…”