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

Abstract

In early 2022, the Japanese encephalitis virus (JEV) was identified as the cause of stillborn and mummified piglets in pig farms in southeastern Australia. Human cases and additional pig farms with infected piglets were subsequently identified across a widespread area encompassing four states. To inform surveillance and control programs, we synthesized existing information on Australian vectors of JEV, much of which was generated in response to incursions of JEV into the northern state of Queensland between 1995 and 2005. Members of the Culex sitiens subgroup, particularly Culex annulirostris, should be considered the primary vectors of JEV in Australia, as they yielded >87% of field detections of JEV, were highly efficient laboratory vectors of the virus, readily fed on pigs and birds (the key amplifying hosts of the virus) when they were available, and are widespread and often occur in large populations. Three introduced species, Culex quinquefasciatus, Culex gelidus and Culex tritaeniorhynchus may also serve as vectors, but more information on their geographical distribution, abundance and bionomics in the Australian context is required. Mosquitoes from other genera, such as Aedes and Verrallina, whilst considered relatively poor vectors, could play a regional or supplemental role in transmission, especially facilitating vertical transmission as a virus overwintering mechanism. Additional factors that could impact JEV transmission, including mosquito survival, dispersal and genetics, are also discussed. Possible directions for investigation are provided, especially in the context of the virus emerging in a region with different mosquito fauna and environmental drivers than northern Australia.

Keywords: Australia; Japanese encephalitis virus; host feeding patterns; mosquitoes; vector competence; virus detection.

Figure 1

Map of Australia and Papua New Guinea showing locations discussed in the text. The states and territories that are shaded pink had reported human cases of Japanese encephalitis, piggeries with infected pigs and/or positive feral pigs in 2022.

Figure 2

Results of the vector competence experiments conducted to assess the ability of Australian mosquito species to (a) become infected with and (b) transmit the Japanese encephalitis virus. Mosquitoes were allowed to feed on an infectious blood meal containing 10⁶⁻⁷ infectious units of virus per milliliter and tested at days 10–14 post-exposure. The orange and blue circles represent experiments conducted with genotype 1 and genotype 2 viruses, respectively, and the bars represent the means of data derived from separate virus exposures. Previously published studies [45,46,47,48] provided the source data. A minimum of five mosquitoes was required for inclusion in the analysis. Mansonia septempunctata and Ae. aegypti were not tested for their ability to transmit the virus.

Figure 3

Identification of the vertebrate source of blood meals of Australian species of Culex, with a focus on animal groups of importance to Japanese encephalitis virus transmission cycles. Source data were from previously published studies [56,57,58,59,60,61,62,63,64].