“The Perfect Swarm” – Flooding across Northern Victoria leads to intensified mosquito breeding and subsequent re-emergence and transmission of Murray Valley encephalitis virus during the 2022-23 mosquito season

Abstract

Background The sporadic yet explosive nature of Murray Valley encephalitis virus (MVEV) in the southeast of Australia has led to several hypotheses to explain the viruses temporal and spatial patterns in the region. These theories, relying on the presence of flooding events, include the role of migratory waterbirds as carriers of MVEV from endemic areas in northern Australia or the re-appearance of MVEV from isolated, cryptic habitats. Here we describe the environmental, climatic and entomological factors that led to the development of the “perfect swarm” and re-emergence of MVEV in Victoria during the 2022–23 mosquito season, allowing greater scrutiny of the proposed theories for MVEV outbreaks in the southeast of Australia. Methodology Mosquitoes were collected using Encephalitis Virus Surveillance Carbon Dioxide (EVS CO2) traps that were set weekly across 17 northern Victorian Local Government Authorities (LGAs). Trapped mosquitoes were counted, speciated and screened for known viruses of public health significance. Mosquito abundance and species composition was compared with climatic variables including temperature and rainfall. Results A positive Southern Oscillation Index and a significant rainfall event in October 2022 led to flooding in central and northern Victoria that persisted for many months. Dominance of the mosquito fauna by Culex australicus in October to December 2022, followed by Culex annulirostris over January to March 2023 provided the vectors for amplification and subsequent transmission of MVEV within hosts, leading to spill-over into human populations for the first time in Victoria in almost 50 years. Mosquito surveillance systems detected MVEV in 48 mosquito traps over a 13-week period commencing in the first week of 2023. Virus positive species-specific pools included Cx. australicus and Cx. annulirostris, implicating both species as playing a role in MVEV amplification and/or transmission. Typing of MVEV was determined to be closely related to MVEV serotype G1A sub-lineage, which had previously only been detected in northwest Australia and from a human case in the Northern Territory. Conclusions The detection of MVEV during the 2022–23 mosquito season in south-eastern Australia provides greater context for understanding the re-appearance of MVEV in this region, with increased evidence implicating the role of migratory waterbirds, in response to flooding events, as carriers of MVEV into south-eastern Australia.