Weather extremes associated with increased Ross River virus and Barmah Forest virus notifications in NSW: learnings for public health response

Objective: To examine the sequence of environmental and entomological events prior to a substantial increase in Ross River virus (RRV) and Barmah Forest virus (BFV) notifications with a view to informing future public health response. Methods: Rainfall, tidal, mosquito and human arboviral notification data were analysed to determine the temporality of events. Results: Following two extremely dry years, there was a substantial increase in the abundance of mosquitoes along coastal New South Wales (NSW) two weeks after a significant rainfall event and high tides in February 2020. Subsequently, RRV and BFV notifications in north east NSW began to increase eight and nine weeks respectively after the high rainfall, with RRV notifications peaking 12 weeks after the high rainfall. Conclusions: Mosquito bite avoidance messaging should be instigated within two weeks of high summer rainfall, especially after an extended dry period. Implications for public health: Intense summertime rain events, which are expected to increase in frequency in south-east Australia with climate change, can lead to significant increases in arboviral disease. These events need to be recognised by public health practitioners to facilitate timely public health response. This has taken on added importance since the emergence of Japanese encephalitis virus in southeastern Australia in 2022.