Epidemiologic, Entomologic, and Virologic Factors of the 2014-15 Ross River Virus Outbreak, Queensland, Australia.

Australia experienced its largest recorded outbreak of Ross River virus (RRV) during the 2014-15 reporting year, comprising >10,000 reported cases. We investigated epidemiologic, entomologic, and virologic factors that potentially contributed to the scale of the outbreak in Queensland, the state with the highest number of notifications (6,371). Spatial analysis of human cases showed that notifications were geographically widespread. In Brisbane, human case notifications and virus detections in mosquitoes occurred across inland and coastal locations. Viral sequence data demonstrated 2 RRV lineages (northeastern genotypes I and II) were circulating, and a new strain containing 3 unique amino acid changes in the envelope 2 protein was identified. Longitudinal mosquito collections demonstrated unusually high relative abundance of Culex annulirostris and Aedes procax mosquitoes, attributable to extensive freshwater larval habitats caused by early and persistent rainfall during the reporting year. Increased prevalence of these mosquitoes probably contributed to the scale of this outbreak.

Mosquito bloodmeals can be used to determine vertebrate diversity, host preference, and pathogen exposure in humans and wildlife.

The surveillance and detection of zoonotic pathogens in animals is essential for predicting disease transmission pathways and the risks of spillover, but challenges include the costs, ethics and technical expertise required for vertebrate trapping, serum sampling and antibody or virus screening. Surveillance using haematophagous arthropods as a sampling tool offers a unique opportunity to obtain blood samples from a wide range of vertebrate species, allowing the study of host-mosquito associations, and host exposure to pathogens. We explored vertebrate diversity and potential Ross River virus (RRV) transmission pathways by analysing blood-fed mosquitoes collected in Brisbane, Australia. Host origins were identified using barcode sequencing, and host exposure to RRV was assessed using a modified plaque reduction neutralisation test. In total, 480 blood-fed mosquitoes were collected between February 2021 and May 2022. The host origins of 346 (72%) bloodmeals were identified, with humans (73%) and cattle (9%) comprising the dominant hosts. RRV seroprevalence was high in both vertebrate species with evidence of RRV exposure in 70% (21/30) of cattle and 52% (132/253) of humans. This is a novel, non-invasive method of estimating seroprevalence in vertebrate host populations. Our results highlight the potential of blood-fed mosquitoes to provide species-specific insights into pathogen transmission dynamics.

Uncovering the genetic diversity within the Aedes notoscriptus virome and isolation of new viruses from this highly urbanised and invasive mosquito.

The Australian backyard mosquito, Aedes notoscriptus, is a highly urbanised pest species that has invaded New Zealand and the USA. Importantly, Ae. notoscriptus has been implicated as a vector of Ross River virus, a common and arthritogenic arbovirus in Australia, and is a laboratory vector of numerous other pathogenic viruses, including West Nile, yellow fever, and Zika viruses. To further explore live viruses harboured by field populations of Ae. notoscriptus and, more specifically, assess the genetic diversity of its virome, we processed 495 pools, comprising a total of 6,674 female Ae. notoscriptus collected across fifteen suburbs in Brisbane, Australia, between January 2018 and May 2019. Nine virus isolates were recovered and characterised by metagenomic sequencing and phylogenetics. The principal viral family represented was Flaviviridae. Known viruses belonging to the genera Flavivirus, Orbivirus, Mesonivirus, and Nelorpivirus were identified together with two novel virus species, including a divergent Thogoto-like orthomyxovirus and an insect-specific flavivirus. Among these, we recovered three Stratford virus (STRV) isolates and an isolate of Wongorr virus (WGRV), which for these viral species is unprecedented for the geographical area of Brisbane. Thus, the documented geographical distribution of STRV and WGRV, both known for their respective medical and veterinary importance, has now been expanded to include this major urban centre. Phylogenies of the remaining five viruses, namely, Casuarina, Ngewotan, the novel Thogoto-like virus, and two new flavivirus species, suggested they are insect-specific viruses. None of these viruses have been previously associated with Ae. notoscriptus or been reported in Brisbane. These findings exemplify the rich genetic diversity and viral abundance within the Ae. notoscriptus virome and further highlight this species as a vector of concern with the potential to transmit viruses impacting human or animal health. Considering it is a common pest and vector in residential areas and is expanding its global distribution, ongoing surveillance, and ecological study of Ae. notoscriptus, together with mapping of its virome and phenotypic characterisation of isolated viruses, is clearly warranted. Immanently, these initiatives are essential for future understanding of both the mosquito virome and the evolution of individual viral species.

Associations Between Ross River Virus Infection in Humans and Vector-Vertebrate Community Ecology in Brisbane, Australia.

Transmission of vector-borne pathogens can vary in complexity from single-vector, single-host systems through to multivector, multihost vertebrate systems. Understanding the dynamics of transmission is important for disease prevention efforts, but is dependent on disentangling complex interactions within coupled natural systems. Ross River virus (RRV) is a multivector multihost pathogen responsible for the greatest number of notified vector-borne pathogen infections in humans in Australia. Current evidence suggests that nonhuman vertebrates are critical for the maintenance and spillover of RRV into mosquito populations. Yet, there is a limited knowledge of which mosquito vector species and amplifying vertebrate host species are most important for transmission of RRV to humans. We conducted field surveys of nonhuman vertebrates and mosquitoes in the RRV endemic city of Brisbane, Australia, to assess the effect of vector and host community structure on human RRV notifications. Six suburbs were selected across a gradient of human disease notification rates. Differences in vertebrate and mosquito compositions were observed across all suburbs. Suburbs with higher RRV notification rates contained greater vertebrate biomass (dominated by the presence of horses) and higher mosquito abundances. This study suggests that horse-mosquito interactions should be considered in more detail and that vertebrate biomass and mosquito abundance be incorporated into future RRV modeling studies and considered in public health strategies for RRV management.