Mosquito-borne flaviviruses comprise many important vertebrate pathogens with worldwide distribution. Insect-specific flaviviruses (ISFs) only infect mosquitoes and are of particular interest since they may regulate transmission of vertebrate-infecting flaviviruses (VIFs). We have discovered two novel flaviviruses that will be instrumental in characterising the evolution, host-restriction and transmission dynamics of ISFs, and elucidating mechanisms of regulating VIF transmission in mosquitoes.
Bamaga virus (BgV) was isolated from Culex annulirostris – the main vector of arboviral encephalitis in Australia – collected from Cape York in 2001 and 2004. Inoculation and growth kinetics in simian, human, leporid, rodent, marsupial and avian cell lines revealed that vertebrate cells were weakly susceptible to BgV infection, indicative of an ISF. However, phylogenetic analysis of BgV genome grouped it separately from other ISFs. BgV was shown to be genetically and antigenically more closely related to Edge Hill virus in the yellow fever virus group. Based on its phylogenetic position we hypothesise that BgV may have recently adapted to a mosquito-only transmission cycle and may serve as a useful model to study ISF evolution and biology. We are currently assessing its ability to replicate in mice.
The complete viral genome of a second novel flavivirus, Karumba virus (KRBV), was detected at high frequency in Anopheles meraukensis mosquitoes collected from northern Australia between 2000 and 2014. Phylogenetically KRBV clusters with other ISFs but forms a separate lineage, consistent with it being the first ISF exclusive to an Anopheles host. While KRBV failed to replicate in a range of cell lines, including those of Aedes and Anopheles origin, KRBV sequence can be easily amplified from mosquito homogenates by RT-PCR targeting both positive- and negative-sense RNA templates, demonstrating the existence of viral RNA in a replicative dsRNA form. We have injected KRBV into live An. farauti mosquitoes to assess replication in the whole insect.