Insect-specific viruses that do not replicate in vertebrate cells, but persist and spread in some mosquito populations are highly prevalent in nature. These viruses are of special interest since they may naturally regulate the transmission of arboviral pathogens in co-infected mosquitoes. Following the recent isolation of the first Australian insect-specific flavivirus (ISF), Palm Creek virus (PCV), we investigated routes of infection and transmission of this virus in mosquitoes. When Cx. annulirostris were orally fed with PCV in blood, no mosquitoes became infected. In contrast, when Culex annulirostris, Aedes aegypti and Aedes vigilax were inoculated with PCV into the thorax, an infection rate of 95-100% and high levels of virus replication was observed for all species. However, no virus could be detected in the saliva of infected mosquitoes at 14 days post inoculation. These data indicate that PCV is unable to infect mosquitoes via an infectious blood meal or be transmitted through saliva, even when the midgut barrier is bypassed via inoculation. This supports the theory that PCV does not use a vertebrate host and probably persists in mosquito populations via infected eggs. We are currently investigating this mode of transmission.
Previously we reported that prior in vitro infection of mosquito cells with PCV supressed the replication of West Nile virus (WNV) in a flavivirus-specific manner. To assess whether prior infection of mosquitoes with PCV could also suppress the transmission of WNV, we inoculated Cx. annulirostris with PCV or growth medium and subsequently fed them blood containing WNV. Significantly fewer mosquitoes that were previously inoculated with PCV were capable of being infected with, or transmitting WNV compared to sham inoculated mosquitoes. These findings support a potential role for insect-specific viruses in regulating arboviral transmission and warrant further investigation to elucidate the mechanisms involved and explore their potential as biological control agents.