This work provides the first evidence that the Ross River vector Ochlerotatus vigilax is not only susceptible to infection, but also capable of transmitting Barmah Forest virus (BFV) to vertebrates. Ochlerotatus vigilax mosquitoes were fed on blood containing BFV isolated from this species. They were found to be susceptible to infection, with an ID50 of 102.6 CCID50 per mosquito. Infection and transmission rates for mosquitoes fed 103.5 CCID50 virus per mosquito varied from 58 to 100% and 36 to 100%, respectively, between days 3 and 13 post infection. Titres in infected mosquitoes were high by 5 d post infection and did not vary substantially thereafter. Virus transmission to suckling mice occurred from 3 d post infection.
Of the Aedes aegypti, Culex sitiens, Culex annulirostris and Culex quinquefasciatus mosquitoes fed blood containing BFV, only Cx. annulirostris was susceptible to infection, with an ID50 of 103.36 CCID50 per mosquito. Aedes aegypti and Cx. quinquefasciatus were experimentally infected but at rates of less than 9%. Culex sitiens did not become infected. Infection rates for Cx. annulirostris fed 103.5 CCID50 virus per mosquito, varied from 9 to 50% between 2 d and 13 d. Virus transmission to suckling mice by Cx. annulirostris occurred from 2 d after infection. Transmission of BFV by Cx. annulirostris was 10% at 2 d after infection and did not exceed 8% thereafter. Although Cx. annulirostris may be infected with BFV and is able to transmit the virus to suckling mice, it is a relatively inefficient vector of the virus. This work establishes that Oc. vigilax is a capable potential vector of BF virus. In contrast, Culex annulirostris was found to be only moderately susceptible to laboratory infection with BFV, but transmitted the virus to suckling mice at rates of 10% or lower.
Host feeding patterns of common mosquitoes were assessed through the identification of bloodmeals collected from urban sites in Brisbane. The results were analysed using the Feeding Index to examine mosquito feeding-patterns according to host availability for most species. Ross River and BFV vector species were found under natural conditions to feed primarily on dogs, but also on brushtail possums and humans, providing important linkage between Cx. annulirostris, Oc. vigilax and Oc. notoscriptus and urban hosts.
Brushtail possums, Trichosurus vulpecula, were experimentally exposed to Ross River virus (RRV) or BFV using Oc. vigilax mosquitoes. Eight of 10 possums exposed to RRV developed neutralising antibody, and three possums developed high viraemia for less than 48 h after infection, sufficient to infect 30% of recipient mosquitoes. Two of the viraemic possums became ill, and subsequently one died and the other was euthenased. Pathology confirmed that RRV was present in the tissues of the dead animals.
Two of 10 possums exposed to Barmah Forest virus developed neutralising antibody. Both infected possums maintained detectable neutralising antibody to BFV for at least 45 d post infection (Log Neutralisation Index > 2.0 at 45 days). Eight possums did not develop neutralising antibody to BFV despite exposure to infected mosquitoes. These experimental infections of T. vulpecula and the relatively high infection rate of recipient Oc. vigilax mosquitoes suggests that they are competent hosts for RRV, potentially acting as a reservoir in urban areas. For BFV, the resulting low infection rate in recipient mosquitoes indicates that brushtail possums are not competent hosts for this virus.
In order to determine whether dogs and cats were potential reservoirs of RR or BF viruses, young female dogs and cats were experimentally exposed to the viruses using Oc. vigilax mosquitoes. Only one of the 10 dogs and one of the 10 cats exposed to RRV developed neutralising antibody. None of the animals developed detectable viraemia or clinical signs. One dog and three cats exposed to BFV developed neutralising antibody. These results suggest that although dogs and cats are exposed naturally to these viruses, and can become infected, they are unlikely to be important urban reservoirs of either virus.
Sera collected from dogs, cats, horses, flying foxes and brushtail possums in southeast Queensland from 1997 to 1999, analysed by microneutralisation assay for antibodies to RRV and BFV, showed that brushtail possums are often exposed to RRV and BFV in the field, with 18% and 11% of wild individuals having antibodies to the viruses. Approximately 22% of dog serum tested had antibodies to RRV. Horses and flying foxes also had a relatively high prevalence of antibodies to RRV, indicating that they may also be involved in transmission cycles. The overall prevalence of BFV antibody in the animals surveyed was low (0 to 11% compared to 11 to 26% for RRV).
This study, therefore, supports the urban conduit hypothesis of Ryan et al (1997) that on arrival in urban backyards, flying foxes are fed on by common mosquitoes, and that brushtail possums are likely to play a role in the urban ecology or RRV. In contrast, because they are common hosts of mosquitoes, and because the data indicate that they remain aviraemic, this study suggests that dogs may exert a zooprophylactic effect to divert transmission of RRV away from humans in urban areas.