Abstract
Rift Valley fever phlebovirus (RVFV) is an arbovirus of medical and veterinary importance causing severe disease and mortality in humans and ruminants in endemic regions in Africa and the Arabian Peninsula. Understanding the capability, and limiting factors, of temperate British mosquitoes to support replication and transmission of RVFV is critical in order to understand the potential for RVFV establishment were it introduced to the UK. Using in vitro cell culture the effect of temperature on viral replication kinetics independently of the mosquito was investigated; demonstrating temperatures below 20˚C negatively affect RVFV replication. The full replication cycle was supported at 20˚C in vitro, and this was confirmed within in vivo mosquito experiments with wild-caught Aedes detritus demonstrating a transmission potential for RVFV at 20˚C and 25˚C. Experiments with two colonised lines of Cx. pipiens further demonstrated the transmission potential for RVFV by mosquitoes present in the UK. A novel RNA in situ hybridisation technique substantiated this result showing widespread dissemination of virus from the primary site of infection and evidence of secondary sites of replication within a single mosquito. Characterisation of the consensus sequence of RVFV propagated within these British mosquitoes in comparison to an in vivo mouse model showed potential for virus adaptation when switching between disparate hosts. Reproducible changes at the consensus level within each host had not previously been shown in early passages of RVFV in studies utilising in vitro models of replication. This suggests that RVFV replication generates genomic variation that may lead to adaptations that could promote potential survival in temperate regions. Taken together these findings indicate that transmission of RVFV within the UK by indigenous mosquitoes is possible. However, factors affecting mosquito survival including temperatures greater than 20˚C and ingestion of the higher virus dose (10^7 PFU/mL) will limit the likelihood of such events occurring.