Assessment of vector competence of UK mosquitoes for Usutu virus of African origin

Luis M. Hernández-Triana, Maria Fernández de Marco, Karen L. Mansfield, Leigh Thorne, Sarah Lumley, Denise Marston, Anthony A. Fooks, Nick Johnson

Usutu virus (USUV) is an emerging zoonotic virus originally from sub-Saharan Africa. It has been introduced into Europe on multiple occasions, causing substantial mortality within the Eurasian blackbird (Turdus merula) population. It is transmitted by the mosquito species Culex pipiens in Europe and Africa. Vector competence studies indicate that European strains of USUV are readily transmitted by indigenous Cx. pipiens. However, there is limited information on the ability of an African strain to infect European mosquitoes. We evaluated the ability of African strain SAAR-1776 to infect two lines of Cx. pipiens colonised within the United Kingdom (UK). Mosquitoes were fed blood meals containing this virus and maintained at 25 °C for up to 21 days. Individual mosquitoes were tested for the presence of virus in the body, legs and an expectorate saliva sample. Changes to the consensus of the virus genome were monitored in samples derived from infected mosquitoes using amplicon based next generation sequencing. Infection, dissemination and the presence of virus in saliva in one mosquito line was observed, but no evidence for dissemination in the second mosquito line. This suggests a strong barrier to infection in UK Cx. pipiens for this strain of USUV. When comparing the genome of input virus within the blood meal with USUV recovered from an infected mosquito, we observed limited changes in the consensus genome sequence. The evaluation of vector competence of UK populations of Cx. pipiens for Usutu virus suggests a limited susceptibility to infection with USUV strain SAAR-1776 of African origin. However, within a single mosquito there was complete dissemination and expectoration of USUV, indicating that infection, and potentially transmission, is possible. Sequence changes were observed that may represent early adaption to the mosquito host and could reflect the early events of USUV establishment in European mosquito populations.