Vertical transmission in field-caught mosquitoes identifies a mechanism for the establishment of Usutu virus in a temperate country

Abstract

Abstract Usutu virus (USUV) is an emerging zoonotic flavivirus in Europe and the first zoonotic mosquito-borne virus to be confirmed in animal hosts in the United Kingdom (UK). Repeated detection of USUV in the years following its initial detection in 2020 indicated that the virus is overwintering in the UK and should now be considered endemic within southeast England. Surveillance of avian hosts and mosquito vectors has been insufficient to elucidate the mechanism by which USUV has persisted through temperate winters. Through enhanced targeted vector surveillance at the index site between 2021 and 2024 inclusive, we detected USUV RNA in host-seeking adult female Culex pipiens s.l. as well as in adults reared from field-caught larvae. This is, to our knowledge, the first description of vertical transmission of USUV in an arthropod vector that should consequently be considered a viable mechanism for the persistence of USUV in temperate areas.