Seroprevalence of West Nile, Usutu and Tick borne encephalitis viruses in equids from southwestern France in 2023

The circulation of West Nile virus (WNV), Usutu virus (USUV) and Tick-borne encephalitis virus (TBEV) was investigated in southwestern France within the first six months of 2023 following the emergence of WNV in equids in Gironde county in 2022. Blood samples were collected from 494 horses located in Gironde County divided into three zones: the Confluence, the Intermediate zone and the Arcachon basin and tested for WNV, USUV and TBEV specific antibodies. An overall seroprevalence of 14% (95% CI [11%-18%]) for orthoflavivirus antibody was detected in Gironde county. The highest seroprevalence rate for WNV and USUV were detected in the Confluence zone (9%, 95% CI, [6%-13%] and 5%, 95% CI, [3%-8%] respectively). The type of housing (animals kept in pasture only) and the distance to the nearest bird special protection area (SPA) were identified as risk factors for WNV seropositivity. This paper presents the first seroprevalence study investigating WNV, USUV and TBEV infections, conducted in equids located on the Atlantic coast of France and demonstrates an intense circulation of WNV in this region of France as well as equine USUV specific infection.