Seroprevalence of West Nile, Usutu and tick-borne encephalitis viruses in equids from south-western France in 2023

Abstract

The circulation of West Nile virus (WNV), Usutu virus (USUV), and tick-borne encephalitis virus (TBEV) was investigated in south-western France during the first six months of 2023, following the emergence of WNV in equids in Gironde, a département in south-western France, in 2022. Blood samples were collected from 494 horses located in the Gironde département and divided into three zones: the Confluence zone, the Intermediate zone and the Arcachon Basin. Samples were tested for WNV-, USUV- and TBEV-specific antibodies. An overall seroprevalence of 14% (95% CI [11-18%]) for orthoflavivirus antibodies was detected in Gironde. The highest seroprevalence rates for WNV and USUV were observed in the Confluence Zone (9%, 95% CI [6-13%] and 5%, 95% CI [3-8%], respectively), where the type of housing (animals kept in pasture only) and proximity to a special bird protection area were identified as risk factors for WNV seropositivity. This study presents the first seroprevalence investigation of WNV, USUV and TBEV infections in equids located on the Atlantic coast of France and demonstrates intense circulation of WNV in this region, as well as evidence of equine USUV-specific infection.

Keywords: Usutu virus; West Nile virus; horse; seroprevalence; tick-borne encephalitis virus.

Figure 1

Map of the study area in south-western France, with black dots representing the stables where horses were sampled, grouped into two main zones: the Confluence and Arcachon zones. The red circles indicate the stables where clinical cases were reported in 2022, and the green polygons indicate the special protection areas for the protection of wild birds.

Figure 2

Distribution of WNV, USUV and/or TBEV seropositive equids in the Confluence zone, the Arcachon Basin and the Intermediate zone in the Gironde département in 2023. The circles represent the stables selected for the study, the size of which is proportional to the number of sampled horses. The darker the color of the circles is, the greater the number of seropositive horses.