Seroprevalence study of West Nile virus and Usutu virus in blood, organs, cells and tissue donors in Italy, 2021-2022.

BACKGROUND: West Nile virus (WNV) and Usutu virus (USUV) are endemic in Italy. Some regions experience high annual circulation, while others report only sporadic or low-level activity. MATERIALS AND METHODS: A national survey was conducted to assess WNV and USUV seroprevalence among blood and organ donors. Between February 2021 and February 2022, serum samples were collected from 9,073 donors (8,829 blood donors and 244 organ donors) across almost all Italian regions. Samples were screened using a WNV ELISA IgG test, with positive results confirmed via plaque reduction neutralization test (PRNT) for WNV and USUV. RESULTS: Of the total samples, 195 (190 blood donors and 5 organ donors) tested positive by ELISA. Among these, 23 samples (20 blood donors and 3 organ donors) were confirmed WNV-positive, and 42 (41 blood donors and 1 organ donor) were USUV-positive by PRNT. Additionally, 39 donors had antibodies to both viruses.Unadjusted national seroprevalence among blood donors was 0.66% (95% CI: 0.50-0.85%) for WNV and 0.90% (95% CI: 0.71-1.11%) for USUV. Among organ donors, WNV seroprevalence was 1.64% (95% CI: 0.45-4.14%) and 0.82% (95% CI: 0.10-2.93%) for USUV. After adjustment by general population weights, national seroprevalence in blood donors was 0.90% for WNV and 1.16% for USUV. DISCUSSION: Seroprevalence was higher in historically endemic northern regions, aligning with known circulation patterns. Blood and organ donors showed similar exposure levels. These findings confirm sustained WNV and USUV presence in Italy and support the continued need for preventive safety measures in the management of Substances of Human Origin (SoHO).