Blood donor West Nile virus screening identifies three autochthonous Usutu virus infections in Spain

Considering the increase of West Nile virus (WNV) circulation in Europe, blood banks perform WNV Nucleic Acid Test testing to ensure transfusion safety during the WNV transmission season. Usutu virus (USUV), an arbovirus related to WNV, has relevant molecular and serological cross-reactivity with WNV.

During the 2024 WNV season in Europe, 15,957 blood donations from the Balearic Islands Blood Bank and 79,400 from the Catalonia Blood Bank were tested for WNV using the Cobas WNV real-time PCR (Roche Diagnostics, USA) and the Procleix WNV/Procleix ArboPlex transcription-mediated assays (Grifols Diagnostic Solutions Inc., USA), respectively. Serological tests for flavivirus, PCR using USUV- and WNV-specific primers, sequencing of viral RNA, and neutralization tests (NT) were performed to confirm positive results.

We identified three donors with USUV infection. In July, the first donor was detected in Majorca (Balearic Islands), whose infection was confirmed by NT. In September, two donors with USUV Africa 3 lineage were identified by RT-PCR and sequencing in Catalonia. There were three USUV-positive cases in 95,357 donations (1 in 31,786; 95% CI: 1 in 10,877 to 1 in 154,083) during the 2024 WNV season in Spain.

We report three cases of autochthonous USUV infection in blood donors from the Balearic Islands and Catalonia, two of which were infected by the USUV Africa 3 lineage. Health authorities should be aware that positive WNV screening may be due to USUV, an emerging zoonotic virus that can be underreported.