West Nile Virus positivity amongst Canadian blood donors and effectiveness of off-season screening.

Background: We sought to examine the prevalence of West Nile Virus (WNV) nucleic acid test (NAT) positivity over time in Canadian blood donors, examine the level of false-positive NAT screening, and understand the effectiveness of travel-related testing outside of WNV seasonality in Canada.

Study design and methods: All blood donations in Canada (except Québec) between 2018 and 2023 were tested by the Roche Cobas WNV-NAT assay during the summer-fall season by Canadian Blood Services. Testing for recent travel outside Canada was performed in the winter-spring season. All 2023 WNV-NAT-positives were confirmed by two independent WNV-specific polymerase chain reaction (PCRs).

Results: While WNV positivity varied by location and time, no increasing trend of positivity was observed over the 6-year study period. No WNV-NAT-positive donors with a history of recent travel outside of Canada in the winter-spring season were identified by donor screening (2018-2023). In total, 2,454,102 samples were tested during summer months and 267,988 in winter months. In 2023, WNV-specific PCR confirmed 10 of 13 (76.9%) screening WNV-NATs. Recent Japanese Encephalitis Virus (JEV) vaccination was identified as a potential false positive for initial WNV screening results.

Discussion: Between 2018 and 2023, WNV-NAT positivity amongst Canadian blood donors remained stable, with no evidence of an increasing positivity trend. The utility of off-season travel-related testing may be limited, as no donors were identified during the six-year study timeframe. Finally, confirmation testing using WNV-specific PCR is recommended to identify true cases and to limit misidentification of other genetically similar viruses, such as JEV.