West Nile virus in 2006 and 2007: the Canadian Blood Services' experience

The Canadian blood supply has been screened for West Nile virus (WNV) since 2003. A strategy for targeted individual-donation nucleic acid testing (ID-NAT) was implemented in 2004 to identify potentially infectious donations that may be missed by minipool (MP) testing. In 2007, Canada experienced a larger epidemic than in previous years providing an opportunity to evaluate the ID-NAT triggering algorithm in higher-risk areas.

A specially created database and internal-external communication identified regions for targeted ID-NAT using MP and community triggers. WNV-positive donations identified by ID-NAT were reexamined in MP to assess the efficacy of targeted ID-NAT in identifying potentially infectious donations that may have been missed by MP testing. WNV-positive donation data from 2006 and 2007 were analyzed to examine temporal and geographic trends. A telephone survey about symptoms was carried out after the 2007 season.

In total 78 WNV-positive donations were identified (66 true-positives and four false-positives being in 2007). Most positive donations were in the late summer, concentrated in the same western provinces as community cases. Fifty-two donations were identified by ID-NAT and 46% were consistently positive in MP. Of the other 54%, 74% were immunoglobulin (Ig)M- and/or IgG-positive. Fifty-six percent of donors experienced mostly mild symptoms before or after donation (but all said they were well at the time of donation).

WNV-positive donations correspond geographically with the epidemic. MP testing identifies most potentially infectious donations with a smaller potential benefit from targeted ID-NAT. Mild symptoms are common but may not deter donation.