Rationale:

• Viremic persons may be asymptomatic, and the virus can be transmitted through blood products.

Evidence:

• In an investigation of 60 cases of WNV illness in blood product recipients, 23 had evidence of contracting the virus through transfusion (18).
• More than 1000 potentially viremic donations have been screened out by national blood donor screening programs. Approximately 5% to 10% of infectious donations may be missed by mini-pool screening. To minimize the risk of transmission, blood centers may switch to single-unit screening in areas with ongoing transmission to humans. All 30 known cases of transfusion-transmitted WNV have resulted from IgM-negative donations (19).

Comments:

• As of March 2006, seven cases of transfusion-associated WNV transmission from screened blood had been reported. In all instances, the currently employed mini-pool screening format used by the blood banks failed to identify the infectious donor (20). Physicians should continue to report transfusion recipients who subsequently develop WNV illness to their state or local public health departments.