Transfusion-associated transmission (TAT) of West Nile virus was first recognized in the United States, in 2002.  This prompted the rapid development of two nucleic acid testing (NAT) assays to screen donated blood for WNV viremia.  By July 14, 2004, all blood banks in the United States had implemented testing. 

Due to automation constraints, mosts tests are conducted on minipools  (MP) drawn from 6 or 16 donors. If pooled blood is positive for WNV, the individual donation testing (IDT) is done for those in the pool to determine who is infected.  Some blood centers, especially in epidemic areas, have used IDT.

Between June and December, 2003, approximately 6 million units of blood were tested, resulting in the removal of at least 818 WNV-contaminated blood donations from the blood supply. During 2003, 23 suspected cases of transfusion-associated transmission were reported to the CDC, and 6 of these cases were confirmed.  There have been at least two confirmed cases of WNV encephalitis associated with transfusion of blood that did not react when initially tested by MP NAT. This suggests that testing of mini-pooled blood may not detect all WNV-infected blood donors and that a small danger may exist despite universal testing.  In both confirmed cases, retrospective IDT identified donors with WNV.

Since 2002, health officials have been encouraged to investigate and report to the CDC any reports of WNV infection among patients who had received blood transfusion <4 weeks before onset of illness.  People who are receiving blood transfusions should be aware of the slight risk of WNV infection, but the benefits of receiving transfusions for most people outweigh the potential risk.

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