West Nile virus transmission through blood transfusion--South Dakota, 2006.

West Nile virus (WNV) transmission through blood transfusion was first reported in 2002, prompting rapid implementation of nationwide screening of blood donations for WNV by 2003. Screening strategies were developed using minipool nucleic acid-amplification testing (MP-NAT) based on six or 16 pooled donor samples. To improve sensitivity of WNV detection, blood-collection agencies (BCAs) later implemented enhanced screening by individual donation NAT (ID-NAT), most often used when a given trigger threshold of positive MP-NAT results is reached during the WNV transmission season. This approach has been effective, resulting in the detection and interdiction of approximately 1,400 potentially infectious blood donations during 2003-2005 and a reduction in recognized transfusion-transmission events. A total of 23 confirmed WNV transfusion-transmitted cases were reported in 2002, before screening was implemented; six probable or confirmed cases were detected in 2003 after MP-NAT screening was initiated, one was detected in 2004, and none were detected in 2005. This report describes the first WNV transfusion-transmission cases detected since the initiation of enhanced screening strategies using ID-NAT triggering. In 2006, two immunosuppressed patients had onset of West Nile neuroinvasive disease (WNND) after receiving blood products from a single infected donor despite a negative MP-NAT result at the time of donation. Although risk for transmission has been substantially reduced as a result of routine MP- NAT and triggered ID-NAT screening, clinicians should be reminded that transfusion-transmitted WNV infections can still occur, and that immunosuppressed patients are more likely to have onset of WNND.