West Nile virus among blood donors in the United States, 2003 and 2004.

Background: West Nile virus first appeared in the United States in 1999 and has since spread throughout the contiguous states, resulting in thousands of cases of disease. By 2002, it was clear that the virus could be transmitted by blood transfusion, and by the middle of 2003, essentially all blood donations were being tested for West Nile virus RNA with the use of investigational nucleic acid amplification tests; testing was performed on individual samples or on "minipools" of up to 16 donations.

Methods: We analyzed data from the West Nile virus testing program of the American Red Cross for 2003 and 2004 to identify geographic and temporal trends. In areas with a high incidence of infection, individual donations were tested to increase the sensitivity of testing. Donors with reactive results participated in follow-up studies to confirm the original reactivity and to assess the natural history of infection.

Results: Routine testing in 2003 and 2004 identified 540 donations that were positive for West Nile virus RNA, of which 362 (67 percent) were IgM-antibody-negative and most likely infectious. Of the 540 positive donations, 148 (27 percent) were detectable only by testing of individual donations, but only 15 of the 148 (10 percent) were negative for IgM antibody. The overall frequencies of RNA-positive donations during the epidemic periods were 1.49 per 10,000 donations in 2003 and 0.44 per 10,000 in 2004. In 2004, 52 percent of the positive donations were from donors in four counties in southern California.

Conclusions: Rapid implementation of a nucleic acid amplification test led to the prospective identification of 519 donors who were positive for West Nile virus RNA and the removal of more than 1000 potentially infectious related components from the blood supply of the Red Cross. No cases of transfusion-transmitted infection were confirmed among recipients of the tested blood.