West Nile Virus

Overview

West Nile virus (WNV), a mosquito-borne flavivirus, first appeared in the U.S. in 1999, and has since become endemic, with high activity during the warmer months. The virus is primarily maintained through a bird-mosquito cycle but can also infect humans and other animals. According to the Centers for Disease Control and Prevention, from 1999-2024 there were 60,992 human cases of WNV reported and 3,134 deaths.

Created in 2006 through a partnership with FDA and CDC, the AABB WNV Biovigilance Network supports compliance with the recommendations in the 2009 FDA guidance, Use of Nucleic Acid Tests to Reduce the Risk of Transmission of West Nile Virus from Donors of Whole Blood and Blood Components Intended for Transfusion including the recommendation that collection facilities in overlapping or adjacent collection regions establish a communication plan to share relevant WNV activity data.

Background

The risk of WNV transmission through blood transfusion and organ transplant was first recognized in 2002, particularly during the viremic phase of infection when individuals show no symptoms. Because approximately 80 percent of people infected with WNV are asymptomatic, the only way to detect infected blood donors is to screen a sample of their blood using tests based on nucleic acid amplification technology. Retrospective studies later confirmed that WNV could be transmitted through blood transfusion and organ transplantation, prompting large-scale nationwide clinical trials under FDA’s IND regulations beginning in 2003 to evaluate investigational NAT assays.

By 2005, FDA had approved two NAT assays to screen for WNV RNA in donors of blood, organs, cells and tissue as well as non-heart-beating donors. These assays are designed for both individual donor testing (ID-NAT) and testing in minipools (MP-NAT) containing up to 6 or 16 donations, depending on the system used. When a reactive result is found in a minipool, and subsequent ID-NAT of the individual donations confirms the reactivity, the unit should be considered potentially infectious. Follow-up evaluation has shown that repeat ID-NAT testing, along with WNV antibody testing, has a high predictive value for confirming infection. Notably, approximately 10% of initially reactive donations that are non-reactive upon repeat NAT testing still show evidence of infection via antibody detection, highlighting the importance of supplementary testing for effective donor counseling and accurate diagnosis.