BACKGROUND: Starting in 1999, the West Nile virus (WNV) epidemic represents the largest outbreak of arboviral encephalitis ever recorded in the U.S. The effective means to determine an infection are detection of viral nucleic acid and/or viral specific immunoglobulin, IgM and/or IgG.
OBJECTIVE: To develop a highly sensitive and specific TaqMan RT-PCR assay for the detection and quantification of WNV RNA of lineage 1 and lineage 2.
STUDY DESIGN: A TaqMan RT-PCR primer-probe was designed to perfectly match target sequences of all sequenced WNV strains and isolates, which added a layer of protection against false-negative results due to strain variability. In addition, the inclusion of a low level RNA internal control (IC) in the assay increased the precision and accuracy of the assay.
RESULTS: By optimizing the RNA preparation procedure for increased WNV RNA recovery, together with optimizing the primer-probe and TaqMan conditions for improved amplification efficiency, we developed a highly sensitive assay with the detection limit of 10 copies/mL. To evaluate the assay, we tested plasma samples from 12 transfusion-transmitted implicated cases in 2002 and from 68 positive blood donors in 2003. All tested specimens were WNV positive. The viral load of 68 positive blood donor samples collected in 2003 ranged from 10 copies/mL to 67,000 copies/mL with a mean of 8100 copies/mL. Furthermore, high sensitivity of the assay was achieved without compromising specificity. All 100 routine donor samples tested negative.
CONCLUSIONS: The assay results demonstrate that our in-house TaqMan RT-PCR procedure can detect and quantify WNV RNA of lineage1 and lineage 2 in human plasma with high sensitivity and specificity.
