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BACKGROUND: Nationwide 2012 represented the greatest number of reported West Nile virus (WNV) cases since 2003 with a third of all cases reported from Texas. The North Texas Region (Collin, Dallas, Denton and Tarrant counties) accounted for approximately 92 percent of all cases in Texas. The purpose of this study was to estimate the number of WNV infections in the North Texas Region during the 2012 arboviral season utilizing blood donor data as well as confirmed WNV neuroinvasive cases (WNND).
METHODS: WNV viremic blood donors in the North Texas Region were detected by either individual (ID-NAT) or minipool (MP-NAT) nucleic acid amplification testing from April 1, 2012 until November 30, 2012. WNV seasonal incidence rates were calculated separately for ID-NAT and MP-NAT to obtain the WNV seasonal cumulative incidence in donors utilizing a previously derived formula employing the known durations of the ID-NAT and MP-NAT detection window periods and number of donors screened by ID/MP-NAT. Cumulative seasonal incidence was then applied to the projected 2011 North Texas population to estimate the number of WNV infections during the 2012 arbovirus season. In addition, confirmed WNND cases reported to ArboNET with onset dates between April 1, 2012 and November 30, 2012 were utilized in estimating the number of WNV infections at a ratio of 244 WNV infections per case of confirmed WNND.
RESULTS: From April 1, 2012 to November 30, 2012 of 133,727 blood donations screened for WNV RNA, 54 WNV viremic donations were detected (30 via MP-NAT and 24 via ID-NAT). Based on the NAT yield data, WNV incidence during the ~2 months of MP-NAT screening was estimated at 11 infections/10,000, whereas during the ~4 months of ID-NAT screening there were 35 infections/10,000. During the 6 month epidemic period, 100,083 estimated WNV infections occurred in the North Texas population. Based on 181 confirmed WNND cases with onset dates of April 1, 2012 to November 30, 2012, 44,164 WNV infections were estimated.
CONCLUSIONS: The donor NAT yield model resulted in higher projected WNV infections compared to the WNND-based model, in part attributable to incomplete ascertainment and confirmation of WNND cases and differing assumptions in the two modeling strategies. Nonetheless, WNV infections identified via viremic donors, which are detected in real time, are a useful component in WNV surveillance to assist in early identification of human infections.