![]() ![]() Therefore, based on 349 probable and confirmed WNND cases, we estimated 85,156 WNV infections (95% CI 68,302–103,866) or 1.98% (95% CI 1.59%–2.41%) infection proportion during the 2012 epidemic season ( Table 2). Of 356 probable and confirmed WNND case-patients, 7 were <15 years of age. Applying the 4-county area’s 2011 population estimates and the number of days screened by each method to the NAT yield-derived incidence rates resulted in an estimated 31,013 WNV infections (95% CI 19,133–42,893) or 0.72% (95% CI 0.44%–1.00%) infection proportion during the 2012 epidemic season. Incidence was presumed to be 0 outside the WNV season. During the 239-day WNV season, the ratio of blood donations screened by each method was assumed to be equal to the ratio of days screened by each method (because donations per day are roughly constant throughout the season). The incidence rates also differed, estimated as 7.2 WNV infections (95% CI 3.5–10.9) per 10,000 donor-months (MP-NAT screening periods) and 24.7 WNV infections (95% CI 13.3–36.0) per 10,000 donor-months (ID-NAT screening periods). The time at risk for donors differed detection period is estimated as 19.6 days for ID-NAT and 10.7 days for MP-NAT ( Technical Appendix). †Person-months = number of donations tested multiplied by WNV RNA detection period/30.5. *NAT, Nucleic acid amplification technology CIs were obtained by applying Taylor series expansion ( 9), based on a Poisson distribution for the WNND cases and the estimated variance of the ratio of WNV cases to WNND cases as reported ( 6). To estimate the number of WNV infections by age and gender, we used confirmed and probable WNND cases in persons >16 years of age reported during the WNV season to the Texas Department of State Health Services and included in ArboNET, a national surveillance system which monitors WNV activity ( 6). The WNV seasonal incidence rate in blood donors and days screened per method were then applied to the estimated 2011 population of the 4 counties who were age-eligible for blood donation ( >16 years of age) ( 8) to estimate the number of WNV infections in that area during the 2012 WNV season. CIs were obtained assuming a Poisson distribution for ID-NAT and MP-NAT yields. WNV seasonal incidence rates were obtained using a previously derived formula ( 5) by using rates of detection and durations of ID-NAT and MP-NAT WNV RNA detection periods. For this analysis, we derived new estimates for the duration of the MP-NAT and ID-NAT detection periods ( Technical Appendix). The length of time WNV RNA is detectable by MP-NAT has been previously reported ( 7). These data were used to derive the WNV seasonal incidence rate in 2012.Ĭalculations were performed separately for ID-NAT– and MP-NAT–screened donations. Carter BloodCare accounts for >95% of blood donation centers in the North Texas Region. The aim of this study was to estimate the number of WNV infections in this area during the 2012 arboviral season using 2 models: blood donor NAT yield and WNND-based models ( 5, 6).Ĭounts of screened blood donations and confirmed WNV viremic donations detected by MP-NAT or ID-NAT from northern Texas residents during the WNV season (April 1, 2012, through November 30, 2012, the WNV surveillance period used by AABB for triggering ID-NAT screening ) were obtained from Carter BloodCare and Creative Testing Solutions, the area blood collection organization and donor screening laboratory, respectively. Approximately 48% of cases in Texas were in 4 counties: Collin, Dallas, Denton, and Tarrant, located in the northern area of the state. Nationwide, the largest WNV epidemic since 2003 occurred in 2012, and approximately one third of cases were reported from Texas. With the introduction of targeted ID-NAT, estimates of WNV infections from viremic blood donors must account for differential ID-NAT and MP-NAT screening during epidemic seasons. West Nile neuroinvasive disease (WNND) reports were then used to approximate the number of infections relative to WNND cases ( 5). Approximately 25% of viremic blood donors can be detected by ID-NAT ( 4).Įstimates of WNV infections in 2003 were derived from viremic blood donor rates detected by MP-NAT throughout the United States. During 2004, screening algorithms expanded, including triggered individual donation NAT (ID-NAT) ( 3). Despite the success of MP-NAT screening of samples from blood donors, WNV transmission from infected donors continued. In 2003, identification of transfusion-transmitted WNV infections ( 2) led to screening of the blood supply for WNV by using nucleic acid amplification technology (NAT) assays in mini-pools (MP-NAT) ( 3). ![]() From the first reported cases of West Nile Virus (WNV) in North America in August of 1999 through 2013, more than 39,000 cases of West Nile virus (WNV) were reported in the United States ( 1). ![]()
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