We used a WNV syndrome ascertainment ratio, the ratio of WNF case counts over WNND counts, to characterize the completeness of WNF ascertainment in the United States from 2003 through 2005. Policies, practices, and capacities of health departments' epidemiology and laboratory programs were linked to syndrome ascertainment ratios for their jurisdictions. These univariate analyses identified commercial testing unavailability (2003), fewer restrictions on WNV testing by the health department laboratories (2003 and 2005), four or more health department surveillance activities to enhance reporting (2004 and 2005), and receipt of most case reports from testing outside the public health laboratory system (2005) as potentially related to increased WNF ascertainment. Although chance findings are possible, the timing of these significant policies and practices are consistent with the changing laboratory and surveillance capacities of health departments' laboratory and epidemiology programs and the overall response to the emergence of WNV in the United States. For example, commercial enzyme immunoassay availability would be expected to be most significantly related to WNF ascertainment in 2003 when a market for the tests was initially being established. Other year-specific inconsistencies, especially the lack of measurable significance for the effects of fewer restrictions on WNV testing by the health department laboratories in 2004 and surveillance activities to enhance reporting in 2003, are likely explained by the small numbers of jurisdictions in the analyses. Although survey response rates were not optimal, response rates did not differ when stratified by population size, cerebrospinal fluid testing rates for WNV, WNND incidence, or the syndrome ascertainment ratio itself.
Because of the potential severities of WNV-related febrile illness and its sequelae,2–6
there is a need for an accepted measure of WNF surveillance sensitivity. Ratio indicators for evaluating surveillance sensitivity have been previously introduced and applied,20,22
but this study is the first use of a WNV syndrome ascertainment ratio. Its application provided a simple and effective measure of the relative frequency of observed syndromes among states and large city/county public health programs, enabling sources of WNF ascertainment variability to be identified each year. Importantly, we specifically refer to the relative completeness of ascertainment because all cases included in these analyses had to have been tested, confirmed, and reported regardless of syndrome. However, among the unknown cases not included in the dataset (i.e., cases not ascertained), the syndrome ascertainment ratio is not intended to determine the frequency or number of cases that were untested, tested but unconfirmed, or tested and confirmed but not reported. Instead, the effectiveness of measuring the relative frequency of observed syndromes across jurisdictions or subpopulations depends on two assumptions. First, WNND ascertainment is assumed to be stable because testing, confirmation, and reporting are relatively complete. Persons with WNND are likely to be hospitalized (> 90%) because of disease severity,4
but the proportion of people with WNND who are diagnosed as having WNV infection is unknown. A meningitis or encephalitis clinical manifestation should raise the index of suspicion, particularly during seasonal or epidemic WNV and in the context of increasing publicity about WNV, making etiologic diagnosis of WNND more likely than WNF. Although CSTE has described WNND reporting as reasonably complete,19
we did not find any published studies of the completeness of WNND or WNV reporting. Second, WNF ascertainment varies, but persons with symptomatic, febrile illness are eligible for ascertainment because healthcare is sought and WNV infection is considered in the differential diagnosis. Estimates of the proportion of persons with WNF seeking healthcare or requiring hospitalization vary, but are substantial.1–4,31
Our estimates of the reduced likelihoods of WNF ascertainment among Blacks and Hispanics relative to non-Hispanic Whites should be interpreted cautiously because a significant portion of the race data (26%) and the ethnicity data (41%) were missing. Ignoring records with missing data would have substantially reduced our sample size and would have assumed that data are missing completely at random, which is unlikely. Instead, missing data uncertainty was appropriately represented by generating multiple, analogous datasets in which imputed race and ethnicity vary. The surveillance data were augmented for imputation by using county-level race and ethnicity data from the U.S. Census. The underlying assumption that racial and ethnic composition of an area likely correlates with one's own race and ethnicity is supported by the realities of U.S. neighborhood racial segregation.32
Multiple imputation also assumes that the extent to which data are missing is statistically independent of one's own race or ethnicity after accounting for the remaining fully observed data (i.e., data are missing at random). In these analyses, this assumption was presupposed because it is impossible to validate without supplemental data.33
More complicated adjustments are possible, but require more detailed, problem-specific statistical development and would not be available in standard software.
Given the overall health disparities, specific disparities in infectious diseases rates, and disparate healthcare among racial and ethnic minorities,34–36
the possibility that WNF ascertainment was 44% less frequent among Blacks and 31% less frequent among Hispanics relative to non-Hispanic Whites, after accounting for missing data uncertainty and program-level sources of ascertainment variability, is worrisome. Specifically, we found that testing and reporting between Blacks and non-Hispanic Whites was unequal only when most reports originated from outside the public health laboratory system in 2005. This finding suggests that differential accessibility and use of commercial tests may play a role in the inequality. Also, the combined effects of residency in regions where WNF ascertainment was less rigorous, together with higher population percentages of Blacks in those areas, may create more missed opportunities for WNF ascertainment in Blacks. In 2003, WNF ascertainment was twice as likely in West and Midwest regions compared with Southern and Northeast regions. All eight states with ≥ 20% of the population comprised of Blacks were in the Southern region, and two-thirds of jurisdictions with 10–20% Black residents were in Southern or Northeast regions. Hispanic population percentages were more evenly distributed among regions (jurisdictions with ≥ 15% Hispanic residents were located in Southern [19%], Northeast [25%], Midwest [19%], and Western [38%] regions).
Other factors might also explain the racial and ethnic disparities in WNF ascertainment we observed. A study of 172 WNV cases that were hospitalized in Houston determined that attack rates were relatively similar for Blacks (5.7 per 100,000) and non-Hispanic Whites (6.6 per 100,000), but substantially lower in Hispanics (3.5 per 100,000).37
One explanation for this finding is that prior flavivirus exposure among Hispanics creates an increased frequency of cross-protective antibodies, which could mitigate WNV disease severity. Nevertheless, additional research and program evaluation are needed to explain these observed differences by directly investigating their underlying mechanisms. In particular, active case-finding could produce a representative cohort of case-patients for such analyses. Collectively, reduced ascertainment and incomplete race/ethnicity data38
undermine surveillance objectives to monitor disease in populations that often have the greatest risk.39
This baseline analysis of WNF ascertainment completeness demonstrated that multilevel analyses can provide estimates of the relative importance of multiple sources of ascertainment variability. Previous evaluations have established that reporting completeness varies,15
but surveillance ascertainment variability had not been jointly measured at the individual and public health systems levels. We identified availability of laboratory testing in the public domain, active surveillance, and surveillance staffing rates as program characteristics that are particularly associated with successful WNF ascertainment. We also found that ascertainment variability was associated with surveillance and infectious disease control staffing rates (2003–2005), Census regions (2003), and population percentages of Blacks and Hispanics (2003). These findings not only suggest that improvements in monitoring human disease can be achieved by agencies operating at optimal capacities, but also reinforce concerns raised about decreased federal funding for prevention and control of WNV and other emerging infectious diseases.