Relating West Nile virus case fatality rates to demographic and surveillance variables

Objective. The purpose of this ecological study was to relate West Nile virus (WNV) human case fatality rates to county-level demographic and surveillance variables, thereby characterizing the populations to which WNV poses the greatest threat. Methods. The authors acquired data on human, avian, and mosquito WNV infections for the 13 states in which there were 100 or more human cases during 2003. The data on avian and mosquito infections were converted into surveillance variables using empirical Bayes methodology. A preliminary logistic regression model was formulated to relate these surveillance variables and demographic variables to case fatality rates. The statistical technique of backward elimination was applied to obtain a final model in terms of the variables most useful for predicting case outcomes. Results. The probability of a fatal outcome depends on the poverty rate for the county in which the infected person lives (p=0.0283), the average temperature (p<0.0001), and surveillance variables reflecting the fractions of Culex pipiens and Culex restuans mosquitoes among infected mosquitoes (p=0.0079; p=0.0076). Conclusions. Effective WNV educational programs and control measures are vital, especially in poverty-stricken areas. A uniform protocol for disseminating county-level data could facilitate timely responses to WNV outbreaks and to emerging infectious diseases more generally.