Figure 2. Relationship between average per capita income and West Nile virus prevalence.

Results are shown for vectors in Orange County, California, for 2004, 2005, and 2008. Prevalence is measured as MLE. Dashed lines indicate the bifurcation between high and low prevalence values as determined by tree regressions. Horizontal lines indicate mean values of prevalence for points above and below this bifurcation (Wilcoxon rank-sum tests for these means were significant for each year, p<0.001). Although absolute measures of WNV prevalence varied between years, relationships between predictors (per capita income in this case) and WNV prevalence were stable throughout the study period.