of WNV disease occurred in Chicago and other cities near the Great Lakes, Mississippi, and Louisiana in 2002 (O’Leary et al., 2004); like the 1999 New York City outbreak, this outbreak occurred during a heat wave, as did outbreaks that spanned much of the Midwest in 2003, and again in 2005 and 2006 (CDC, 2007a). By 2004, WNV had become endemic in much of the United States and had reached the West Coast. Outbreaks occurred in desert locations, such as Phoenix, Arizona; this seems remarkable until one realizes that these places have been transformed by humans—who have built golf courses, swimming pools, and reservoirs—into urban oases capable of supporting extensive mosquito breeding. An analysis of data from 2002 through 2006 showed that North and South Dakota, Wyoming, Colorado, and Nebraska had a cumulative incidence of human neuroinvasive disease of more than 15 per 100,000 population, while Montana, Louisiana, and Mississippi had cumulative incidences from 10 to 14 per 100,000. These same states historically have had high incidence of St. Louis encephalitis (SLE) virus, a related flavivirus with similar ecology to WNV. A ranking of the counties in terms of mean annual incidence for WNV neuroinvasive disease reveals a pattern of persistent high incidence of disease in counties along the Platte and Missouri Rivers and the southern Mississippi River.
As illustrated by the epidemic curve for WNV in 2006 (Figure 2-7), human case incidence increases very quickly in mid-summer; often, human epidemics