FIGURE 2-5 Equine and human West Nile virus neuroinvasive disease cases, by year, United States.

by health departments across the country. Between 1999 and 2006, ArboNET has detected 62 species of mosquitoes that are positive for WNV—a huge number for an arbovirus—although more than 98 percent of infected mosquitoes collected belong to the genus Culex (CDC, 2007a). WNV is associated with an enormously high prevalence of virus in mosquitoes, such that during WNV outbreaks, infection rates are measured in percents of mosquitoes infected, rather than per thousand, a more typical rate of infection for an arboviral vector.

From 1999 through 2006, 317 species of WNV-positive dead birds were reported to ArboNET (CDC, 2007a). In 2006, American crows and blue jays accounted for 62 percent of the total reported (however, some reporting bias is likely as both species are relatively common in urban settings, large, and morphologically distinct). Recent data indicate that declines in crow and other susceptible bird populations have accompanied the introduction of WNV into an area (LaDeau et al., 2007). Figure 2-5 illustrates trends in cases of neuroinvasive WNV disease among humans and horses. The introduction of an equine WNV vaccine before the 2003 transmission season has markedly decreased the incidence of disease in horses.

A Hidden Epidemic

Table 2-1 summarizes reports of human WNV cases in the United States to date, including more than 9,900 cases of neuroinvasive disease in the form of



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