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. 2010 Feb;7(2):494-508.
doi: 10.3390/ijerph7020494. Epub 2010 Feb 11.

Hydrologic conditions describe West Nile virus risk in Colorado

Jeffrey Shaman  1 , Jonathan F Day, Nicholas Komar
Affiliations

Hydrologic conditions describe West Nile virus risk in Colorado

Jeffrey Shaman et al. Int J Environ Res Public Health. 2010 Feb.

Abstract

We examine the relationship between hydrologic variability and the incidence of human disease associated with West Nile virus (WNV; family Flaviviridae, genus Flavivirus) infection (hereafter termed "human WN cases") in Colorado from 2002 to 2007. We find that local hydrologic conditions, as simulated by the Mosaic hydrology model, are associated with differences in human WN cases. In Colorado's eastern plains, wetter spring conditions and drier summer conditions predict human WN cases. In Colorado's western mountains, drier spring and summer conditions weakly predict human WN cases. These findings support two working hypotheses: (1) wet spring conditions increase the abundance of Culex tarsalis vectors in the plains, and (2) dry summer conditions, and respondent irrigational practices during such droughts, favor Cx. pipiens and Cx. tarsalis abundance throughout Colorado. Both of these processes potentially increase the local vector-to-host ratio, favoring WNV amplification among competent avian hosts and bridging to humans.

Keywords: Culex mosquitoes; West Nile Virus; amplification; hydrology; spatial autocorrelation; transmission.

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Figures

Figure 1.
Figure 1.
Climatology of monthly Mosaic model root zone soil moisture (RZSM) simulated conditions (2001–2007) throughout Colorado. RZSM estimates are units of kg/m2. The mountainous western portion of Colorado has a larger seasonal cycle with greater variations in RZSM due to the large amounts of snowfall that typically occur there in late fall, winter and early spring.
Figure 2.
Figure 2.
Plot of the 0.125 resolution RZSM estimates for January 2001 and the interpolation of this gridded surface to the census tract and county centroids (blue dots).
Figure 3.
Figure 3.
Mean annual monthly Mosaic model estimates of hydrology (RZSM in kg/m2) during 2002–2007 for: top left) eastern Colorado (105.5−102.0 °W) site-years with 1 or more human WN cases per 100,000 people (n = 977) and site-years with fewer than 1 human WN cases per 100,000 people; middle left) eastern Colorado site-years with 10 or more human WN cases per 100,000 people (n = 969) and site-years with fewer than 10 human WN cases per 100,000 people; bottom left) eastern Colorado site-years with 100 or more human WN cases per 100,000 people (n = 251) and site-years with fewer than 100 human WN cases per 100,000 people. The right column shows the same groupings for western Colorado (109.0−106.0 °W) with respective Group 1 numbers of n = 178, 168 and 26 from top to bottom. Corresponding 2 standard error intervals for each grouping are indicated by the color-coded dashed lines. For all plots the blue lines show the grouping of monthly RZSM associated with higher human WN case rates (Group 1).
Figure 4.
Figure 4.
Monthly per capita human WN cases during 2002–2007 in eastern Colorado (105.5−102.0 °W) and western Colorado (109.0−106.0 °W).
Figure 5.
Figure 5.
Gridded monthly per capita human WN cases and corresponding SAR GLM best-fit estimates of monthly per capita human WN cases during 2002–2007 in eastern and western Colorado modeled as a function of local monthly RZSM conditions within that year. The observations and model estimates are ordered in pairs by the latter variable, from least to greatest.
See this image and copyright information in PMC

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