Environmental Factors and Risk Areas of West Nile Virus in Southern California, 2007–2009

Abstract

The West Nile virus (WNV) may post a significant health risk for mammals, including humans and insects. This study examines the spatial-temporal effects of environmental factors on WNV dissemination with a case study of ten counties in the southern California, where the epidemic was recently most prevalent within the USA. WNV surveillance data were obtained from the California Vectorborne Disease Surveillance System and Centers for Disease Control and Prevention. Remote sensing and Geographic Information Systems (GIS) techniques were combined to derive environmental variables. Principal component analysis was performed to select the most relevant environmental variables. Two ecological zones were identified based on the selected variables. Identification of risk areas for WNV was limited to a zone with 95% mosquitoes surveillance records. Three time windows, the epidemiological weeks 18-26, 27-35, and 36-44 in each year of 2007-2009, were examined in details with risk area mapping. It is found that the southern part of San Joaquin Valley in Kern County and Los Angeles County (especially its southern part) were the most vulnerable locations for WNV outbreak. Main factors contributing to the WNV propagation included summer mean temperature, annual mean deviation from the mean temperature, land surface temperature, elevation, landscape complexity, landscape diversity, and vegetation water content. The result of this study improves understanding of WNV ecology and provides tools for detecting, tracking, and predicting the epidemic. The holistic approach developed for this study, which integrated remotely sensed, GIS-based, and in situ-measured environmental data with landscape metrics, may be applied to studies of other vector-borne diseases.

Environmental and Socioeconomic Factors Associated with West Nile Virus

“…In this study, the environmental factors temperature and precipitation were significantly related to the incidence of human West Nile cases, and this finding is consistent with previous studies (12,14,31,32). Mosquitoes need water for larvae to develop, and insects grow more rapidly in warmer temperatures (33–35).…”

“…Several variables that previous studies found associated with WNV include temperature, rainfall, and habitat. In southern California, summer mean temperature, land surface temperature, elevation, landscape diversity, and vegetation water content were principle environmental factors that contributed to WNV propagation (12). High temperature has been consistently associated with outbreaks of WNV (13).…”

Environmental Drivers of West Nile Fever Epidemiology in Europe and Western Asia—A Review

“…In their study in northern Colorado, Eisen et al . [76] found that mosquito species richness is highest in plains habitats at elevations below 1,600 m. Later, Liu and Weng [77] showed in southern California that areas with lower elevations tended to be more susceptible to WNV invasion. This finding may be explained by the higher landscape diversity usually associated with multiple land cover types such as urban, grass, and water.…”

A comparison of least squares regression and geographically weighted regression modeling of West Nile virus risk based on environmental parameters

“…Spectral indices acquired from satellite imagery provide information about environmental characteristics like temperature, vegetation cover, and moisture (Rodgers & Mather, 2006). Liu & Weng (2012) in a study on WNV risk in southern California found that one of the main factors contributing to the WNV propagation included land surface temperature. They related higher temperature to viral replication in mosquitoes for WNV to be disseminated throughout the year.…”