Academic journal article URISA Journal

West Nile Virus in the Greater Chicago Area: A Geographic Examination of Human Illness and Risk from 2002 to 2006

Academic journal article URISA Journal

West Nile Virus in the Greater Chicago Area: A Geographic Examination of Human Illness and Risk from 2002 to 2006

Article excerpt

BACKGROUND

West Nile virus (WNV) is a mosquito-borne disease agent primarily associated with the Culex genus of mosquito as vectors and several species of birds as reservoir hosts. First introduced to North America in New York in 1999, it has since emerged as a major zoonotic pathogen. Human cases of illness from WNV now have been reported throughout the continental United States, as well as in Canada and Mexico, and it is expected that the virus cycle will continue with occasional human and animal outbreaks (CDC 2009, Elizondo-Quiroga et al. 2005, Petersen and Hayes 2004, Public Health Agency of Canada 2009). Although the disease often presents only mild flu-like symptoms in humans, it can manifest itself in a more severe neuroinvasive form, which may result in death (Hayes and Gubler 2006). Because of the absence of a vaccine for WNV, reduction in the abundance of mosquito vectors and personal protection from mosquito bites remain the primary options for WNV prevention in humans (Zeller and Schuffenecker 2004).

Since 1999, 47 states have reported human illness from WNV. During the period from 1999 to the end of 2009, nearly 29,000 human WNV infections have been reported in the United States, four percent of which have resulted in death (CDC 2010). Illinois consistently experienced high numbers of cases of human illness and deaths between 2002 and 2006, ranking first in 2002, second in 2005, and sixth in 2006 (Hamer et al. 2008). When the first large outbreak was experienced in Illinois in 2002, 686 of the total 884 cases of human illness were reported in the greater Chicago area, with some neighborhoods exhibiting significantly higher rates than did others (Ruiz et al. 2004). Although 2002 was the largest outbreak year to date, 362 more cases were reported in this area during the years 2003 to 2006, with the second largest outbreak (182 cases) occurring in 2005. Our objective is to determine the environmental risk factors associated with human illness in the Chicago area from 2002 to 2006 through an ecological statistical analysis that accounts for any spatial autocorrelation. This area has had enough cases of illness to allow for spatial statistical analysis of the data and has been the subject of other studies of transmission of the virus, allowing for a more in-depth discussion of the results of the ecological analysis.

Risk of illness from WNV has been estimated using a variety of approaches. Case data and individual characteristics and behaviors point to higher rates of severe illness in older people and male patients and to greater risk among those who do not use insect repellent or who are outdoors during peak mosquito hours (O'Leary et al. 2004, Komar 2003, Gujral 2007, Warner et al. 2006). Surveillance of birds to predict human risk has yielded mixed results. Yiannakoulias et al. (2006) found that infected bird data contributed little to their model of geographic variations of human WNV illness in Alberta, Canada, while others have reported successful prediction of human risk with this approach (Theophiledes et al. 2002, Theophiledes et al. 2006, Guptill et al. 2003). Other risk studies focus on mosquito infection or mosquito habitat (Gibbs et al. 2006, Ozdeneral et al. 2008, Trawinski and MacKay 2008, Zou et al. 2006, Diuk-Wasser et al. 2006, Tachiiri et al. 2006), and through combinations of approaches (DeGroote et al. 2008, Bell et al. 2006, Neilsen et al. 2008, Winters et al. 2008). Evaluations of environmental risk factors for human illness from WNV have included considering the amount or types of vegetation, the density of human settlement, the neighborhood housing and socioeconomic characteristics, the bird diversity, and the dynamic weather-related conditions (Landesman et al. 2008, Platonov et al. 2008, Allan et al. 2009). Equine illness from WNV also has been considered as a potential marker of risk to humans (Corrigan et al. 2006, Ward and Scheurmann 2007).

Considering the variety of results from these studies, we note that differences in the behavior and the habitat of the mosquito vectors are found in different places, which make direct comparisons difficult. …

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