West Nile virus (WNv), first reported in the United States in 1999, infects many species of birds as well as humans, equids, and other mammals. Though new to North America, the virus had circulated in Africa, Europe and the Middle East for some time prior to 1999; and other outbreaks, including those in southern Russia, Romania and Israel were indications of a change in the range of the virus [1–4]. Mosquitoes transmit the virus between hosts, with Culex species most often implicated as the primary amplification vector  and bridge vector [6, 7]. Many competent avian hosts have been identified both in the lab and field , and recent work in parts of North America have focused on the possible important role of American Robins (Turdus migratorius) in contributing to virus amplification and maintenance in the sylvatic cycle [9–11]. The continued risk for this sometimes severe and even fatal disease prompted establishment of annual surveillance programs of virus infection in mosquito populations. Now that several years have passed since the introduction of the WNv in North America, longitudinal data from testing of mosquitoes and host species (including records of human and equine illness) reported through systematic surveillance are available for development of models of the risk of infection. These records can be used to examine differences in infection between and within years and among locations to better understand the risk of transmission of the virus and to predict the possibility of place and time-specific outbreaks.
Though many published reports characterize associations between climatic and landscape factors and WNv occurrence, broad patterns have remained elusive as inconsistent results make generalization difficult. A review of 15 publications identified common landscape variables used to predict risk of WNv transmission, including distance to riparian corridor, vegetation measures, slope, elevation, human population numbers, housing and road density, type of urban land use, race, income, housing age, and host community structure [12–26]. Each predictor variable had a significant relationship with the WNv response variable in at least one study, but the directions of the effects were inconsistent among studies. From our review, we consider these inconsistencies to result largely from differences in climatic factors and mosquito vectors among geographic locations, heterogeneous temporal and spatial resolution of the analyses, and the different response variables used to measure risk.
In studies focused on spatial or temporal risk of illness from WNv, the response variable typically includes data from human disease cases [13, 14, 16, 20, 27–29], vector abundance or infection [15, 17, 30–34], evidence of infection in non-human hosts [21–24, 26], or a combination of vector and host data [1, 12, 25, 35–37]. When human case data are the response variable, inconsistencies between WNv risk and environmental factors have been attributed to: exposure occurring away from the home residence; time outdoors; use of insect repellent; socioeconomic differences, and even variable case definition [38–41]. An analysis in Indianapolis, IN, with similar climatic features and spatial scale as the current study, identified a relationship with both low precipitation and higher temperatures and increased numbers of human WNV cases .
For analysis of the transmission of mosquito-borne arboviruses, vector abundance and infection data are key elements, and the two are not always proportional in space or time [42, 43]. For this reason, the fine scale patterns relevant to production and dispersal of adult female mosquitoes and the association between WNv infection in mosquitoes and weather in the context of other landscape features are in need of further investigation. When analyses include different vectors, results will often be inconsistent [16, 29]. For example, the population size of Culex pipiens., the primary enzootic and epidemic vector in the eastern U.S. north of 36 degrees latitude, is often impacted negatively by large rain events due to the flushing of catch basins, a primary urban larval habitat, and the reduction of organic content in all ovipositing sites [44, 45]. By contrast, the vector Culex tarsalis generally responds positively to heavy precipitation, which provides the typical larval habitat in rural areas in the western part of the U.S [28, 33]. In semi-permanent wetlands, drought conditions can increase abundance of some vector populations as they result in more larval breeding sites with fewer competitors and mosquito predators . Early season drought with subsequent wetting and low water table depth preceded amplification episodes for both WNv and St. Louis encephalitis virus in peninsular Florida, where Culex nigripalpus functions as the main vector [47–49].
Increased temperature is known to increase growth rates of vector populations , decrease the length of the gonotrophic cycle (interval between blood meals), shorten the extrinsic incubation period of the virus in the vector and increase the rate of virus evolution [50–54]. Kunkel et al.  showed a correlation between the number of days when daily maximum temperature exceeded a threshold (degree days), timing of a seasonal shift to a higher proportion of Culex pipiens among all Culex species, and the onset of the amplification phase of WNv transmission seasonally in Illinois. Other studies considered less proximate weather conditions, such as increased rainfall in the preceding year . Warmer winter temperatures and warmer March and April may lead to larger summer mosquito populations . Temperature has also been linked to the rate of evolution of the virus and warmer temperatures facilitated the displacement of the WNv NY99 genotype by the WN02 genotype . Bertolotti et al.  discovered high genetic variation of WNv at fine temporal and spatial scales, with variation in local temperature offered as one explanation for it. At the same time, in one study, very high temperatures (above 30°C) reduced larval Culex tarsalis survival .
Finally, the choice of geographic scale and units can impact profoundly the outcome of an analysis. Coarse geographic scales such as the county level can obscure fine spatial patterns in a heterogeneous landscape . The census tract provides a somewhat finer scale geographic unit, but while logistically useful, is not a biological meaningful unit. In fact, any use of a spatial unit introduces the modifiable area unit problem [59, 60], where spatial units of analysis become arbitrary or possibly even introduce systematic bias. In addition to the importance of fine spatial scales, coarse temporal scales could obscure some temporal patterns [31, 32].
Our analysis focuses on greater Chicago, Illinois, where WNv infection in mosquitoes, horses and birds was first noted in 2001, and where human illness has been reported every year from 2002 to 2009 [, http://www.idph.state.il.us/envhealth/wnv.htm]. We consider temporal and spatial patterns of infections in mosquitoes for the years 2004 to 2008, years for which comprehensive mosquito testing data were available in Illinois. We have focused, in particular, on the meteorological conditions that precede or are concurrent with amplification, when a sharp increase is seen in the infection rate in mosquito populations. Our three research questions are: 1) Inter-annually: what are the conditions associated with higher mosquito infection in some years compared to others? 2) Intra-annually: what temporal characteristics of rainfall and temperature precede changes in mosquito infection and with what temporal lag? 3) Spatially: can the patterns of rainfall and temperature help explain the differences in mosquito infection across space? We used surveillance data from the Illinois Department of Public Health (IDPH) and publicly available meteorological readings, and consider the heterogeneity of urban land cover through an analysis of digital spatial data to identify and forecast favorable conditions for WNv amplification in the greater Chicago area.