Daily weather conditions associated with the summertime onset of West Nile Virus in central Illinois

The West Nile Virus (WNV) was first identified in the United States in 1999 and reached epidemic proportions by the summer of 2002. It is a mosquito-borne virus. Two mosquito species, Culex restuans, the white-spotted mosquito, and Culex pipiens, the northern house mosquito, are believed to maintain the natural transmission cycle of West Nile Virus (WNV) between birds and mosquitoes. The population of northern house mosquitoes, the primary suspect for WNV transmission to humans, is low in spring but becomes the dominant species later in the summer, as the white-spotted mosquito decreases in abundance. This “crossover” period is defined as the time when the relative proportion of the two species is equal. Culex restauns and Culex pipiens mosquito populations have been sampled in the Urbana-Champaign area since 1988 by the Illinois Natural History Survey. Over the last 21 years, the cross-over has occurred in central IL, from early July to mid-September. The peak infection rate in mosquitoes is usually about two to three weeks after the northern house mosquito becomes the dominant species. This is the period of greatest risk of transmission to humans, horses, and wildlife.

Mosquito populations have been found to be sensitive to daily temperature and in fact, predictive models for the cross-over date based on maximum temperature and growing degrees have been developed (Kunkel et al., 2006) and are currently being run operationally at the Midwestern Regional Climate Center (http://mcc.sws.uiuc.edu/research/westnile/index_anim.htm). The model inputs are based on daily data for Urbana IL, but the results are generally representative of the central Midwest. The crossover date is generally well predicted (to within about 7 days) except for distinguishing years where the mosquito populations exhibit multiple crossover dates or where there is no crossover date. The pre-crossover period in the years with early cross-over dates (July 10-31 / 9 cases) are typified by warmer than average maximum and minimum temperatures, and later cross-over dates by cooler than average temperatures (Aug 15 – Sep 11 / 5 cases), as expected from earlier studies. The timing and amount of precipitation, as well as more subtle temperature variations will be examined to determine how they impact the expected timing of the cross-over of mosquito populations. These results will be presented.