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Historical Maps by County - Historical Maps by Climate Division - All Maps by Year


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There is evidence of mosquito infection from WNV in Illinois every year since 2001. However, locations with higher WNV mosquito infection rates change from year to year, varying with climatic conditions. In the estimates provided here, Mosquito Infection Rate (MIR) is predicted with statistical models that use rainfall and temperature specific to the nine climate divisions of Illinois. The statistical models are based on observed relationships between climate and weather parameters and measured MIR from previous years, using a methodology developed for a model for Cook – DuPage County area (Ruiz, M.O. et al., 2010), but for the nine Illinois Climate Divisions (CD). Because of uneven differences in sampling and population, the models were based on 4 latitudinal zones: North (CD 01, 02), North-Central (CD 03, 04, 05), South-Central, (CD 06-07), and South (CD 08-09). Climate Divisions in the North region are treated individually. While weather specific to the individual CDs 03 to 09 are used, combined parameters and MIR characteristics are used for each of the other 3 regions.

Model results show predicted values of MIR over time, with the prediction based on the prior week's temperature and precipitation, plus the 10-day forecast of temperature and the 3-day forecast of precipitation, day length, and precipitation and temperature from the previous summer, fall, winter and spring. For comparison, historical time series from prior years are provided. The historical MIR is based on mosquito test results carried out by various agencies throughout Illinois from data organized by the Illinois Department of Public Health (IDPH).  The historical MIR is calculated using guidelines from the U.S. Centers for Disease Control and Prevention. MIR is computed as the ratio of mosquito pools testing positive for WNV to the number of mosquitos tested (N POS pools / N individuals *1000).

Although there is no specific MIR value that tells us that human illness from WNV is certain, its relative value is helpful to estimate the amount of WNV in an area and whether it is increasing or declining.  The number of human cases per week for the climate division were those cases reported to the IDPH. Note that the number of human cases are found as early as June, but they often peak in August (at or after Epi-Week 34). The peak often occurs within 0-3 weeks of the peak in MIR. Also, note that in the heaviest case years, temperatures are often warmer and conditions are often drier than normal for the summer months, and in the lowest case years, temperatures are often cooler and surface conditions often wetter than normal. 

Click on a Climate Division for graphical displays of season-to-date MIR model results for that climate division.
Click on a link in the submenu for historical MIR maps by county, climate division, or by year.


Ruiz, M. O., L. F. Chaves, G. L. Hamer, T. Sun, W. M. Brown, E. D. Walker, L. Haramis, T. L. Goldberg, and U. D. Kitron. 2010. Local impact of temperature and precipitation on West Nile virus infection in Culex species mosquitoes in northeast Illinois, USA. Parasites & vectors 3: 19.

Shand, L., W. M. Brown, L. F. Chaves, T. L. Goldberg, G. L. Hamer, L. Haramis, U. Kiltron, E. D. Walker, M. O. Ruiz, 2016. Predicting West Nile infection risk from the synergistic effects of rainfall and temperature. Journal of Medical Entomology, 53(4): 935-944.