Predicting the Effects of Climate and Weather on the Occurrence of West Nile Virus in Nortern Cook County IL

West Nile Virus (WNV) spread into northern Illinois in 2001. Illinois led the nation in human cases of WNV in 2002, had the second highest number of cases in 2005, and the sixth highest in 2006. The majority of cases are from the highly populated Cook County, where WNV-infection rates in vector mosquitoes tend to be among the highest in the eastern United States. Mosquito Abatement Districts (MADs) in Cook County IL routinely conduct surveillance for the abundance of the primary vectors (Culex specis) and their rate of infections. In addition, the Illinois Department of Public Health reports the date and general location of birds, animals and humans affected by WNV. The onset, severity and duration of the mosquito season vary by year, as does the number of affected mosquitoes, birds and mammals. This seasonal database includes almost 10 years of data which spans years with major outbreaks (2002, 2005, 2006), those with an intermediate number of human cases (2007), and years with low levels of transmission to humans (2003, 2004, 2008, 2009, 2010, 2011). During this same span of years, climatic factors varied considerably, from record low to record high temperatures, and from very wet to very dry conditions. It is well established that warm, dry conditions are favorable for the vector Culex development and for the replication rate of the virus in the mosquito. Previous studies by our group have demonstrated the impact of temperature on the seasonal abundance of the primary vector, Culex pipiens. The spring and summer of 2012 have had unusually early and continuous above normal temperatures that has promoted some of the earliest detections and the greatest numbers of WNV positive mosquitoes. This study examines the impact of daily weather and average pre-season and in-season climate conditions on the occurrence of WNV at several locations within Cook County. The locations chosen for study are sites where mosquitoes have been sampled on a regular basis for 7-10 years and are in close proximity to weather data collection sites. The pre-season climate may determine the onset and initial dynamics of the mosquito season, and in-season climate may dictate the overall intensity and duration of the mosquito season. However, the temporal variation in mosquito populations and WNV risk may also be affected by other weather events, particularly the presence or absence of precipitation and precipitation intensity occurring during the mosquito season. This study will present for the period 2002-2012, both the seasonal climate conditions and daily weather conditions (temperature, relative humidity, winds, and precipitation) and relate those to the onset, magnitude, and temporal variation in mosquito abundance, number of infected mosquitoes, and number of infected birds, animals and humans. Climatic and weather factors are expected to provide a means of predicting the possible consequences of future climate trends on the dynamics of this major aboviral threat in the US.