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When the first West Nile Virus-positive birds were confirmed in Illinois in September 2001, the event was indicative of the slow but steady march of the disease westward from its east coast introduction in 1999. Though public health officials knew the virus would likely return in 2002, none of the experts could have predicted that Chicago would be a West Nile Virus hotspot in 2002 with more than 175 cases confirmed through mid-October.

"We knew that West Nile Virus as endemic to the Chicago area after 2001," said Dr. William Paul of the Chicago Department of Public Health. "And we were prepared for it to resurface in 2002, but I don't think anyone thought we'd see this many cases during the 2002 outbreak."

West Nile Virus is a mosquito-borne disease that can overwinter in mosquitoes, which means the infected mosquitoes have the potential to reintroduce the virus the following year. At the end of the mosquito season in 2001, the Chicago Department of Public Health worked with Clarke Mosquito Control to develop a comprehensive mosquito habitat survey and aerial map to prepare for the occurrence of West Nile Virus. This map cataloged Chicago's surface water sites, flood plains, and retention ponds and was instrumental in developing potential truck adulticiding routes.

The Chicago Department of Public Health developed an integrated mosquito management plan for 2002, including increases surveillance, early-season larviciding programs targeting areas where positive mosquitoes were found and the groundwork was laid for the possibility of adulticiding.

West Nile Virus was detected early in the Chicago area in 2002, with infected bird and mosquitoes identified in the spring. On August 15, the first human case of West Nile Virus was reported in Chicago, which triggered a citywide larviciding effort undertaken by the Department of Sewers, in close consultation with the Department of Public Health. The first human case also triggered a redoubling of efforts at educating the public on personal precaution, as well as city effort to identify and eliminate standing water.

The rapid increase of human cases spurred the Chicago Department of Public Health to consult with the Illinois Department of Public Health and the U.S. Centers For Disease Control and Prevention to prepare an appropriate response. In early September, Chicago moved forward with the next phase of mosquito control-adulticiding.

Public information officials prepared to address any concerns from residents about adulticiding, as this phase was more high profile and had not taken place city-wide since the St. Louis Encephalitis outbreak in 1975. Meetings across all city departments integrated responses from Mayor Daley's office, the Chicago Park District, Public Health, Streets & Sanitation, the Chicago Housing Authority and Department of the Environment.

Two large areas of Chicago were identified as West Nile "hot spots"-with high rates of infection, and those areas were treated twice over the course of 10 days. Ultra-low-volume applications of Anvil, an adulticide, were applied to residential streets and alleys between dusk and dawn by a fleet of 20-30 trucks each evening.

Public and media response to the applications was largely positive, and the evening applications occurred without significant incident and were completed ahead of schedule each evening. Initial surveillance showed that a single application of Anvil dramatically impacted mosquito populations, significantly reducing the public health threat. "We were definitely satisfied with the effectiveness of the adulticiding," Dr. Paul added. "After these adulticiding applications, we saw a substantial drop in mosquito populations in our traps."

Though we may never know what caused the concentration of West Nile cases in the Chicago area, it's certain that the steps taken to reduce the population of mosquitoes in affected areas helped to control the outbreak.