Chicago is the 3rd largest city in the country with well over 2.5 million residents. It covers over 225 square miles and has several hundred thousand catch basins, miles of ditches, mud holes, empty swimming pools, and traffic! The City’s program has changed over the years but the primary responsibilities remain the same. The core of City’s Mosquito Surveillance and Control Program is to oversee the treatment of approximately 120,000 catch basins within the public way and to oversee the trapping and testing of all mosquitoes for the presence of WNV and SLE.

Each spring catch basin crews get a GPS unit, a backpack full of larvicide, and a water bottle or two. On average each crew treats about 5,000 catch basins per day and completes all citywide treatments in just about 5 weeks. In 2012, the City treated approximately 90,000 catch basins with FourStar 180 Day residual briquets which proved effective in controlling larval emergence.

Simultaneously, the City begins the surveillance portion of the program. They hire college interns for the program and divide the City into 8 zones, assigning each student a zone. All trapping is with gravid traps and they set approximately 76 traps twice each week. As the season progresses, they add/move traps as needed. All mosquitoes are brought to the CDPH lab where they are identified, pooled and tested for the presence of WNV and SLE. In addition, mosquitoes are collected from the O’Hare airfield and these collections are taken to the CDPH lab for identification and testing. In 2012, Chicago collected 36,021 Culex mosquitoes, tested 2,478 pools, and detected 437 WNV positive pools in 40 separate community areas.

Additionally, each week throughout the season, the surveillance teams sample several hundred catch basins and monitor for the effectiveness of the larvicide applications. When found, larvae are brought back to the lab to check for emergence. If too many are emerging, they know the catch basin treatments have failed and retreatment of the basins in the area is necessary.

There are a host of other “miscellaneous” responsibilities that are a part of the program. For example, the program picks up dead birds for WNV testing as reported. They take calls from the City’s 311 hotline and get called upon to help with a host of other insect problems as well as inspect complaint properties as requested. They also do some larviciding in ditches and other Culex breeding sites.

In order to reduce the risk of human infections in 2012, the program utilized adulticiding to decrease the number of WNV infected mosquitoes. Adulticide (Zenivex) was used on 11 occasions in July through late August. Following the adulticiding, the density of WNV infected mosquitoes fell sharply indicating that the adulticide treatments were effective. Overall, 72 human cases and 2 deaths were reported in Chicago in 2012. These numbers could have been much higher if it weren’t for the intuitive decision making done by the City’s Mosquito Surveillance and Control Program.

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