The district — which covers Harlem Avenue to County Line Road and North Avenue to 87th Street, plus Oak Park — was to spray this week in Oak Park to eliminate mosquitoes, which can carry the disease. Fogging is done between dusk and midnight, said Paul Geery, assistant manager and biologist.
"We're doing the whole district. A couple more evenings and we're done with the first round," he said last week.
The number of rounds depends on what is found in the traps to catch mosquitoes laying eggs, he said. In the past, there have been as many as five rounds of fogging in the district, which includes Lyons, Oak Park, Proviso, Riverside and River Forest townships with 31 communities.
Geery said 98 percent of the work is larval control — killing the mosquitoes before they become adults. They catch about 50 mosquitoes per pool of water and then they are ground up and tested.
"We have maps that show where water lies," he said. "It helps us determine whether and where we do mosquito control."
This year, they have found 126 mosquito pools positive for West Nile virus out of 1,454.
"Last year, we had quite a bit more viruses. It was the second highest since the 2002 epidemic," Geery said.
That year, there were 884 human cases of West Nile in Illinois with 66 deaths, according to the district's website.
In 2013 nationwide, there have been 174 human cases and seven deaths, according to the Centers for Disease Control and Prevention.
Geery said he is not totally sure why last year there were 290 human disease cases, including the 187 "most-nasty" neuroinvasive cases, in Illinois and none so far this year. The virus has been building more slowly, partially due to colder temperatures this summer, he said. Mosquitoes develop faster when temperatures are hot.
Dead birds collected in La Grange, Oak Lawn, Glenview, Arlington Heights and Oak Park have tested positive for the virus, according to local officials and the Cook County Department of Public Health. That does not mean residents in those communities are more at risk for the virus than others, according to a county spokesman.
The virus was discovered in the United States in New York City in 1999 and it has spread south and west, according to the district website. Crows, blue jays, eagles, hawks and other raptors are particularly susceptible to the disease. Mosquitoes, especially the genus Culex pipiens, spread the virus from bird to bird, and birds to humans. The virus is not transferred from person to person.
West Nile goes back and forth between mosquito and bird and builds up in the salivary glands of the mosquitoes before the bite, Geery said. Those over age 50 are the most susceptible to the virus but all ages are at risk, the district website says. Those affected get low-level, flu-like, symptoms, with low fever, headache, body aches and swollen lymph glands.
"They don't feel good and it goes away," Geery said.
Less than 1 percent of those infected will develop symptoms of a high fever, neck stiffness, muscle weakness, stupor, a coma, tremors and paralysis. Death occurs in 3 percent to 15 percent of severe infections.
Community officials recommend limiting times outdoors between dawn and dusk, when mosquitoes are most active; wearing long sleeves and pants; and wearing insect repellent containing DEET, Picaridin or oil of lemon eucalyptus.
Anywhere water can stand for a week can become a potential breeding source for Culex mosquitoes. Potential sites are curbside storm water catch basins, off-road storm water catch basins, discarded tires, buckets and other artificial containers, rain gutters, bird baths, unused swimming pools, ditches and ponds.
Geery also recommends checking window screens and air conditioning units for holes that could let mosquitoes inside.