The North Shore Mosquito Abatement District has scheduled adult mosquito spraying operations in response to one of the largest outbreaks of West Nile virus in U.S. history.

The adult mosquito control program is “only undertaken when absolutely necessary,” according to the district.

Six human cases of West Nile virus have been reported in Evanston this year, compared to none last year, according to the Evanston Health Department. As of noon Tuesday, 48 people are infected statewide, according to the Illinois Department of Public Health. Most cases are located in the northern lakeside counties of Lake, Cook and DuPage.

Nationwide, 1118 cases have been reported to the Centers for Disease Control and Prevention.

NSMAD spokesman Dave Zazra said although the agency has been conducting mosquito larvae control and source reduction since March, a hot, dry summer has helped virus-carrying mosquito populations flourish.

“In Evanston we’ve had more cases this year than probably since 2005,” Zazra said. “The CDC are saying that up until this point in August, this is the most human West Nile virus cases this early in the year that has ever been reported to them. This is a significant thing.”

Evonda Thomas, director of the Evanston Health Department, said the number of cases in Evanston depends on the existence of standing water in the community. Any form of stagnant water, whether it’s in potholes, gutters, flowerpots, pools or birdbaths, can breed mosquitoes. Current weather conditions then accelerate both the mosquitoes’ growth and reproduction of the virus.

“We had a very mild winter, which means that those mosquitoes probably did not die off in the winter,” Thomas said. “And with the hot weather that we’ve seen in the county and the state, for this space of time we’ve had a double portion, if you will, of adult mosquitoes.”

Not only are mosquito populations higher this year than in previous years, but also those carrying West Nile virus are in abundance. Although NSMAD regularly performs larvae control by targeting known mosquito breeding sites throughout the city, West Nile infection rates in mosquitoes must hit a certain threshold before the agency determines the need to spray adult populations.

NSMAD tests mosquitoes for West Nile virus by collecting them in traps dispersed throughout the city. Usually when about 30 mosquitoes from any one trap test positive, that’s an indication the agency should conduct control operations. This year, some of Evanston’s traps have yielded over 500 mosquitoes, including one that tested 90 percent positive for West Nile virus, Zazra said.

Throughout the summer, the Evanston Health Department has urged residents to take preventative measures by clearing any standing water around their houses. However, Zazra noted that motivating citizens to do their part in preventing disease can be difficult when people don’t grasp the severity of the outbreak.

“It’s like first-hand experience,” he said. “If people aren’t being bitten by mosquitoes they don’t tend to think there are mosquitoes around. And convincing people of that is difficult, especially in an area like Chicago where there’s a lot of news stories that are happening that may take precedence in the media versus what’s happening with mosquitoes.”