West Nile activity yet to be documented this year in historical hot spot
Friday, July 08, 2016
by Zak Koeske
In 2002, during the largest mosquito-borne encephalitis outbreak in state history, Oak Lawn and Evergreen Park accounted for 52 human cases of West Nile virus and three deaths — a rate of infection more than six times greater than the rest of the county.
Last year, the first West Nile-positive mosquito batches in all of northern Illinois were collected in Oak Lawn on May 21 and Evergreen Park on May 26.
Evergreen Park Mayor James Sexton has even contracted the virus.
So it's come as a surprise to experts that, as of Friday, the South Cook County Mosquito Abatement District, which handles mosquito control for the 340-square-mile swath of Cook County south of 87th Street, had yet to collect a single mosquito carrying West Nile.
"I was kind of looking to see some West Nile activity right now," said Douglas Wright, who has served as general manager of the district for the past decade. "We've run (more than 90) tests so far and all have been negative."
Wright, who said he could not remember a time this late in the year that his mosquito traps hadn't picked up a single insect infected with the virus, was at a loss to explain the phenomenon.
He and representatives from the Northwest and Des Plaines Valley mosquito abatement districts, which also have had lower-than-normal West Nile activity, invoked weather patterns as a possible explanation for the paucity of infected mosquitoes in the area.
The cool, wet spring, they surmised, may have delayed the development of West Nile-carrying Culex mosquitoes, which prefer hot, dry weather. Why the recent spate of dry 90-degree days hadn't since jump-started the mosquitoes, however, remains a mystery.
"It's kind of hard to tell why," Wright said. "To be honest with you, with the temperatures the way they were, I expected to see more positive mosquito batches."
Linn Haramis, an entomologist with the Illinois Department of Public Health, said multiple factors, including weather, topography and the quality of a mosquito abatement agency, can affect West Nile's impact on a community.
Wooded suburban communities like Oak Lawn and Evergreen Park, where virus-carrying birds come in contact with the virus-spreading mosquitoes in cemeteries and on golf courses, and where underground pipes and culverts offer the infected insects haven during the winter months, are typically considered prime locations for potential West Nile outbreaks, Haramis said.
It's difficult to say why, in spite of communities with those risk factors, Wright's district had yet to find a West Nile-carrying mosquito this year, he said.
"It would be something you'd have to look at in detail," said Haramis, noting that it's not uncommon to see an uneven distribution of West Nile cases for reasons that aren't immediately obvious. "It's essentially a research project."
State and county health officials warned that just because West Nile-positive mosquitoes had not been identified in south Cook County to date this year, it did not mean they weren't out there or that area residents could afford to drop their guard expecting a down season for the virus.
"Now is the time to use (insect repellent) because we're entering the more risky part of the summer where the infection rate of mosquitoes tends to blow up and the risk to humans becomes greater," Haramis said, citing a past situation in Lake County where the mosquito infection rate exploded in a matter of weeks, jumping from no sampled mosquitoes in early July to 35 percent by August.
"That's what's dangerous about West Nile when the conditions are right," he said. "We've still got plenty of time if we hit a hot, dry period. You could just have the activity blow up really, really quickly."
Currently, about 2 percent of the mosquito samples tested this year by Cook County's four mosquito abatement districts have come back positive for West Nile.
The North Shore Mosquito Abatement District, which is responsible for mosquito control in north Cook County, has accounted for the vast majority of the county's West Nile-positive mosquitoes and about half of the entire state's positive mosquitoes so far this year, according to district statistics. Fourteen of the area's 20 positive batches were collected in Skokie, Winnetka and Glenview/Golf, district communications manager Dave Zazra said.
Fifteen counties statewide have reported at least one case of West Nile infection in a mosquito, bird or human this year.
The only two human cases this year have involved an adolescent in west central Illinois and another person in Grundy County, Illinois Department of Public Health spokeswoman Melaney Arnold said.