Mosquito breeding off to a head start
Christopher Xamplas of North Shore Mosquito Abatement District looks to see if a Gravid Trap has caught any mosquitos. Alyssa Schueneman~Sun-Times Media
Updated: July 8, 2012 8:13AM
Ten years ago, an unusually hot summer allowed the West Nile virus to strike the Chicago area with a vengeance.
In Cook County alone, there were 634 human cases and 41 deaths from the emerging mosquito-borne virus, which only three years earlier had first appeared in New York.
The tally from the Illinois outbreak, 884 human cases and 66 deaths, was the worst in the country.
No year since has come anywhere close to that toll. But mosquito populations have gotten an earlier start this year thanks to a mild winter and warm spring. If the summer turns out to be hot and dry, that jump start could dramatically increase the number of culex pipiens, the northern house mosquito that is primarily responsible for transmitting the West Nile virus from birds to humans.
“We have seen a heavier amount of mosquitos earlier in the season,” said Christopher Xamplas, the ecologist with the North Shore Mosquito Abatement District, which serves 13 communities from Evanston north to the Lake County border.
“Whether that’s because the mosquitoes didn’t die off, or they were just able to multiply sooner is anybody’s guess, really.”
Xamplas said conditions so far create the potential for a bad year for the West Nile virus. “It is all dependent on the weather. It could change next week, or a month from now,” he said.
More Culex mosquitoes have been found in the mosquito traps used to collect pools for counting, identifying and testing.
Cooler temperatures, like those experienced late last week, can slow down the population growth. Rain helps too, flushing out catch basins underneath sewer grates where culex mosquitoes often lay their eggs.
In northern climates like Illinois, mosquitos either die off during the winter or go into a hibernation-like state. The surviving culex females typically emerge from winter hiding around early April and go looking for a blood meal, preferring birds but sometimes settling for humans. The mosquitoes pick up the virus when they bite an infected bird. They pass it along to other birds and mammals, including humans.
According to the Illinois Department of Public Health, the first positive bird case, a crow, was found in Chicago on May 16, well ahead of last year’s schedule. Mosquito pools also have tested positive from samples collected May 17 in Cook and DuPage counties.
Last year, the first positive mosquito samples did not show up until June 8.
“Although it is a little earlier in the season than we normally find West Nile virus positive mosquito pools and birds, it is not unheard of,” said Illinois Department of Public Health Director Dr. LaMar Hasbrouck. No human cases have yet been reported.
The surge in human cases typically comes in the latter part of August and September. However, about 80 percent of people who are infected show no symptoms and others have only a slight fever or headache. In more severe cases, victims show a sudden high fever with head and body aches, disorientation, tremors and convulsions.
Culex mosquitos thrive in well-populated urban areas where there are many containers in which to lay eggs.
“Anything on your property that holds water is a potential breeding source,” said Dave Zazra, communications manager for the North Shore Mosquito Abatement District. “Residents play a huge role in all of this.”
Residents are advised to clear away containers that collect standing water and regularly change the water in bird baths and swimming pools. Citizens also are urged to take personal precautions by covering up exposed skin when going outdoors and using a repellent that contains DEET, picaridin, oil of lemon eucalyptus or IR 3535 according to label instructions.