Press Release

May 13, 2011


Floodwater Mosquitoes vs. House Mosquitoes

West Nile virus typically not carried by floodwater mosquitoes  

SPRINGFIELD, Ill. – Weeks of rain and flooding in southern Illinois have made conditions ripe for mosquitoes. Floodwater mosquitoes (Aedes vexans and other species) typically appear approximately two weeks after heavy rains and flooding. While floodwater mosquitoes can be a nuisance, they are rarely infected with West Nile virus (WNV). However, as floodwaters recede into ditches, catch basins or other areas where water sits stagnant, house mosquitoes (Culex pipiens) will typically start to appear. House mosquitoes, in areas that have seen WNV in recent years, are often infected with the virus.

“With the floodwaters and increasing temperatures, we’re going to start seeing increased mosquito activity,” said Illinois Department of Public Health Director Dr. Damon T. Arnold. “It is important to protect yourself against mosquito bites by wearing insect repellent and taking other precautions.”

Last year, 30 of the state’s 102 counties were found to have a West Nile positive bird, mosquito, horse or human case. The first positive bird was collected on May 8, 2010 in Carroll County and the first positive mosquito batches were collected on June 8, 2010 in Tazewell County. A total of 61 human cases of West Nile disease were reported in Illinois last year, the first reported on August 31.

There have been no confirmed cases of West Nile virus so far this year

West Nile virus is transmitted through the bite of a mosquito that has picked up the virus by feeding on an infected bird. Most people with the virus have no clinical symptoms of illness, but some may become ill three to 15 days after the bite of an infected mosquito. The first human case in Illinois is not usually reported until July or later.

Only about two people in 10 who are bitten by an infected mosquito will experience any illness. Illness from West Nile is usually mild and includes fever, headache and body aches, but serious illness, such as encephalitis and meningitis, and death are possible.

Persons older than 50 years of age have the highest risk of severe disease.

The best way to prevent West Nile disease or any other mosquito-borne illness is to reduce the number of mosquitoes around your home and to take personal precautions to avoid mosquito bites. Precautions include:

  • Avoid being outdoors when mosquitoes are most active, especially between dusk and dawn.
  • When outdoors, wear shoes and socks, long pants and a long-sleeved shirt, and apply insect repellent that includes DEET, picaridin, oil of lemon eucalyptus or IR 3535 according to label instructions. Consult a physician before using repellents on infants.
  • Make sure doors and windows have tight-fitting screens. Repair or replace screens that have tears or other openings. Try to keep doors and windows shut, especially at night.
  • Eliminate all sources of standing water that can support mosquito breeding, including water in bird baths, ponds, flowerpots, wading pools, old tires and any other receptacles. In communities where there are organized mosquito control programs, contact your municipal government to report areas of stagnant water in roadside ditches, flooded yards and similar locations that may produce mosquitoes.

Public health officials believe that a hot summer could increase mosquito activity and the risk of disease from West Nile virus.

Additional information about West Nile virus and mosquitoes can be found on the Illinois Department of Public Health Web site at and

idph online home
idph online home

Illinois Department of Public Health
535 West Jefferson Street
Springfield, Illinois 62761
Phone 217-782-4977
Fax 217-782-3987
TTY 800-547-0466
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