A virus new to this part of the world has found its way to Illinois. West Nile virus (WNV) has been detected in dead crows in Cook and DuPage counties. Although both sites are in northeastern Illinois, a case was also detected in a horse near Lexington, Kentucky, in late August. It has been found in 21 states, the District of Columbia, and Ontario province in Canada since detected in New York in 1999. Its original home is portions of Africa, Asia, and the Middle East. This virus is transmitted through mosquito bites to humans, birds, horses, and other mammals. It can be fatal to humans, crows, blue jays, hawks, falcons, and horses.

Various mosquitoes are capable of transmitting WNV, but Culex species are most commonly involved. The northern house mosquito, Culex pipiens, is the most common human-biting member of that group. This mosquito is a small, medium brown, quiet biter that is most common in Illinois from mid-June through the rest of the summer and fall. A quiet biter means that it lands softly on the skin and the bite is painless enough in many people that they do not notice it. It may not buzz around your ear; and if it does, the hum of its wings is not very loud. It is usually not present in large numbers, but a few are present in most areas almost daily.

Culex are container-breeding mosquitoes. Several black eggs are laid in a mass about 1/8 inch across that floats on the water surface. The water typically has a large amount of decaying organic matter in it, frequently giving the water a dark color and putrid smell. Eggs are laid on water that has collected in tree cavities, clogged gutters, old tires, pet water dishes, birdbaths, wading pools, old tin cans, and any other structure lying around that holds water after a rain. The eggs hatch into legless, slender, wriggling, whitish larvae that feed on fine debris and tiny organisms in the water. The larvae hang down from the water surface with a posterior tube sticking up through the water surface to collect air. Full-grown larvae, about 1/4 inch long, transform into aquatic, comma-shaped, tumbling, nonfeeding pupae. Adult mosquitoes emerge from the pupae and stand on the water surface before flying off in search of food. Both male and female adult mosquitoes feed on flower nectar, but females require a high-protein blood meal to produce a large number of fertile eggs.

The northern house mosquito does not fly far from where it grows up. You are likely to be bitten by northern house mosquitoes that grew up on your property or elsewhere in your neighborhood. By keeping gutters clean of fallen leaves, removing old tires and other water-collecting debris, stocking water lily ponds with goldfish or minnows, and emptying and cleaning wading pools, birdbaths, and pet water bowls weekly, you can greatly reduce the number of these mosquitoes in your yard.

This mosquito mainly bites at dawn and in the evening, so restrict your outdoor activities at those times. Wear a hat, long-sleeved shirt, long pants, shoes, and socks to reduce the amount of exposed skin. If you are outdoors when mosquitoes are biting, apply an insect repellent containing DEET. Although fine for skin application, DEET should not be ingested. For young children who are unable to understand that they shouldn't lick the material off their hands or arms, use another product such as repellents advertised for child use or other repellents that don't contain DEET. Although less effective, they should be less toxic if ingested.

The northern house mosquito feeds on birds as well as mammals. A female mosquito obtains WNV particles while feeding on an infected bird. The virus particles are injected by the infected mosquito into the blood of another bird, human, horse, or other mammal at a later feeding, thus transmitting the virus. Because birds are very mobile, the virus has been spreading quickly across the country.

Most people who become infected have no symptoms, but some may become ill 3 to 15 days after being bitten. Typical symptoms are a fever and headache. In some, particularly elderly people, WNV can cause serious disease that includes muscle weakness, inflammation of the brain (encephalitis), stiff neck, stupor, disorientation, tremors, convulsions, paralysis, coma, or death.

Horses infected with WNV can also experience brain inflammation and die. Vaccinations available for horses for encephalitic diseases do not protect against WNV. Dogs, cats, and other mammals can also get WNV; but as with humans and horses, most make a full recovery.

Additional information on West Nile Virus is on the Illinois Department of Public Health Web site at http://www.idph.state.il.us/envhealth/wnvnews.htm. Information on mosquito control is available from the University of Illinois Extension at http://www.ipm.uiuc.edu/publications/infosheets/105mosquito/cmm.html.