June 30, 2000

SPRINGFIELD, IL – For the past quarter century, the Illinois Department of Public Health has coordinated a little known early warning system for mosquito-borne disease.

Ever since 1975, when an epidemic of St. Louis encephalitis in Illinois caused 47 deaths and sickened nearly 600 people, a complex surveillance network has been activated from mid-June until Oct. 31, or until two weeks following the first killing frost. Hundreds of persons throughout the state monitor reports of suspect or confirmed cases of encephalitis or other mosquito-borne diseases and screen blood samples collected from wild bird populations for the presence of viruses carried by mosquitoes.

Information gathered through these various sources focuses and guides prevention activities, including public alerts that can result in actions to reduce the risk of exposure to mosquitoes. These actions may include cleaning up sites where tires are stored or discarded, requesting more intensive surveillance by the state's medical community or recommending personal protections, such as the use of repellents.

"While most mosquitoes are merely nuisances, some can transmit serious diseases," said Dr. John R. Lumpkin, state public health director. "This safety net of public health authorities and the medical community every year works diligently behind the scenes to prevent or control an outbreak of mosquito-borne disease."

The surveillance system includes infectious disease physicians, hospital laboratory directors and infection control practitioners, local health departments and the Department's laboratory, environmental health and communicable disease staffs who test for and report suspect or confirmed cases of aseptic meningitis, meningoencephalitis or encephalitis, all of which can be caused by mosquito-borne viruses.

In addition, the program monitors wild bird populations in about 30 counties for the presence of mosquito-borne viruses. This year, monitoring was expanded to include West Nile virus, which was first identified in the United States last year. Blood samples collected from about 5,000 birds will be checked this year by technicians in the Department's Chicago laboratory for antibodies to arboviruses or diseases caused by mosquitoes. Detection of antibodies in more than 5 percent of the wild bird samples indicates the possibility of an increased risk of human disease.

Wild birds are caught in nets by local and state public health staff in counties where mosquito-borne diseases have historically occurred, a blood sample is taken and the birds are released. Blood samples also are collected from sentinel and privately-owned chicken flocks. Mosquitoes may also be collected and tested for the presence of viruses.

West Nile virus is a mosquito-borne disease that is genetically and clinically similar to St. Louis encephalitis. It was first isolated in Uganda in 1937 and was reported for the first time in the Western Hemisphere in August 1999 in metropolitan New York City. The outbreak there resulted in 62 confirmed or probable cases and seven deaths. The primary hosts of West Nile virus are various species of birds; the virus can cause high mortality in crows and other bird species.

Due to concerns about West Nile virus, the Department this year will obtain bird blood samples from additional sources. The U.S. Department of Agriculture Wildlife Services will collect blood samples from waterfowl during field work, the Illinois Department of Natural Resources will monitor crows for unusual mortality and the Illinois Natural History Survey will test mosquitoes for West Nile virus.

Symptoms of St. Louis encephalitis and California encephalitis, historically the most common mosquito-borne diseases in Illinois, and West Nile virus are similar and usually begin five to 15 days following a bite by an infected mosquito. The symptoms range from a slight fever or headache to rapid onset of severe headache, high fever, muscle aches, stiffness in the back of the neck and disorientation.

California encephalitis occurs more often in children, while St. Louis encephalitis and West Nile virus are more common among older adults. Most victims recover fully, although some may suffer permanent neurological damage. California encephalitis is commonly transmitted by the tree-hole mosquito, which bites during daylight hours in or near shaded or wooded areas, while St. Louis encephalitis is spread by the northern house mosquito that bites from dusk to dawn. The northern house mosquito is the primary carrier of West Nile virus, but the virus also can be transmitted by other mosquito species.

Last year in Illinois, there were three confirmed cases of California encephalitis, one each in LaSalle, Peoria and Winnebago counties. No cases of St. Louis encephalitis have been identified in the state since 1995.

During the mosquito season, Dr. Lumpkin recommended the following precautions:

• When outdoors and mosquitoes are present, wear shoes and socks, long pants and a long-sleeved shirt;

• Use mosquito repellent when it is necessary to be outdoors, applied sparingly to exposed skin or clothing, as indicated on the repellent label;

• Check residential screens, including porches and patios, for tears and other openings; and

• Eliminate stagnant water in bird baths, ponds, flowerpots, old tires and any other receptacles in which mosquitoes might breed.

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