IDPH 125th


Illinois suffered the nation’s worst epidemic of St. Louis encephalitis in 1975, resulting in 578 cases and 47 deaths. It was the first major outbreak of a mosquito-borne disease in the state since 1930, when malaria sickened more than 300 Illinoisans.

Although the Department had anticipated and prepared for a small outbreak of the disease, no one had imagined that Illinois would lead the nation in the number of cases and deaths. One of the state’s first cases of St. Louis encephalitis that year was an 8-year-old from Cook County. While it is believed that the child was exposed to an infected mosquito during a trip to Greenville, Miss., other cases, these originating in Illinois, were soon reported.

When it became apparent a few days before the Labor Day weekend that the state was facing a major outbreak of St. Louis encephalitis, the Department turned to the news media to inform the public and provided daily updates. Agency director Joyce Lashof alerted physicians in the state and asked them to order tests for arbovirus infections in patients with appropriate neurological symptoms.

As the scope of the outbreak expanded, Dr. Lashof increased the Department’s response

team to more than 100 public health workers. Staff were pulled from other areas of the agency and reassigned to the Division of Disease Control, where they assisted in following up on positive lab results, transmitting information from local health departments, collating data and writing summaries. Other staff were reassigned to the virology lab to help with testing the 13,000 specimens that were processed during the outbreak.

Still, staff were strained: At the state and local level, many people routinely worked 14- to 16-hour days, seven days a week, for the two months the outbreak lasted. In fact, one technician slept on a cot in the lab in order to save the time spent commuting.

Mosquito control, initiated and supervised by Department engineers and sanitarians, helped to curb the outbreak. However, by the time the outbreak had subsided in mid-October, St. Louis encephalitis had sickened residents in two-thirds of the state’s 102 counties. Almost half of the cases occurred in Cook County, an area where the disease had never before been identified.

The state’s first ever case of St. Louis encephalitis was recorded in 1932 in eastern Illinois, in Paris, the Edgar County seat. The following year there were 185 cases, most of them in Madison, Morgan and St. Clair counties, and 59 deaths. Across the Mississippi River, the city and county of St. Louis reported 1,114 cases and 224 deaths. It was during this 1933 outbreak that the virus was first isolated, identified and named. Other, smaller outbreaks of St. Louis encephalitis have occurred in Illinois in 1955, 1964 and 1968.

In the wake of the 1975 outbreak, the Department moved quickly to establish a mosquito- borne disease early warning system and, in 1976, hired an arbovirologist to coordinate the effort. The surveillance system remains in place today.

A complex network that is activated annually from May until Oct. 31 (or two weeks after the first killing frost), the surveillance system draws information from hundreds of persons throughout the state who monitor reports of suspect or confirmed cases of encephalitis or other mosquito-borne diseases.

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

The system also relies on blood samples collected from wild bird populations, mosquito pools and sentinel bird flocks that are screened for the presence of mosquito-borne viruses.

Information gathered through these various sources guides prevention activities, including public alerts that feature precautions on how to reduce the risk of exposure to mosquitoes, such as wearing long pants and long-sleeved shirts and using repellents appropriately. Other prevention efforts entail cleaning up sites where tires are stored or discarded, requesting more intensive surveillance by the state’s medical community and providing guidance for mosquito control efforts at the local level.

... Years Ago in Public Health

A Timeline of the Illinois Department of Public Health

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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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