Skip Navigation LinksSkip Navigation Links
Centers for Disease Control and Prevention
Safer Healthier People
Blue White
Blue White
bottom curve
CDC Home Search Health Topics A-Z spacer spacer
Blue curve MMWR spacer

Rapid Assessment of Vectorborne Diseases During the Midwest Flood - - United States, 1993

Heavy spring and summer rainfall during 1993 caused the most extensive flash and riverine flooding ever recorded in the upper midwestern United States. In portions of the flood region, * standing water provided large expanses of habitat capable of producing large populations of the mosquitoes Culex pipiens and Cx. tarsalis. These species can rapidly amplify transmission of the arboviruses that cause St. Louis encephalitis (SLE) and western equine encephalitis (WEE). Although information from state health departments in the disaster area indicated minimal SLE or WEE activity in the region before the flooding, large vector populations in certain areas following the flooding increased the potential for exposure of residents and emergency workers to arboviral infection. To determine the risk for arboviral disease in the disaster area, CDC, in collaboration with state and local health departments, conducted surveillance during August-September 1993. This report summarizes the results of the surveillance activity.

The risk for SLE or WEE amplification was low in the northern part of the flood region because flooding occurred during late summer, vector population densities were moderate, and nighttime temperatures were below 50 F (10 C). To verify the low risk, mosquito-based surveillance was conducted in Iowa, Minnesota, Nebraska, North Dakota, and South Dakota during August 2-7. Because larger mosquito populations and higher average temperatures (that may facilitate virus amplification) were observed in the southern part of the flood region, intensive surveillance for SLE and WEE was conducted in Illinois, Iowa, Kansas, and Missouri from August 1 through September 21. Mosquitoes were collected in carbon dioxide-baited light traps and sorted by species. Known vector species were grouped into pools of up to 100 mosquitoes and tested for the presence of SLE antigen (using an antigen-capture enzyme-linked immunosorbent assay) and/or WEE virus (using a Vero cell culture plaque assay).

WEE virus was detected in one pool of Cx. tarsalis collected in Deuel County, South Dakota; no evidence of SLE activity was detected in any of the 186,501 mosquitoes tested from throughout the region (Table_1). In Iowa, state-based sentinel chicken surveillance revealed no evidence of SLE or WEE activity. In Illinois, state-based wild bird surveillance identified SLE virus in one of 2073 birds tested. Two human cases of SLE were reported from the nine-state area; one occurred within the disaster area. Sporadic cases of SLE frequently occur in the Midwest; these cases were not related to flooding in 1993. Because surveillance data indicated minimal risk for arboviral disease above background levels in the disaster area, contingency plans for large-scale mosquito adulticiding were not implemented.

Reported by: J Anders, Div of Microbiology, LA Shireley, MPH, State Epidemiogist, North Dakota State Dept of Health and Consolidated Laboratories. L Volmer, Office of Communicable Disease Prevention and Control, K Forsch, State Epidemiologist, South Dakota State Dept of Health. MT Osterholm, PhD, State Epidemiologist, Minnesota Dept of Health. WL Schell, JP Davis, MD, State Epidemiologist, Div of Health, Wisconsin Dept of Health and Social Svcs. WA Rowley, Dept of Entomology, Iowa State Univ; R Currier, LA Wintermeyer, MD, State Epidemiologist, Iowa Dept of Public Health. WL Kramer, TJ Safranek, MD, State Epidemiologist, Nebraska Dept of Health. D Alfano, Bur of Disease Control, Kansas Dept of Health and Environment. LD Haramis, Vector Surveillance and Control, Illinois Dept of Public Health. W Kottkamp, St. Louis County Dept of Health-Vector Control, St. Louis; CL Frazier, Southeast Missouri State Univ, Cape Girardeau; FT Satalowich, Vector Control, Missouri Dept of Health. US Navy Disease Vector Ecology and Control Center, Naval Air Station, Jacksonville, Florida; US Navy Disease Vector Ecology and Control Center, Alameda, California; US Navy Environmental and Preventive Medicine Unit No. 2, Norfolk, Virginia. US Air Force Reserve 910 AG/DOS. Emergency Response Coordination Group, Vienna, Ohio. National Center for Environmental Health; Medical Entomology/Ecology Br, Div of Vector-Borne Infectious Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: Although natural disasters that result in flooding often are followed by a proliferation of mosquitoes, in the United States such disasters are rarely followed immediately by epidemics of arboviral disease. Surveillance data in this report confirmed that, in 1993, flood-related risk for epidemic mosquitoborne arboviral infections was low in the upper midwestern United States.

Despite the presumed low risk for mosquitoborne arboviral disease after flood-related natural disasters, surveillance for arboviruses can assist in determining prevalence in large vector populations and the need for mosquito control. Because the 1993 Midwest flood was more widespread than previous floods in the region, the risk for arboviral disease was unknown. Surveillance provided an accurate determination of the risk for transmission of arboviral infection and obviated the expense of large-scale mosquito control. For example, the total allocation for arbovirus surveillance in the disaster area was approximately $390,000 (range: $32,275 {Illinois} to $150,000 {Missouri}). If surveillance had not been implemented in the area, prophylactic mosquito control most likely would have been conducted. The estimated cost of mosquito control for the St. Louis metropolitan area alone was $1.6 million. If other metropolitan areas in the flood region also were treated, the total estimated cost of prophylactic mosquito control would have exceeded $10 million. These findings suggest that arbo- virus surveillance programs to determine public health risk can prevent unnecessary expenditures associated with application of insecticides.

In addition to large-scale application of insecticides, the primary public health interventions to prevent mosquitoborne arboviral outbreaks include community alerts that warn residents to avoid mosquito exposure during twilight hours by staying inside screened or air-conditioned buildings or by using repellents or other personal protection measures. The decision to use large-scale application of insecticides to reduce vector population densities is complex and depends on many factors, including detection of early-season arbovirus transmission, indicating increased risk for human infection. Timely intervention, however, requires an active program of mosquito and avian surveillance and appropriate mosquito-control measures.

Reasons also may exist for emergency control of mosquitoes that are not related to disease transmission after a disaster. Pest (i.e., nonvector) mosquito species may cause severe nuisance problems that compromise emergency-response operations. CDC recommends control of pest mosquitoes when 1) emergency-response or reconstruction efforts are substantially hampered by large populations of mosquitoes, 2) normal civil services (e.g. police, fire, emergency medical services, power, and water and sewage services) in the disaster area are substantially disrupted, or 3) large nuisance mosquito populations place additional stress on the human population (1). Surveillance protocols and control methods vary by the mosquito species. Decisions to control pest mosquitoes are based on criteria that differ from those to control vector mosquitoes. No large-scale emergency control of pest mosquitoes was conducted in the 1993 flood disaster.

In the disaster area, the risk for epidemic transmission of arboviruses during 1994 is being monitored by human, bird, and mosquito surveillance. Winter snows and spring rains contributed to flooding and standing water in some areas of the midwestern United States that experienced flooding in 1993. As a result, mosquitoes in these localities may be more abundant than usual during the 1994 arbovirus transmission season.


  1. CDC. Emergency mosquito control associated with Hurricane Andrew -- Florida and Louisiana, 1992. MMWR 1993;42:240-2.

* Illinois, Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota, South Dakota, and Wisconsin.

Note: To print large tables and graphs users may have to change their printer settings to landscape and use a small font size.

TABLE 1. Mosquitoes tested for arboviruses in states affected by the Midwest
floods -- August 2-7, 1993
                   No.           No.      Positive pools
State             pools      mosquitoes     (SLE/WEE *)
Illinois            618         30,900         0/ +
Iowa                 20          1,230         0/0
Kansas              139          6,258         0/0
Minnesota            45          3,505         0/0
Missouri          1,759        123,863         0/ +
Nebraska             31          2,082         0/0
North Dakota         53          3,502         0/0
South Dakota        172         15,161         0/1

Total             2,837        186,501
* St. Louis encephalitis/western equine encephalitis.
+ Not done.

Return to top.

Disclaimer   All MMWR HTML versions of articles are electronic conversions from ASCII text into HTML. This conversion may have resulted in character translation or format errors in the HTML version. Users should not rely on this HTML document, but are referred to the electronic PDF version and/or the original MMWR paper copy for the official text, figures, and tables. An original paper copy of this issue can be obtained from the Superintendent of Documents, U.S. Government Printing Office (GPO), Washington, DC 20402-9371; telephone: (202) 512-1800. Contact GPO for current prices.

**Questions or messages regarding errors in formatting should be addressed to

Page converted: 09/19/98


Safer, Healthier People

Morbidity and Mortality Weekly Report
Centers for Disease Control and Prevention
1600 Clifton Rd, MailStop E-90, Atlanta, GA 30333, U.S.A


Department of Health
and Human Services

This page last reviewed 5/2/01