Lake-Associated Outbreak of Escherichia coli O157:H7 -- Illinois, 1995

On July 5, 1995, the Winnebago County Health Department (WCHD) in northern Illinois received a report from the local hospital of five cases of Escherichia coli O157:H7 infection among children who resided in Rockford. Interviews of the children's parents revealed no common food source; however, on June 24-25, they all had visited an Illinois state park with a lake swimming beach. On July 6, the Illinois Department of Public Health (IDPH) closed the swimming beach because of suspected transmission of infection through lake water. WCHD and IDPH investigated the outbreak to assess risk factors for illness and determine the source of infection. This report summarizes the findings of the investigation, which indicate that ingesting contaminated and untreated lake water can result in infection.

Epidemiologic and Laboratory Investigation

A case was defined as onset of at least one of the following in a resident of or a visitor to Rockford, Illinois, during June 24-July 1: diarrhea with culture-confirmed E. coli O157:H7 infection; diarrhea with serologically confirmed E. coli O157:H7 infection by antibody testing; hemolytic uremic syndrome (HUS); or bloody diarrhea. Cases were identified by reporting by telephone from hospital laboratories, reporting by telephone from physicians, telephone calls to the health department by persons after extensive news media coverage, and telephone calls to persons who had camped at the park. Isolates of E. coli O157:H7 cultured from stool samples obtained from six persons who swam in the lake were sent to CDC for both Shiga toxin testing and for pulsed-field gel electrophoresis (PFGE). Shiga toxins were detected in all six isolates, and all six had the same PFGE pattern. Acute-phase serum samples from 11 persons were tested for immunoglobulin G and immunoglobulin M antibody titers to E. coli O157:H7; these samples were obtained from persons for whom appropriate stool samples had not been obtained (nine) or for whom stool cultures were negative (two).

A total of 12 cases were identified, including seven with culture-confirmed E. coli O157:H7, three with positive serology, one with HUS and had culture-confirmed E. coli O157, and one with culture-negative bloody diarrhea. Seven patients were male; ages ranged from 2 to 12 years. The median period from swimming in the lake to onset of illness was 4 days (range: 0-6 days). Cultures of stool from eight persons with confirmed E. coli O157:H7 infection were negative for Salmonella, Shigella, and Campylobacter. Two families each had two children with E. coli O157:H7. Bloody diarrhea occurred in nine cases; three cases (in children aged 2, 4, and 5 years) developed HUS and were hospitalized for at least 1 month each.

The first of two case-control studies was conducted July 6-13 to assess whether swimming in the lake was associated with risk for disease. The parents of seven ill children were asked to provide the name of one adult neighbor or friend with a child who was within 1 year of age of the ill child (three cases were identified later in the investigation, and the parents of two refused to participate). A questionnaire was administered regarding activities during the week preceding illness, including restaurants visited, foods eaten from a concession stand at the lake, recreational activities, and park exposure (including swimming in the lake). A matched analysis of information for the case-patients and controls indicated that swimming at the park was the only risk factor for illness (the seven case-patients swam at the lake and the seven controls did not swim at the lake; matched odds ratio {OR}=undefined; 95% confidence interval {CI}=4.0-undefined).

The second case-control study was conducted July 14-28 to assess specific risk factors for infection among persons who swam in the lake. Cases included 10 ill persons who had visited the lake for 1 day. Controls were selected by identifying families who camped at the state park the same weekend the ill persons had visited the lake and swam in the same lake. Two controls were matched to each case in two age categories (1-6 years and 7-12 years). Case and control parents were asked about their child's drinking-fountain water consumption, concession stand purchases, recreational activities (e.g., boating, hiking, and fishing), and swimming behaviors (e.g., area of the lake in which they swam, duration of stay in the water, whether they submerged their heads, and whether they put water in their mouths or swallowed water). Analysis by unmatched odds ratios suggested that risk for illness was associated with taking lake water into the mouth (unmatched OR=9.8, 95% CI=1.03-93.5) and swallowing lake water (unmatched OR=12.4, 95% CI=1.3-118.3).

Environmental Investigation

The park includes two connected lakes. The 50-acre lake with the bathing beach is fed by a stream; the outflow for this lake is connected to the second lake where swimming is not permitted. Water movement from the first lake to the second was limited because of dry conditions. On June 24 and June 25, an estimated 2200 and 2500 persons visited the beach area (approximate attack rate=0.3%). None of the ill persons reported swimming in the second lake. Facilities at the implicated lake included two concrete vault privies, each containing two toilets. To determine whether these privies were watertight, the concrete vaults were filled with water; no leakage was observed. Handwashing facilities were not available. Samples obtained from the potable water supply for the showers and drinking fountain were tested July 10 and 12, but were negative for fecal coliforms. No cattle farms or sewage outlets that would affect water quality were adjacent to the stream that feeds into the swimming lake.

In Illinois, state regulations require facilities to perform fecal coliform testing of swimming beach waters when requested by IDPH and stipulate that a beach be closed when any two consecutive tests detect fecal coliform levels greater than 500 per 100 mL (1). Two samples collected at the beach on June 21 had levels of 660 and 900 E. coliper 100 mL. Four samples collected on July 6, after the lake had been closed to swimming, were not analyzed for E. coli but fecal coliform levels were less than 25 per 100 mL. On July 10, the level of E. coli was greater than 500 per 100 mL in two of six beach water samples; water samples from the lake were cultured for E. coli O157:H7 but were negative. Although many waterfowl (e.g., geese and ducks) were observed at the beach, waterfowl droppings cultured on July 12 were negative for E. coli O157:H7. Because E. coli O157:H7 can survive for up to 1 month in tap water, the bathing beach was closed for the remainder of the 1995 swimming season.

Reported by: M Warrner, Winnebago County Health Dept; K Kuo, L Williams, State Public Health Laboratory; B Adam, C Langkop, R Ruden, B Francis, MD, State Epidemiologist, Illinois Dept of Public Health. T Haupt, Wisconsin Dept of Health and Social Svcs. Div of Bacterial and Mycotic Diseases, National Center for Infectious Diseases; Div of Field Epidemiology, Epidemiology Program Office, CDC.

Editorial Note

Editorial Note: Although most outbreaks of E. coli O157:H7 have been associated with eating contaminated, undercooked ground beef, others have been linked to drinking apple cider, eating vegetables, and swimming in or drinking contaminated water (2-6). This report describes the third outbreak of lake-associated E. coli O157:H7 infections documented in the United States since 1991 (7,8). Investigations of the two previous outbreaks concluded that infected swimmers had contaminated the water with E. coli O157:H7.

Bathing beaches usually are regulated by the environmental division of each state health department. The fecal coliform level used as a threshold for closing beaches varies among states. Methods used for testing for fecal coliforms are insensitive to low concentrations of pathogens, and the infectious dose for E. coli O157:H7 is low (2). The sensitivity of routine microbiologic testing of swimming water also may be limited because contamination caused by swimmers may be transient. The fecal coliform test usually indicates fecal contamination from warm-blooded mammals but is not specific for pathogenic organisms or a human source (9). In the lake in Illinois, fecal coliform levels were high on July 10 after no one had been swimming in the water for 4 days, possibly indicating contamination of water by nonhuman sources.

Measures to decrease risks associated with swimming in unchlorinated water should be directed toward reducing the likelihood of fecal contamination of swimming water. Such measures could include providing changing tables for infants in locker rooms, providing adequate toilet and handwashing facilities, posting signs warning against drinking lake water and defecating in the lake, recommending that children with gastrointestinal illness or dirty diapers not swim in the lake, and prohibiting children who are not toilet trained from swimming.

References

1. Illinois Department of Public Health. Illinois swimming pool and bathing beach code. Springfield, Illinois: Illinois Department of Public Health, 1990.

2. Bell BP, Goldoft M, Griffin PM, et al. A multistate outbreak of Escherichia coli O157:H7-associated bloody diarrhea and hemolytic uremic syndrome from hamburgers: the Washington experience. JAMA 1994;272:1349-53.

3. Swerdlow DL, Woodruff BA, Brady RC, et al. A waterborne outbreak in Missouri of Escherichia coli O157:H7 associated with bloody diarrhea and death. Ann Intern Med 1992;117:812-8.

4. Brewster DH, Brown MI, Robertson D, Houghton GL, Bimson J, Sharp JCM. An outbreak of Escherichia coli O157 associated with a children's paddling pool. Epidemiol Infect 1994;112:441-7.

5. Besser RE, Lett SM, Weber JT, et al. An outbreak of diarrhea and hemolytic uremic syndrome from Escherichia coli O157:H7 in fresh-pressed apple cider. JAMA 1993;269:2217-20.

6. Cieslak PR, Barrett TJ, Griffin PM, et al. Escherichia coli O157:H7 infection from a manured garden. Lancet 1993;342:367.

7. Keene WE, McAnulty JM, Hoesly FC, et al. A swimming-associated outbreak of hemorrhagic colitis caused by Escherichia coli O157:H7 and Shigella sonnei. N Engl J Med 1994;331:579-84.

8. Ackman D, Birkhead G, Root T, Caldwell M, Marks S, Mack P. Swimming-associated hemorrhagic colitis due to Escherichia coli O157:H7 infection {Abstract K67}. In: Program and Abstracts of the 35th Interscience Conference on Antimicrobial Agents and Chemotherapy. Washington, DC: American Society for Microbiology, 1995.

9. American Public Health Association, American Water Works Association, Water Environment Federation. Standard methods for the examination of water and wastewater. New York: American Public Health Association, 1992:9-52.

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