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

Cryptosporidium Infections Associated with Swimming Pools -- Dane County, Wisconsin, 1993

In March and April 1993, an outbreak of cryptosporidiosis in Milwaukee resulted in diarrheal illness in an estimated 403,000 persons (1). Following that outbreak, testing for Cryptosporidium in persons with diarrhea increased substantially in some areas of Wisconsin; by August 1, 1993, three of six clinical laboratories in Dane County were testing routinely for Cryptosporidium as part of ova and parasite examinations. In late August 1993, the Madison Department of Public Health and the Dane County Public Health Division identified two clusters of persons with laboratory-confirmed Cryptosporidium infection in Dane County (approximately 80 miles west of Milwaukee). This report summarizes the outbreak investigations.

On August 23, a parent reported to the Madison Department of Public Health that her daughter was ill with laboratory-confirmed Cryptosporidium infection and that other members of her daughter's swim team had had severe diarrhea. On August 26, public health officials inspected the pool where the team practiced (pool A) and interviewed a convenience sample of patrons at the pool. Seventeen (55%) of 31 pool patrons interviewed reported having had watery diarrhea for 2 or more days with onset during July or August. Eight (47%) of the 17 had had watery diarrhea longer than 5 days. Four persons who reported seeking medical care had stool specimens positive for Cryptosporidium.

On August 31, public health nurses at the Dane County Public Health Division identified a second cluster of nine persons with laboratory-confirmed Cryptosporidium infection while following up case-reports voluntarily submitted by physicians. Seven of the nine ill persons reported swimming at one large outdoor pool (pool B). Because of the potential for disease transmission in multiple settings, a community-based matched case-control study was initiated on September 3 to identify risk factors for Cryptosporidium infection among Dane County residents.

Laboratory-based surveillance was used for case finding. A case was defined as Cryptosporidium infection that was laboratory-confirmed during August 1-September 11, 1993, in a Dane County resident who was also the first person in a household to have signs or symptoms (i.e., watery diarrhea of 2 or more days' duration). During the study interval, 85 Dane County residents with stool specimens positive for Cryptosporidium were identified. Sixty-five (77%) persons were interviewed; 36 (55%) had illnesses meeting the case definition. Systematic digit-dialing was used to select 45 controls, who were matched with 34 case-patients by age group and telephone exchange. All study participants were interviewed by telephone using a standardized questionnaire to obtain information on demographics, signs and symptoms, recreational water use, child-care attendance, drinking water sources, and presence of diarrheal illness in household members.

The median age of ill persons was 4 years (range: 1-40 years). Reported signs and symptoms included watery diarrhea (94%), stomach cramps (93%), and vomiting (53%). Median duration of diarrhea was 14 days (range: 1-30 days). Swimming in a pool or lake during the 2 weeks preceding onset of illness was reported by 82% of case-patients and 50% of controls (matched odds ratio {MOR}=6.0; 95% confidence interval {CI}=1.4-25.3). Twenty-one percent of case-patients and 2% of controls (MOR=7.3; 95% CI=0.9-59.3) reported swimming in pool A. Fifteen percent of case-patients and 2% of controls (MOR=undefined {6/0}; p=0.02, paired sample sign test) reported swimming in pool B. When persons reporting pool A or B use were excluded from the analysis, the association with recreational water use was not statistically significant (MOR=3.4, 95% CI=0.8-15.7). Child-care attendance was reported for 74% of case-patients aged less than 6 years and 44% of controls (MOR=2.9; 95% CI=0.8-10.7). Two case-patients reported child-care attendance and use of pool A or pool B. No case-patients reported travel to the Milwaukee area during the March-April outbreak, and no associations were found between illness and drinking water sources.

To limit transmission of Cryptosporidium in Dane County pools, state and local public health officials implemented the following recommendations: 1) closing the pools that were epidemiologically linked to infection and hyperchlorinating those pools to achieve a disinfection (CT *) value of 9600; 2) advising all area pool managers of the increased potential for waterborne transmission of Cryptosporidium; 3) posting signs at all area pools stating that persons who have diarrhea or have had diarrhea during the previous 14 days should not enter the pool; 4) notifying area physicians of the increased potential for cryptosporidiosis in the community and requesting that patients with watery diarrhea be tested for Cryptosporidium; and 5) maintaining laboratory-based surveillance in the community to determine whether transmission was occurring at other sites (e.g., child-care centers and other pools).

On August 27, pool A was closed and hyperchlorinated for 18 hours; on September 3, pool B closed early for the season. Because many control measures were initiated less than 1 week before many pools closed for the season (after September 5), their impact on transmission could not be evaluated adequately.

Reported by: J Bongard, MS, Dane County Public Health Div, Madison; R Savage, MS, Madison Dept of Public Health; R Dern, MS, St. Mary's Medical Center, Madison; H Bostrum, J Kazmierczak, DVM, S Keifer, H Anderson, MD, State Epidemiologist for Occupation and Environmental Health, JP Davis, MD, State Epidemiologist for Communicable Diseases, Bur of Public Health, Wisconsin Div of Health. Div of Parasitic Diseases, National Center for Infectious Diseases; Div of Field Epidemiology, Epidemiology Program Office, CDC.

Editorial Note

Editorial Note: Person-to-person, waterborne, and zoonotic transmission of Cryptosporidium has been well documented (2). A marked seasonality has been reported, with peaks occurring in North America during late summer and early fall (3,4). Cryptosporidiosis associated with use of swimming pools has been reported previously (5-7) but is probably underrecognized. Infection with Cryptosporidium resulting from recreational water use may contribute to the observed seasonal distribution.

The March-April 1993 Milwaukee waterborne outbreak stimulated increased testing for Cryptosporidium in Dane County, increasing the likelihood of outbreak detection. However, the number of cases described in this report was not sufficient to conduct a stratified matched analysis. Confounding of the associations found for child-care attendance and pool use is possible, although child-care attendance was reported in only one case for each implicated pool.

Cryptosporidium oocysts are small (4-6 u), are resistant to chlorine, and have a high infectivity. The chlorine CT of 9600 needed to kill Cryptosporidium oocysts is approximately 640 times greater than required for Giardia cysts (8). The ability of pool sand-filtration systems to remove oocysts under field conditions has not been well documented, but would not be expected to be effective. Results of an infectivity study suggest that the infective dose among humans for Cryptosporidium is low (H. DuPont, University of Texas Medical School at Houston, personal communication, 1994). Because of the large number of oocysts probably shed by symptomatic persons, even limited fecal contamination could result in sufficient oocyst concentrations in localized areas of a pool to cause additional human infections.

This investigation underscores the potential for transmission of Cryptosporidium in swimming pools. Health-care providers should consider requesting Cryptosporidium testing of stool specimens from persons with watery diarrhea, and public health departments should consider establishing surveillance for Cryptosporidium to facilitate prompt recognition of outbreaks. Maintaining the high levels of chlorine necessary to kill Cryptosporidium in swimming pools is not feasible; therefore, such recreational water use should be recognized as a potential increased risk for cryptosporidiosis in immumocompromised persons, including those with human immunodeficiency virus infection, in whom this infection may cause lifelong, debilitating illness (9).


  1. Mac Kenzie WR, Hoxie NJ, Proctor ME, et al. A massive outbreak in Milwaukee of Cryptosporidium infection transmitted through the public water supply. N Engl J Med 1994;331:161-7.

  2. Casemore DP. Epidemiologic aspects of human cryptosporidiosis. Epidemiol Infect 1990;104:1-28.

  3. Wolfson JS, Richter JM, Waldron WA, Weber DJ, McCarthy DM, Hopkins CC. Cryptosporidiosis in immunocompetent patients. N Engl J Med 1985;312:1278-82.

  4. Skeels MR, Sokolow R, Hubbard CV, Andrus JK, Baisch J. Cryptosporidium infection in Oregon public health clinic patients, 1985-1988: the value of statewide laboratory surveillance. Am J Public Health 1990;80:305-8.

  5. Sorvillo FJ, Fujioka K, Nahlen B, et al. Swimming-associated cryptosporidiosis. Am J Public Health 1992;82:742-4.

  6. Bell A, Guasparini R, Meeds D, et al. A swimming pool-associated outbreak of cryptosporidiosis in British Columbia. Can J Public Health 1993;84:334-7.

  7. CDC. Surveillance for waterborne disease outbreaks -- United States, 1991-1992. MMWR 1993;42(no. SS-5):1-22.

  8. Current WL, Garcia LS. Cryptosporidiosis. Clin Microbiol Rev 1991;4:305-8.

  9. Navin TR, Juranek DD. Cryptosporidiosis: clinical, epidemiologic, and parasitologic review. Rev Infect Dis 1984;6:313-

    • CT=pool chlorine concentration (in parts per million) multiplied by time (in minutes).

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