Cryptosporidium infections associated with swimming pools - Dane County, Wisconsin, 1993

Morbidity and Mortality Weekly Report, August 12, 1994

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 daughters 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 <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 96000;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 l week before many pools closed for the season (after September 5), their impact on transmission could not be evaluated adequately.