Outbreak of cryptosporidiosis linked to an indoor swimming pool

Objective: To determine the extent and source of a community outbreak of cryptosporidiosis. Design: Questionnaire-based survey and matched case-control study. Setting: Sutherland area in southern Sydney, September 1994 to January 1995. Participants: 70 patients reported by pathology laboratories to have stool specimens positive for cryptosporidia, of whom 43 were surveyed; 35 were compared with age- and neighbourhood-matched controls. Main outcome measures: Demographic characteristics and potential risk factors in the two weeks before onset of illness. Results: Laboratories reported 70 cases of cryptosporidiosis between September 1994 and January 1995. We found no association between illness and foods consumed or contact with people with diarrhoea or sick animals in the two weeks before onset. Seventeen of the case group (49%) reported swimming in a particular indoor swimming pool, compared with only seven controls (20%) (odds ratio, 3.7; P=0.015). Cryptosporidial oocysts were detected in water from the swimming pool in January 1995. Conclusions: The outbreak of cryptosporidiosis was probably associated with ingestion of water from the indoor swimming pool, presumably contaminated by infected bathers. Recommendations: As it is difficult to eradicate cryptosporidia from swimming pools by either disinfection or filtration, we recommend that: People with recent diarrhoea should avoid public swimming pools; and Non-toilet-trained and faecally incontinent swimmers should be provided with alternative swimming facilities with separate water and filtration systems. To enable appropriate public health responses: Doctors and pathology laboratories should consider cryptosporidiosis in patients with diarrhoea lasting longer than three days; and Laboratory reporting of cryptosporidia to local health departments should be mandatory in all States and Territories.