Objective: To describe an outbreak of Cryptosporidium gastroenteritis in a swimming pool in Melbourne in early 1998 that was not detected through routine surveillance, and discuss difficulties in identifying such outbreaks.
Methods: The Water Quality Study (WQS) was a large community-based study of gastroenteritis. Following suspicion of an outbreak of cryptosporidiosis within the study group, due to pool "X", a nested case control study was performed. Each case of Cryptosporidium gastroenteritis was matched with six controls and data from weekly Health Diaries from the WQS were reviewed. The Department of Human Services also instigated active surveillance among patrons at pool "X" using a systematic sample of 50 people from the pool's swim-school enrollment list.
Results: There were seven cases of Cryptosporidium gastroenteritis in the case control study. Five cases and eight controls swam at pool "X" during the outbreak period. The adjusted odds of developing cryptosporidial diarrhoea if an individual swam at pool "X" was 34.5 (CI 2.3-2548). DHS identified another 11 laboratory confirmed cases associated with pool "X" as well as cases not linked to pool "X". 125 cases were identified throughout Melbourne with the suspected involvement of seven swimming pools.
Conclusions: Despite a high odds ratio of developing cryptosporidiosis this outbreak was not detected by routine surveillance methods. Current outbreak detection methods lack sensitivity, specificity or timeliness.
Implications: Improved surveillance systems are required if outbreaks of gastroenteritis are to be detected early so an intervention can be instigated to reduce the amount of subsequent illness.