Cryptosporidium spp. and Giardia intestinalis have been found in swimming pool fi lter backwash during outbreaks. To determine baseline prevalence, we sampled pools not associated with outbreaks and found that of 160 sampled pools, 13 (8.1%) were positive for 1 or both parasites; 10 (6.2%) for Giardia sp., 2 (1.2%) for Cryptosporidium spp., and 1 (0.6%) for both. Giardia sp. and Cryptosporidium spp. are gastrointestinal parasites spread through the fecal-oral route. In 2003–2004, these parasites were responsible for 61.2% (Cryptosporidium spp. 55.6%; Giardia sp. 5.6%) of gastroenteritis outbreaks associated with treated swimming venues (e.g., swimming pools, water parks) in the United States (1). Cryptosporidium’s key role in these outbreaks is likely because of its small size, low infectious dose (2), and high tolerance to chlorine (3), which is the major disinfectant used in swimming pools. Despite frequent outbreaks, little is known about these parasites ’ occurrence in swimming pools in the absence of outbreaks. Although the frequency of contamination is unknown, 4.4 % of formed feces recovered from non–outbreak-related pools were positive for Giardia sp. and 0 were positive for Cryptosporidium spp. (4). In the Netherlands, 7 pools sampled for>1 year had a prevalence of 5.9 % for Giardia sp., 4.6 % for Cryptosporidium spp., and 1.3 % for both pathogens (5). In Italy, 1 study found 28.6 % (2/7) of tested pools were positive for both Giardia sp. and Cryptosporidium spp. (6) and another study found 40 % (4/10) of tested pools positive for either parasite (7). No data exist on the occurrence of these parasites in US pools. Further data on pool contamination would reinforce existing US pool codes and support code changes designed to reduce the level of parasite contamination, particularly chlorine-resistant Cryptosporidium spp. During the past 2 decades, Cryptosporidium spp. and Giardia sp. have been associated with increasing outbreak
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