Everybody Out of the Pool!

Abstracts & Commentary

Synopsis: Prolonged outbreaks due to cryptosporidiosis resulted from exposure while swimming in chlorinated pools.

Sources: CDC. Protracted outbreaks of cryptosporidiosis associated with swimming pool use—Ohio and Nebraska, 2000. MMWR Morb Mortal Wkly Rep. 2001;50:406-410;
CDC. Prevalence of parasites in fecal material from chlorinated swimming pools—United States, 1999. MMWR Morb Mortal Wkly Rep. 2001;50:410-412.

After reports of cases of cryptosporidiosis came to their attention, the Delaware City/County Health Department (Ohio) found 700 patients meeting their clinical case definition, all of whom had been in central Ohio from June 17 to August 18, 2000. Of 268 stool samples from these individuals, 186 (70%) were found to contain Cryptosporidium parvum. A case-control study found that swimming at a club pool was strongly associated with illness, particularly among those involved in activities that increased the risk of pool water entering the mouth.

In an outbreak in Douglas County, Nebraska, 65 of 229 (29%) individuals meeting the clinical or laboratory definition were laboratory confirmed as cryptosporidiosis in the summer of 2000. A case-control study found that illness was strongly associated with swimming at a particular club pool and with having been splashed in the pool.

The demographics and severity of illness were similar in the 2 episodes. In the Ohio outbreak, the median age of case patients was 6 years (range, 1-46 years) and the median duration of illness was 7 days (1-36 days). In the Nebraska outbreak, the median age was 10 years (range, < 1-77 years) and the median duration of illness was also 7 days (range, 1-44 days). Approximately two-fifths at each site reported vomiting; the diarrhea was intermittent in almost one-half.

During the Nebraska outbreak, approximately 18% of case patients swam while they were experiencing symptoms, including diarrhea. "Fecal accidents" had been observed in both Nebraska and Ohio—at least 5 at the latter site, one of which was diarrheal.

Separately, the CDC examined the prevalence of detectable C parvum and Giardia intestinalis in formed stools collected after fecal accidents at 47 swimming pools, water parks, or aquatic centers throughout the United States. Samples were tested on unconcentrated samples using an antigen detection system with positives confirmed using immunofluorescent staining. While Giardia was identified in 13 (4.4%) of the samples, none of these formed stools contained detectable Cryptosporidium.

Comment by Stan Deresinski, MD, FACP

Cryptosporidium oocysts, at 2-4 mm in diameter, are sufficiently dimunitive that they may escape pool filtration systems and, in additon, are remarkably resistant to chlorine. While less than 1 hour of exposure is required to inactivate G intestinalis and only less than 1 minute to inactivate Escherichia coli at a concentration of 1 mg/mL of free available chlorine, approximately 7 days are required to inactivate C parvum.1

Several factors contributed to the prolonged nature of the outbreaks discussed here. In both cases diarrhea was commonly intermittent and affected individuals continued to use the pool facilities, despite symptoms of cryptosporidiosis. In addition, oocyst excretion commonly persists for a week after resolution of symptoms and may continue intermittently for as long as 2 months.2 Thus, in addition to the ability of C parvum to resist standard measures of pool hygiene, episodes of contamination undoubtedly continued until the pools were closed. These factors combined with the intensity of oocyst excretion (as high as 9.2 ´ 105/mL of stool in adult AIDS patients) and the low infectious dose (median infective dose of 132 oocysts in healthy adult volunteers), conspired to create these prolonged outbreaks affecting large numbers of individuals.3,4 Additional factors may play roles. Variation in infectivity among strains has been demonstrated and has variation in host susceptibility relating to the presence or absence of pre-existing IgG antibody against C parvum.5,6

These are not the only 2 intestinal infections associated with exposure to swimming pool water. Perhaps the most devastating have been outbreaks of infection due to Escherichia coli 0157:H7, such as one that occurred at a waterpark in Georgia in 1998.7 (Editor’s Note: The CDC maintains a web site providing information about the prevention of recreational water illness.8)


1. Carpenter C, et al. Chlorine inactivation of Escherichia coli 0157:H7. Emerg Infect Dis. 1999; 5:461-463.

2. Jokipii L, Jokipii AM. Timing of symptoms and oocyst excretion in human cryptosporidiosis. N Engl J Med. 1986;315:1643-1647.

3. Goodgame RW, et al. Intensity of infection in AIDS-associated cryptosporidiosis. J Infect Dis. 1993;167:

4. DuPont HL, et al. The infectivity of Cryptosporidium parvum in healthy volunteers. N Engl J Med. 1995; 332:855-859.

5. Okhuysen PC, et al. Virulence of three distinct Cryptosporidium parvum isolates for three healthy adults. J Infect Dis. 1999;180:1275-1280.

6. Chappell CL, et al. Infectivity of Cryptosporidium parvum in healthy adults with pre-existing anti-C parvum serum immunoglobulin G. Am J Trop Med Hyg. 1999;60:157-164.

7. Gilbert L, Blake P. Outbreak of Escherichia coli 0157:H7 infections associated with a waterpark.
Georgia Epidemiol Report. 1998;14:1-2.

8. http://www.cdc.gov/healthyswimming.