Swimming-associated cryptosporidiosis - Los Angeles County

Morbidity and Mortality Weekly Report, May 25, 1990

From July 13 through August 14, 1988, 44 persons in five separate swimming groups developed a gastrointestinal illness after using a swimming pool in Los Angeles County. The outbreak began several days after an unintentional human defecation in the pool during the first week of July. When the outbreak was reported to the Los Angeles County Department of Health Services (LACDHS) in early August, LACDHS initiated an epidemiologic investigation.

The affected groups had repeated pool contact in July and included a high school water polo team, a SCUBA class, a "masters" group, an elementary school group, and the pool lifeguards. Sixty (73%) of 82 persons from the five groups were interviewed. A case was defined as any person with watery diarrhea or diarrhea plus cramping and/or fever during July or August.

The overall attack rate was 73% (44/60) and ranged from 47% to 100% by group (Table 1). Illness was characterized by watery diarrhea (88%), abdominal cramps (86%), and fever (60%) and was often protracted (median duration: 5 days; range: 1-30 days). Two persons, both from the SCUBA class, were hospitalized. Cryptosporidium was identified in stool specimens by modified acid-fast staining from seven of 11 patients tested. Results of other laboratory examinations, including bacteria culturing for Salmonella, Shigella, and Campylobacter and testing for ova and parasites, were negative. Assessment for viral agents was not performed.

For all persons with pool contact during the outbreak period, the attack rate was highest among those with extensive (>3 total hours) water exposure (p<0.01, Fisher's exact test; relative risk=2.2; 95% confidence interval=1.1-4.4). No other common exposures or risk factors were identified. Review of surveillance data revealed no increase of cryptosporidiosis or diarrheal illness during July or August in Los Angeles County or the community affected by the outbreak. Pool water was not tested for Cryptosporidium, and the person who fecally contaminated the pool was not examined for Cryptosporidium infection.

The pool implicated in this outbreak is a 100,000-gallon pool at a school in Los Angeles County. Inspection of the pool during the outbreak period confirmed adequate chlorine levels (2 ppm) but detected a 30% diminished filtration flow rate and established that one of three diatomaceous earth (DE) filters was inoperative. The filtration system was repaired on August 3, and no additional cases of diarrhea were subsequently identified among newly exposed swimmers.

Reported by: FJ Sorvillo, MPH, K Fujioka, PhD, M Tormey, MPH, R Kebabjian, RS, W Tokushige, L Mascola, MD, S Schweid, M Hillario, SH Waterman, MD, Los Angeles County Dept of Health Svcs. Parasitic Diseases Br, Div of Parasitic Diseases, Center for Infectious Diseases, CDC.

Editorial Note: Outbreaks of giardiasis, Norwalk gastroenteritis, and adenovirus types 3 and 4 associated with swimming pool contact have been reported [1-4]. In each outbreak, inadequate pool maintenance was an important contributing factor.

The clinical features and laboratory findings in this investigation are consistent with an outbreak of cryptosporidiosis. Moreover, the investigation suggests that Cryptosporidium may be acquired through recreational water contact. Resistance of Cryptosporidium to chlorination [5], an inadequately maintained filtration system, and repeated and prolonged exposure may have contributed to the size and extent of

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this outbreak. Continued pool use and possible ongoing contamination by infected persons, many of whom continued to swim despite their illness, could also have sustained transmission.

Cryptosporidium oocysts are resistant to chlorine. Because Cryptosporidium oocysts are small (4-6 [mu]), rapid sand filters commonly used in swimming pools may not be effective in filtering oocysts. However, evidence suggests that a well-maintained, fine-grade DE filtration system may remove Cryptosporidium [6]. Further study is needed to assess the capability of different filtration devices to remove Cryptosporidium oocysts from swimming pool water.

Recommendations for managing swimming pools that have been fecally contaminated include prohibition of swimming until the chlorine level and contact time are sufficient to kill Giardia cysts [1]. Given the ineffectiveness of chlorine against Cryptosporidium, greater consideration should be given to control strategies that use effective filtration (e.g., DE filters) or to draining the pool and replacing contaminated filter media in filters not considered effective against Cryptosporidium. In systems that use DE filters, one option may be to close contaminated pools until relatively complete filtration has occurred (typically three turnovers or approximately 1 day).

References

[1] Porter JD, Ragazzoni HP, Buchanon JD, Waskin HA, Juranek DD, Parkin WE. Giardia transmission in a swimming pool. Am J Public Health 1988;78:659-62.

[2] Kappus KF, Marks JS, Holman RC, et al. An outbreak of Norwalk gastroenteritis associated with swimming in a pool and secondary person-to-person transmission. Am J Epidemiol 1982;116:834-9.