AN OUTBREAK OF CRYPTOSPORIDIUM PARVUM IN A SURREY POOL WITH DETECTION IN POOL WATER SAMPLING

Introduction

Cryptosporidium parvum is a protozoal pathogen that causes acute, watery diarrhea. Outbreaks of cryptosporidiosis have been associated with exposure to contaminated drinking water, recreational water, consumption of apple cider and produce, and contact with infected individuals in health care and child day-care settings⁽¹⁾. Cryptosporidium oocysts are highly resistant to the chemical disinfectants used to purify drinking water and recreational water sources, such as pools and water parks.

An unusually high number of reports of C. parvum infection in Surrey residents, predominantly children, were received between 22 August and 8 September, 2003. During this 17-day period, laboratories reported stool specimens from nine Surrey residents as positive for Cryptosporidium parvum, as compared with zero to two in a similar period during the previous 4 years. Eight of the nine reported cases were children between the ages of 6 and 14 years.

An open-ended hypothesis-generating questionnaire was developed and used to identify risk factors and any potential common exposure. Analysis on 8 September revealed that six of the eight cases in children and the single adult case had all been involved in swimming lessons in the same four-pool complex.

An investigation was initiated to confirm that the pool complex was the source of the outbreak and to identify contributing factors.

Methods

A structured questionnaire was developed to assess symptoms and ascertain the date the illness began, exposure to recreational water, use of the four different pools (main, tot, leisure, and swirl pools) at the pool complex, dates of use, whether there had been head submersion while swimming, consumption of foods sold at the complex, and exposure to animals, farms, child care settings, and other people with diarrhea. The questionnaire was administered to all cases from Surrey and neighbouring areas who had had C. parvum oocysts identified in their stools by microscopic examination at an accredited laboratory and had been reported between 1 August and 30 September, 2003.

A tabulation of clinical cases was kept in order to further estimate the burden of illness associated with the outbreak. A clinical case was defined as an individual whose stools were not examined for Cryptosporidium oocysts but who had experienced loose or watery stools for at least 1 day with illness beginning between 2 and 4 days after use of the pool complex. Clinical cases were identified through self-report or from interviews of laboratory-confirmed cases (e.g. symptomatic family contacts) conducted by the Fraser Health Authority during August and September 2003.

An inspection of the pools and the water treatment system was conducted on 8 September, and a review of the operational records was carried out.

On 9 September, 100-L samples of water were obtained from sand filter backwash from three pools (tot, leisure, and main pools) using a stringwound filter. In addition, 100-L samples were filtered directly from the tot pool and leisure pool. The samples were processed by cutting and washing of the filters with eluting buffer, centrifugation to pellets, and microscopic examination of the pellets using fluorescent monoclonal antibody and viability stains (U.S. Environmental Protection Agency - Information Collection Rule, June 1995).

Results

Twenty-five laboratory-confirmed cases in residents of Surrey and neighbouring areas were identified during the study period. Nineteen (76%) reported swimming in a recreational pool in the 2 weeks before the illness began. Of these, 15 (60%) swam at the suspected pool complex between 14 July and 23 August. Distribution of these cases by age and sex is shown in Table 1 and Figure 1 shows the date of onset of symptoms in 14 laboratory-confirmed cases. While the age range of cases was 0 to 36 years, the median age was 6 years. No individuals from neighbouring health authority areas who had laboratory-confirmed C. parvum infection reported visits to the pool. Although cases had used multiple pools in the complex, only one pool - the leisure pool - was used by all of the cases who reported exposure to the pool complex. No other common exposures were identified.

Table 1. Laboratory-confirmed cases with pool use as most likely exposure (n = 15)

Total	Sex		Age (years)
	Female	Male	0-4	5-9	10-14	14-19	20+
15	7	8	5	7	1	0	2

Figure 1. Onset dates of symptoms in 14 laboratory-confirmed cases with pool as the most likely exposure

Sixteen clinical cases who swam in the pool complex were identified, of whom eight were children (age <= 19 years) and eight were adults. Two additional cases had not used the complex but were close contacts of cases who had.

The water treatment system for each of the pools uses sand filtration, ozonation, and chlorination. All had been operational throughout the likely exposure period of the cases. There had been two fecal accidents noted between June and the end of August; these occurred in the leisure pool on 24 June and 14 July, 2003.

Water samples from filter backwash were described as moderately or very dirty. Consequently, detection limits were < 119.8, 510.5 and 1412.4 oocysts per 100 L for backwash from the leisure, tot and main pools respectively. Detection limits on water sampled directly from the pools were much lower, at < 4.0 and 4.7 oocyts per 100 L for the tot and leisure pools. Cryptosporidium oocysts were detected at a level of 14.0 per 100 L in a sample of leisure pool water (consistent with the detection limit for this water sample). No other water samples or samples of backwash yielded the parasite (Table 2).

Table 2. Test results on pool water samples

Pool	Date sampled	Detection limit	Table 2. Test results on pool water samples
Leisure pool Filter backwash	2003-09-09	<119.8	No Parasites Detected (NPD)
Leisure pool water	2003-09-09	<4.7	14.0
Main pool Filter backwash	2003-09-09	<1 412.4	NPD
Tot pool Filter backwash	2003-09-09	<510.5	NPD
Tot pool water	2003-09-09	<4.0	NPD

Public Health Action

On the basis of the initial epidemiologic evidence - seven of the nine reported cases having pool use as a common risk exposure - the pool complex was closed on 9 September for remediation. Remediation proposed by the pool management was to maintain a chlorine concentration of 20 mg/L for 8 hours, thus meeting a CT 9600 standard (chlorine concentration [mg/L] x time [minutes] = 9600), with return to a normal operating chemical balance before re-opening^(2-6). In consultation with public health personnel, pool management increased the remediation procedure to 20 mg/L x 12 hours (a CT of 14,400). Return to normal chemical balance was achieved for pool reopening on 11 September, having taking longer than anticipated and consequently achieving a higher than targeted CT value. Challenges in maintaining proper pH of 7.2 to 7.5^(2,3) were experienced at times during the remediation process.

Enhanced signage was posted at the pool complex, informing pool users that individuals who had diarrhea or had had diarrhea within the previous 14 days should not enter any of the pools. Enhanced surveillance by pool staff for illness in swimming lesson participants was introduced.

Discussion

A higher than average number of reports of cryptosporidiosis were received until 7 October as compared with previous years. All but one of the affected individuals reported that their last use of the pool had been before 9 September (the date of pool closure and remediation). The exception was an adult who had used the pool both before and after remediation. Although his symptoms began more than one incubation period after his pre-remediation exposure, two family members had used the pool prior to closure and both had experienced mild gastrointestinal illness in the week after pool use. These two children did not seek medical attention and did not have stool testing. The adult family member is thought to have most likely experienced secondary infection from spread within the family.

Even with prompt closure, the total case count associated with the pool in the period before remediation reached 33, of which 31 had direct pool exposure and two were likely due to secondary spread within families.

Late in August, Cryptosporidium oocysts were detected in the stool of one employee who was in the pools on a daily basis. Symptoms began in late July and the employee had worked throughout the symptomatic period. It was reported, though not confirmed, that other employees had had diarrhea but continued to work during the same period. The employer confirmed that there are no provisions for sick time with pay for part-time employees. Public health personnel emphasized the importance of excluding any employees who have diarrhea from using the pool.

The potential contribution to the outbreak of patrons and employees participating in swimming lessons while symptomatic with a diarrheal illness could not be determined in this outbreak. During the course of the outbreak, no fecal accidents were detected or reported for the period during which the pool was being used by symptomatic cases. However, continued disease transmission occurred, and C. parvum was detected in pool water. Diarrheal accidents in public pools may go unnoticed and unreported. In addition, fecal shedding without recognized diarrheal accidents may have contributed to propagation of the outbreak. Although contribution in this manner cannot be quantified, on average about 0.14 g of feces per person may rinse off into, and potentially contaminate, pool water⁽⁷⁾. The continued use of the pool by symptomatic cases demonstrates that all pool users may not be aware of the potential for spread of illness and may benefit from education.

Conclusions

Rapid remediation by super chlorination combined with ozonation was effective in terminating an ongoing outbreak associated with contaminated water in a swimming pool. At the time of pool closure for remediation, additional measures, including draining the pools, scrubbing and disinfecting the surfaces, and changing the filter material in the sand filters, were considered but not undertaken^(8,9). Continued surveillance has not revealed evidence of ongoing infection related to the pool complex.

A need for additional education of pool staff and patrons regarding gastrointestinal illness transmission through recreational water use was identified. All pool staff should be aware of principles and practices related to prevention of recreational water illnesses. Information should be posted at all pools to educate patrons that they should not use the pool while they have diarrheal illness⁽⁴⁾ or for 14 days after symptoms have resolved if the diagnosis was cryptosporidiosis.

Documentation and remediation following fecal accidents in pools is an essential component in a strategy to reduce the spread of recreational water illnesses. Fecal accidents in public pools must be managed in accordance with the recommendations of the Centers for Disease Control and Prevention^(2-5). This involves super chlorination to achieve a CT inactivation value of 9600 following diarrheal (liquid stool) accidents.

Acknowledgement

We would like to thank Mr. Lloyd Lane, Environmental Health Officer, Fraser South, for helping collect some of the pool water for analysis.

References

1. Chin J, ed. Control of communicable diseases manual, 17th ed. Washington, DC: American Public Health Association, 2000: 134-37.

2. Center for Disease Control and Prevention. Notice to readers: responding to fecal accidents in disinfected swimming venues. MMWR 2001;50(20):416-17.

3. Centers for Disease Control and Prevention, Division of Parasitic Diseases. Healthy Swimming. Fecal accident response recommendations for pool staff. URL: <http://www.cdc.gov/healthyswimming/>.

4. Centers for Disease Control and Prevention, Division of Parasitic Diseases. Healthy swimming - for pool staff: 12 steps for prevention of recreational water illnesses (RWIs). URL: <http://www.cdc.gov/healthyswimming/>.

5. Centers for Disease Control and Prevention. Cryptosporidium infections associated with swimming pools - Dane County, Wisconsin, 1993. MMWR 1994;43 (31):561-63.

6. Carpenter C, Fayer R, Trout J et al. Chlorine disinfection of recreational water for Cryptosporidium parvum. Emerg Infect Dis 1999;5(4):579-84.

7. Centers for Disease Control and Prevention, Division of Parasitic Diseases. Health swimming. How are RWIs spread? URL: <http://www.cdc.gov/healthyswimming/>.

8. Meeds D. Cleanup of a community swimming pool after a Cryptosporidium outbreak. Environ Health Rev 1993;37(1):45-6.

9. Bell A, Guasparini R, Meeds D et al. A swimming pool-associated outbreak of cryptosporidiosis in British Columbia. Can J Public Health 1993;84(5):334-7.

Source: K Louie, CPHIC, and L Gustafson, MD, MHSc, Fraser Health Authority, Surrey, BC; M Fyfe, MD, MSc, FRCPC, Epidemiology Services, British Columbia Centre for Disease Control, Vancouver, BC; I Gill, CPHIC, Fraser Health Authority, Surrey, BC; Laura MacDougall, MSc, Epidemiology Services, British Columbia Centre for Disease Control, Vancouver, BC ; L Tom, BSc, RT, Q Wong, BSc, and J Isaac-Renton, MD, DPH, FRCPC, Laboratory Services, British Columbia Centre for Disease Control, Vancouver, BC.

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