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ARCHIVED - An outbreak of Escherichia coli O157:H7 associated with a children's water spray park and identified by two rounds of pulsed-field gel electrophoresis testing

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Volume 31-12
15 June 2005

[Table of Contents]

In July 2004 an unusually high number of Escherichia coli O157:H7 infections were reported in British Columbia (B.C.), many of which were in the Interior of the province. By early August, regional health authorities and the BC Centre for Disease Control (BCCDC) were actively investigating several clusters of E. coli O157:H7. One cluster was linked to a nationally distributed beef product and involved cases identified by a unique pulsed-field gel electrophoresis (PFGE) pattern.

One case in a separate cluster of E. coli O157:H7 infections with a similar PFGE pattern had a strong epidemiologic link to the contaminated beef product. On careful review, we felt that the PFGE patterns associated with these two clusters were identical. On 9 August, we performed a second round of PFGE testing using a different enzyme to confirm that there was only one cluster - not two - of E. coli O157:H7 in the province related to the contaminated beef product.

However, this second round of PFGE testing reclassified isolates into two new clusters with different PFGE patterns. One cluster was connected to the nationally distributed beef product. The second cluster was determined to have a different PFGE pattern, and the majority of infections occurred in the Interior of the province.

In this article, we describe the laboratory findings leading to the identification of this second cluster and discuss the role of PFGE testing with more than one enzyme in the identification of outbreaks. We also describe the results of our outbreak investigation to determine the source of E. coli O157:H7 in this second cluster, and we make recommendations to prevent such outbreaks in the future.

Methods

Microbiologic analysis

Stools from symptomatic patients were submitted either to the BCCDC Laboratory Services Enteric Section or to local hospital and community laboratories for processing for bacterial pathogens. Stool specimens submitted directly to BCCDC were tested for toxin production by Vero cell assay(1,2). If positive, the strain of E. coli was isolated, serotyped, and tested for the presence of toxin genes. Isolates of suspected E. coli O157:H7 identified in local laboratories were sent to BCCDC Laboratory Services Enteric Section for serotyping and toxin gene testing.

Serotyping and polymerase chain reaction testing for toxin genes VT1 and VT2 were performed according to established protocols(3,4). All toxigenic E. coli O157:H7 isolates were subtyped by PFGE according to an established protocol using the restriction enzymes Xba 1 (first round of PFGE testing) and Bln 1 (second round)(5). Accepted criteria were used to identify isolates having the same PFGE pattern(6).

Epidemiologic investigation

We defined cases in two ways(7). A confirmed case was an individual with laboratory-confirmed E. coli O157:H7 infection in stool cultures having the unique PFGE pattern associated with the outbreak strain. A probable case was an individual with an epidemiologic link to the suspect exposure and with compatible clinical symptoms (i.e. bloody or non-bloody diarrhea and abdominal cramps). We relied on identification of confirmed cases through the B.C. laboratory surveillance network. Case identification was assisted by letters sent to physicians in the Interior in mid-July requesting testing of individuals presenting with compatible symptoms. Probable cases were identified from households of confirmed cases only.

Local public health inspectors and medical health officers contacted confirmed cases and administered a modified standard enteric follow-up questionnaire (routinely done for all laboratory-confirmed E. coli O157:H7 infections in the Interior from mid-July onwards). This questionnaire was administered in person or by telephone to collect demographic, clinical, and exposure data. We used Microsoft Excel 2002 (Microsoft Corporation, USA) to collect data, which were analyzed descriptively.

Environmental assessment

Local investigators visited the implicated facility and reviewed operations and maintenance logs. Facility attendants, city officials and municipal workers were interviewed, and engineering plans of the water circulation and drainage systems were examined. Water samples were collected both for microbiological testing and measurement of free chlorine residuals (using a Hach® Test Kit). System connections were traced using fluorescein tracing dye.

The BCCDC Environmental Microbiology Laboratory tested water samples for fecal indicators (fecal coliforms and E. coli) and specifically for E. coli O157:H7. A standardized protocol using a membrane filter technique (with m-FC agar and Nutrient Agar with methylumbelliferyl-ß-D glucuronide) was used to test for fecal indicators(8). To detect potentially small numbers of E. coli O157:H7 bacteria, each water sample was first concentrated by a membrane filtration procedure. The concentrated filter was placed in Doyle's enrichment broth and incubated at 43º C for 24 hours(9). The broth culture was then streaked onto selective differential agar plates (Sorbitol-MacConkey agar) and incubated at 35º C for 24 hours. At least five isolated sorbitol-negative colonies were selected and put through a number of biochemical and serological tests to confirm the identity of E. coli O157:H7.

Results

Microbiologic results

By the first week of August 2004, we had identified eight isolates of E. coli O157:H7 with the same PFGE pattern as the contaminated beef product (pattern XCA1 1016, Figure 1). The second round of PFGE testing conducted on 9 August divided these eight isolates into two clusters, and one additional isolate unrelated to either cluster. By comparison with the PFGE results from other provinces we determined that one cluster (two isolates, pattern ECBN1 0157) was related to consumption of the contaminated beef product. The second cluster (five isolates, pattern ECBN1 0170) shared a new, unique PFGE pattern. Representative PFGE patterns are shown in Figure 2.

By 19 August a total of eight isolates had been identified as the outbreak strain of E. coli O157:H7, pattern ECBN1 0170. We refocused our ongoing epidemiologic investigation on these eight isolates in order to determine the source of the strain of E. coli O157:H7 in this cluster.


Figure 1. Classification of Escherichia coli O157:H7 isolates by two rounds of pulsed-field gel electrophoresis (PFGE) testing

Figure 1. Classification of Escherichia coli O157:H7 isolates by two rounds of pulsed-field gel electrophoresis (PFGE) testing



Figure 2. Pulsed-field gel electrophoresis (PFGE) patterns of Escherichia coli O157:H7 clusters after two rounds of testing (pattern A from round one PFGE, XCA1 1016; pattern B from round two PFGE, ECBN1 0157; and pattern C from round two PFGE, ECBN1 0170)

Figure 2. Pulsed-field gel electrophoresis (PFGE) patterns of Escherichia coli O157:H7 clusters after two rounds of testing (pattern A from round one PFGE, XCA1 1016; pattern B from round two PFGE, ECBN1 0157; and pattern C from round two PFGE, ECBN1 0170)

Epidemiologic results

We identified eight confirmed and two probable cases. Seven cases were children (70%), ranging from 2 to 9 years of age. The remaining three cases (30%) were adults, ranging from 20 to 35 years of age. Half of all cases were male. All cases were residents of (90%) or visitors to (10%) one city in the Interior of B.C. Dates of onset of symptoms were from 26 June to 5 August, 2004 (Figure 3).

All cases reported having diarrhea (80% bloody diarrhea). Additional symptoms included nausea and vomiting (60%), abdominal cramps (50%), and fever (30%). Cases reported significant morbidity from their infection: over half of cases were hospitalized (60%), and in one case hemolytic uremic syndrome subsequently developed. No deaths occurred.

In our review of follow-up questionnaires we noticed that several children had attended a children's festival in the city in the week before their symptoms began. On re-questioning these cases, we discovered that the children had played in a water spray park adjacent to the festival. When all cases were questioned, a total of six of the seven children and all three adults identified in the outbreak had a history of exposure to water in the spray park (90% of all cases). One child - the index case - did not have a history of exposure to the water park before or after onset of infection.


Figure 3. Epicurve of Escherichia coli O157:H7 cases linked to the water spray park in the Interior of British Columbia, 2004 (n = 10)

Figure 3. Epicurve of Escherichia coli O157:H7 cases linked to the water spray park in the Interior of British Columbia, 2004 (n = 10)

Environmental investigation

The spray park was constructed in 1991 and consists of a recirculating water system with disinfection by automatic chlorination and a high-rate slow sand filter. The system was originally designed such that backwashed wastewater from the spray park was discharged to a sanitary sewer.

The documentation by park attendants revealed frequent manual backwash of the system to eliminate excess water after rainfall, suggesting a problem with discharging water from the system. On inspection and review of the spray park's water system, it was determined that backwashed wastewater (including water from daily drainage of the system) was being discharged to a storm sewer and not to a sanitary sewer as originally constructed. This storm sewer had become blocked over time, with backup and overflow of water through peripheral storm grates (catch basins) onto the spray park surface, resulting in collections of standing water and saturation of the surrounding lawn.

Water samples collected from spray jets, filter backwash, surge tank, and storm drains were submitted for microbiological testing. Two samples collected from the storm drain had elevated fecal indicators (sample 1: 2200 fecal coliforms per 100 mL, 1500 E. coli per 100 mL; sample 2: 16 000 fecal coliforms per 100 mL, 14 400 E. coli per 100 mL). E. coli O157:H7 was not identified in any of the submitted water specimens.

Water samples demonstrated a low level of free chlorine residuals (0.4 ppm). This level was lower than the reading obtained using the spray park's chlorine residual testing kit, which indicated a level of 1.5 ppm. While no standard is specified for a chlorine residual for water in spray parks, the standard for swimming pools in B.C. is 0.5 to 1.0 ppm(10).

As a result of this inspection, the city voluntarily closed the spray park on 19 August, 2004.

Discussion

In this article we describe an investigation into a cluster of E. coli O157:H7 that was identified only through a second round of PFGE testing of isolates. Conducting two rounds of PFGE testing of E. coli clusters has been recommended in the literature in order to discriminate between simultaneously occurring clusters or outbreaks of E. coli(11). This has proved helpful when isolates with the same PFGE pattern are observed in geographically distinct areas, or when the history of exposure suggests no epidemiologic connection between cases. The results of this investigation support this practice.

We linked the majority of cases to a single children's water spray park. While formal hypothesis testing was not performed, we felt the result of the environmental assessment strongly supported this association. Following closure of the spray park no further cases of infection with the outbreak strain of E. coli O157:H7 were reported.

The original source of the outbreak strain of E. coli O157:H7 is unknown. The strain may have originated from another source in the community with early introduction into the spray park system through fecal contamination of the water park by infected children and/or through the storm drain system. While we were unable to detect the outbreak strain of E. coli O157:H7 in collected water samples, these samples were collected approximately 2 weeks after the onset of symptoms in the last identified case. It is possible that this pathogenic strain was no longer present in the spray park water system.

This is the first known report of an outbreak linked to a children's water spray park in Canada. Outbreaks of enteric infections related to spray parks or fountains have previously been reported in the US(12-15). Pathogens identified include Cryptosporidium, Shigella, and Norovirus. All outbreaks occurred in spray parks or fountains with recirculating water systems, and in all reports the contributing factor identified was a lack of or inadequate filtration and disinfection.

Unlike these previous reports, we identified a structural problem - the overloaded and blocked storm sewer - to be the most likely contributing factor in the outbreak. This resulted in wastewater backing up onto the surface of the spray park and surrounding lawn, creating standing water in which the children played. As reported in other outbreaks, inadequate disinfection may also have played a role because of the falsely elevated readings on the spray park's test kit for free chlorine residuals.

After closure of the spray park the system was amended, as recommended by the local health authority, such that wastewater from the park would be discharged to the main sanitary sewer for disposal. The local health authority also made the following recommendations: to replace the test kit for detecting chlorine free residuals; to maintain the residual chlorine level at 2 ppm; to regularly submit water samples for bacteriologic analysis; to develop a procedural manual to outline system operation and maintenance for staff; and to improve documentation of operations, maintenance, and incidents.

In B.C., public health inspectors were informed about the findings of this outbreak investigation. As a result, regional health authorities have changed or are considering changes to practice in order to improve inspection of spray parks in their jurisdiction.

Conclusions

We identified an outbreak of infections due to E. coli O157:H7 in the Interior of B.C. during the summer of 2004 that was associated with a children's water spray park. Two rounds of PFGE testing were required to identify the cluster of infections associated with this outbreak.

Structural problems and inadequate filtration or disinfection of recirculating water at spray parks or fountains can lead to outbreaks of enteric infections. As a preventive measure, it would be prudent for public health officials to consider inspection of similar facilities within their jurisdiction. Regulations for spray parks may be less comprehensive than those for swimming pools.

Regulatory changes to improve standards for these facilities should be considered.

Acknowledgements

The authors would like to thank the following people for their assistance with this outbreak investigation: K. Cooper, R. Benzon, G. Komick, R. King, Interior Health Authority, Kelowna; G. Volk, W. Radmoske, R. Johnston, Interior Health Authority, Penticton; R. Birtles, Interior Health Authority, Vernon; M. Ritson, G. Eng, Vancouver Health Authority, Vancouver; and R. Sévigny, BCCDC Laboratory Services Enteric Division, Vancouver. The authors would also like to thank Dr. J. Buxton, BCCDC, Vancouver, and Dr. L. Panaro, Canadian Field Epidemiology Program, Ottawa, for their assistance in editing this manuscript.

References

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  3. Pollard DR, Johnson WM, Lior H et al. Rapid and specific detection of verotoxin genes in Escherichia coli by the polymerase chain reaction. J Clin Microbiol 1990;28(3):540-5.

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  5. Foodborne and Diarrheal Diseases Branch, Division of Bacterial and Mycotic Diseases, National Centre for Infectious Diseases, Centers for Disease Control and Prevention. Standardized molecular subtyping of Escherichia coli O157:H7 by pulsed-field gel electrophoresis: A training manual. Atlanta: CDC, 1996.

  6. Tenover FC, Arbeit RD, Goering RV et al. Interpreting chromosomal DNA restriction patterns produced by pulsed-field gel electrophoresis: Criteria for bacterial strain typing. J Clin Microbiol 1995;33(9):2233-9.

  7. Case definitions for diseases under national surveillance. CCDR 2000;26S3:i-iv, 1-122.

  8. American Public Health Association, American Water Works Association, and Water Environment Foundation. Standard methods for examination of water and wastewater. 20th ed. Washington: APHA, 1998.

  9. Doyle MP, Schoeni JL. Isolation of Escherichia coli O157:H7 from retail fresh meats and poultry. Appl Environ Microbiol 1987;53(10):2394-6.

  10. Swimming pool, spray pool and wading pool regulations. B.C. Reg. 289/72 (1972).

  11. Gupta A, Hunter SB, Bidol SA et al. Escherichia coli O157 cluster evaluation. Emerg Infect Dis 2004;10(10):1856-8.

  12. Outbreak of cryptosporidiosis associated with a water sprinkler fountain - Minnesota, 1997. CCDR 1999;25(2):13-5.

  13. Outbreak of gastroenteritis associated with an interactive water fountain at a beachside park - Florida, 1999. MMWR 2000;49(25):565-8.

  14. Hoebe CJ, Vennema H, Roda Husman AM et al. Norovirus outbreak among primary schoolchildren who had played in a recreational water fountain. J Infect Dis 2004;189(4):699-705.

  15. Fleming CA, Caron D, Gunn JE et al. An outbreak of associated with a recreational spray fountain. Am J Public Health 2000;90(10):1641-2.

Source: M Gilbert, MD, Canadian Field Epidemiology Program and BCCDC Epidemiology Services, Vancouver; L Srour, MD, Interior Health Authority, Kamloops; A Paccagnella, BSc, BCCDC Laboratory Services Enteric Section, Vancouver; L MacDougall, MSc, BCCDC Epidemiology Services; J Fung, MSc, MPH, BCCDC Environmental Microbiology Laboratory, Vancouver; E Nelson, Interior Health Authority, Kelowna; M Fyfe, MD, Vancouver Island Health Authority, Victoria.


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