PU650 Unit 10 Outbreak Investigation Report Example

Eighty-Three Cases, One Splash Pad, Six Weeks: A Unit 10 Outbreak Investigation Report to a Composite County Board of Health, Told in the Order the Investigation Happened

[Student Name]

Purdue University Global

PU650: Infectious Disease Epidemiology

Unit 10 Outbreak Investigation Report

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[Date]

Dunmore County, its board, its cases and its records are composite; the methods, the water treatment standards and the research cited are real.

Summary

Between June 27 and August 9, 83 people in Dunmore County became ill with cryptosporidiosis: 73 primary and 10 secondary cases, 31 confirmed by laboratory testing, with a median age of six. Illness was strongly associated with the Harlan Park splash pad. Among children at a county day camp, the risk of illness was 4.8 times higher for children who went on a park outing than for those who did not, with a gradient by water swallowed. In a community case-control study, cases had 24.6 times the odds of controls of having visited the pad. Every typed specimen was Cryptosporidium hominis. The pad's chlorine levels were in the required range, but it had no secondary disinfection and its filter maintenance had slipped. The pad was closed on July 16, hyperchlorinated and fitted with ultraviolet treatment before reopening on August 14. The report recommends secondary disinfection for every interactive water feature in the county.

Background

Dunmore County, population about 210,000, typically records 12 to 20 cryptosporidiosis reports a year. Fourteen reports arrived between July 1 and July 12. Nationally, treated recreational water venues such as pools and water playgrounds account for more cryptosporidiosis outbreak cases than any other exposure, and reported outbreaks rose by about 13 percent a year from 2009 to 2017 (Gharpure et al., 2019).

Methods

Case finding drew on laboratory reports, a health alert to clinicians on July 9, the day camp roster and interviews with the households of cases. The working case definition, adopted July 10, required three or more days of diarrhea with onset from June 25 and residence in or a visit to the county; confirmed cases had Cryptosporidium detected by microscopy, fluorescent antibody or molecular testing, and probable cases had an antigen-only result or an epidemiologic link. Once the pad was suspected, a narrower definition for the case-control study required onset from June 26 to July 20. Secondary cases were household members with onset more than three days after a primary case and no exposure at the pad. The camp cohort was surveyed by phone. For the case-control study, 40 community cases were matched by age group to 80 controls drawn from the same clinics' patient lists. Environmental assessment covered the pad's operating records, a site inspection and water and filter sampling. Methods lists the definition's revision openly, because a definition that narrowed after the source was suspected is a choice the board should see rather than discover.

Results: Descriptive

Onsets ran from June 27 to August 9, with the main body between July 6 and July 18 and a tail of secondary cases into August. Of 83 cases, 61 were children under 13. Fifty-two were children at the day camp; 21 were community cases with no camp link; 10 were secondary. Seven people were hospitalized, all for dehydration, and none died. The epidemic curve's width, eighteen days of primary onsets against an incubation range about eight days wide, indicated a continuing source rather than a single exposure.

Results: Analytic

Camp cohort: 49 of 164 children on the park outing became ill, 29.9 percent, against 3 of 48 who stayed behind, 6.2 percent, a risk ratio of 4.8, whose 95 percent interval ran from 1.6 to 14.7. Among outing children, the attack rate was higher again for children who played in the spray, 34.1 percent, and highest, 46.9 percent, for the subset who swallowed water. Case-control study: 34 of 40 cases and 15 of 80 controls had visited the pad in the two weeks before onset or the matched reference date, an odds ratio of 24.6 (95 percent confidence interval 8.7 to 69.0). Among visitors, getting water in the mouth carried an odds ratio of 4.8, with an interval of 1.3 to 17.7. No other exposure, including drinking water, the county fair or child care, was associated with illness.

Environmental and Laboratory Findings

The pad recirculates about 9,000 gallons through a sand filter and chlorine feeder. Daily free chlorine logs showed levels within the required range on every day inspected. There was no ultraviolet or ozone treatment. Filter backwashing, scheduled every three days, occurred only twice between June 28 and July 15. No fecal incident was logged by staff, although the pad had no posted procedure for reporting one. Water samples taken after closure did not detect oocysts, which is common after the source event has passed. Fourteen specimens were typed, and all were C. hominis, the species that spreads among people rather than from animals. Oocysts need a chlorine concentration times contact time of at least 10,400 milligram-minutes per liter for a thousand-fold reduction in viability, so chlorine within the required range would not have inactivated them quickly enough to protect children in the spray (Shields et al., 2008).

Discussion

The evidence points to the Harlan Park splash pad as the source: a strong association in two independent study designs, a gradient by water swallowed, a human-specific species and a venue without a second barrier against a chlorine-tolerant parasite. The most likely sequence is a diarrheal contamination event in late June that went unreported, followed by weeks of recirculation through a filter that was not being backwashed on schedule. Limitations include underascertainment, since many people with diarrhea are never tested; recall of swallowing water, which ill children's parents may report more readily; and controls drawn from clinic lists, which may differ from the general population in how often they visit parks.

Control Measures Taken

1. Pad closed July 16.

2. Hyperchlorination at 20 parts per million for 12.75 hours, followed by filter backwash and media replacement.

3. Communication package released July 18, with updates on July 21, July 28 and August 11.

4. Exclusion guidance to child care centers and the day camp.

5. Ultraviolet secondary disinfection installed before reopening on August 14.

Recommendations, Each With an Owner

1. County parks department: install secondary disinfection on the county's two other interactive water features before next season, as the national model aquatic health code recommends for these venues (Centers for Disease Control and Prevention [CDC], 2018).

2. Parks department: written fecal incident procedures and staff training at every aquatic venue, with incidents logged and reported to the health department.

3. Parks department: filter backwash logged and checked weekly by a supervisor.

4. Health department: an annual pre-season letter to all pool and splash pad operators on Cryptosporidium response.

5. Health department: a sign at every venue asking people with diarrhea to stay out of the water, in English and Spanish.

6. Board of Health: consider an ordinance requiring secondary disinfection at all new interactive water features.

References

Centers for Disease Control and Prevention. (2018). The model aquatic health code (3rd ed.). U.S. Department of Health and Human Services.

Gharpure, R., Perez, A., Miller, A. D., Wikswo, M. E., Silver, R., & Hlavsa, M. C. (2019). Cryptosporidiosis outbreaks: United States, 2009-2017. Morbidity and Mortality Weekly Report, 68(25), 568-572. https://doi.org/10.15585/mmwr.mm6825a3

Shields, J. M., Hill, V. R., Arrowood, M. J., & Beach, M. J. (2008). Inactivation of Cryptosporidium parvum under chlorinated recreational water conditions. Journal of Water and Health, 6(4), 513-520. https://doi.org/10.2166/wh.2008.068