PU650 Unit 7 Hypothesis Generation Paper Example

Eleven of Twelve Had Been to the Splash Pad: A Unit 7 Hypothesis Generation Paper Naming a Source for a Composite Cryptosporidium Outbreak Only After the Counting, and Saying How the Claim Could Fail

[Student Name]

Purdue University Global

PU650: Infectious Disease Epidemiology

Unit 7 Hypothesis Generation Paper

[Instructor Name]

[Date]

Dunmore County, Harlan Park and the interview results are composite; the parasite's biology, the water treatment facts and the research cited are real.

What the Counting Established

Three findings came before any source was named. The Unit 3 definition, applied consistently, counted 44 cases by July 15, with onsets from June 27. The Unit 4 curve spread those onsets over eighteen days, much wider than a single incubation period, which pointed to an exposure still open rather than one event. And the Unit 5 camp cohort showed children who went on the Harlan Park outing falling ill at 4.8 times the rate of those who stayed behind, with the rate rising for those who played in the splash pad and higher again for those who swallowed its water.

The Hypothesis

Illness in the Dunmore County outbreak was associated with exposure to the Harlan Park splash pad between June 24 and July 15, and the association was stronger among people who reported getting water in their mouths. The sentence names an exposure, a period and a dose element. It could be wrong in three specific ways: community cases might show no excess of splash pad visits, the association might disappear once age is accounted for, or the dose gradient might be absent outside the camp. A hypothesis is useful only if the next study could show it false, so this one names the exposure, the dates and the gradient a test would look for.

Evidence, Strongest First

Camp gradient. Within one cohort with a full roster, the attack rate rose from 29.9 percent among outing children to 34.1 percent among splash pad players and 46.9 percent among those who swallowed water. A gradient inside a defined group is the strongest evidence available so far, because the comparison groups shared everything except the exposure. What it cannot show: that the same source explains cases outside the camp. Community interviews. Twelve community cases with no camp connection were interviewed with a standard questionnaire covering two weeks before onset. Eleven had visited the Harlan Park splash pad. A county parks survey from the previous summer estimated that about 18 percent of children in the county had used the pad in a given two-week period. If the twelve cases were no more likely than other children to have visited, the chance of eleven or more visits among twelve is the binomial probability with a visit probability of 0.18, which the script puts at about 6 in 100 million, below one in ten million. What it cannot show: the parks survey is a year old and measured a different summer, and cases may recall a memorable visit more readily than controls would. Laboratory typing. All six specimens typed so far were Cryptosporidium hominis. This species spreads almost entirely from person to person, while the related C. parvum also infects cattle and other animals, so the typing points toward a source contaminated by human feces, such as recreational water, rather than an animal one (Xiao, 2010). What it cannot show: typing narrows the kind of source, not the place. Timing. Counting back one incubation period from the peak onset days lands on the holiday weekend, when the pad recorded its highest attendance of the summer. What it cannot show: holidays raise attendance everywhere people gather.

Alternatives, Weighed and Set Aside

Drinking water. Dunmore County's municipal system serves about 70 percent of residents and draws from a reservoir with filtration. A drinking water source would spread cases across the service area and across ages, including adults with no reason to visit a children's water feature. Cases cluster among children and among adults linked to them, the water utility reported normal turbidity throughout June and July, and two cases live on private wells. Set aside, with a request for the utility's daily filter records to close it formally. Animal contact. The county fair opened a petting barn on June 26. Calves are a common source of C. parvum. Only two of the twelve community cases reported visiting the fair, and all typed specimens were C. hominis. Set aside. Child care centers. Child care is a well-recognized setting for person-to-person spread of this parasite, and 57 of the 444 outbreaks reported nationally from 2009 to 2017 were traced to contact with infected people there (Gharpure et al., 2019). Four cases attend three different centers, and none of the centers has more than one case. Child care is likely to amplify the outbreak in the weeks ahead but does not explain its start. Held as a secondary pathway rather than a source.

Mechanism

The biological mechanism comes late on purpose, after the epidemiology, because a plausible mechanism can make a weak association look strong. Cryptosporidium oocysts are highly tolerant of chlorine. In laboratory work at pool-relevant pH, a three-log reduction in viability needed a chlorine concentration multiplied by time of 10,400 to 15,300 milligram-minutes per liter (Shields et al., 2008). At a typical free chlorine level of 1 to 3 milligrams per liter, that is several days of contact, during which a splash pad recirculates the same water past hundreds of children. A single diarrheal accident can seed the water, and a pad's shallow sprays put that water into mouths. The Harlan Park pad uses chlorine alone, with no ultraviolet or ozone treatment as a second barrier, which the national model code recommends for interactive water features precisely because chlorine does not inactivate this parasite quickly (Centers for Disease Control and Prevention [CDC], 2018).

Immediate Action Before the Test

The hypothesis is strong enough to justify closing the pad for remediation now, while it is tested, because the cost of waiting is more cases and the cost of closing a splash pad for two weeks is small. The paper recommends closure, hyperchlorination and a record review, and says plainly that closure does not prove the hypothesis.

How the Hypothesis Will Be Tested

1. A case-control study of community cases and age-matched controls from the same neighborhoods, chosen by random digit dialing or from clinic rosters, asking about the splash pad and the three alternatives with identical questions.

2. Odds ratios for any pad visit and for getting water in the mouth, with confidence intervals and adjustment for age.

3. Water and filter samples from the pad, recognizing that oocysts may no longer be detectable after the source event.

4. Typing of further specimens to see whether one subtype dominates.

5. A review of the pad's operating records: chlorine logs, filter backwash dates and any fecal incidents reported by staff.

The hypothesis will be considered supported if the community odds ratio is clearly above one and rises with water swallowed, and it will be revised if it is not.

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

Xiao, L. (2010). Molecular epidemiology of cryptosporidiosis: An update. Experimental Parasitology, 124(1), 80-89. https://doi.org/10.1016/j.exppara.2009.03.018