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Centers for Disease Control and PreventionCase Studies in Applied EpidemiologyNo. 000-805No Fair Warning:An Outbreak Following the1999 Washington County FairLearning ObjectivesAfter completing this case study, the participant should be able to: List the reasons to investigate an outbreak; List and discuss each step in the investigation of an outbreak; Describe the features of an appropriate case definition; and Construct an epidemic curve.Participant’s GuideThis case study is based on the investigation of an outbreak following the 1999 Washington County(New York) Fair. The investigation was conducted by the EIS Officers Joel Ackelsberg, Sonja Olsen,and Sumathi Sivapalasingam, staff from the New York State Department of Health, several countyhealth departments, and the Centers for Disease Control and Prevention.This case study was developed in 2000 by Richard C. Dicker for Tufts University and was last updatedin 2005 with input from the 2004 EIS Summer Course instructors.U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICESPublic Health Service

CDC EIS: No Fair Warning (000-805) — Participant’s Guide Page 2PART IOn September 3, 1999, the New York StateDepartment of Health (NYSDOH) receivedreports of at least 7 children hospitalized withbloody diarrhea in counties near Albany, NewYork. At least one of the children had developedhemolytic uremic syndrome (HUS), a lifethreateningcomplication, characterized bydestruction of blood cells, damage to the lining ofblood vessels, and in severe cases, kidneyfailure.Question 1:If you took the call at the NYSDOH, what questions would you ask?Question 2:Is this an outbreak?

CDC EIS: No Fair Warning (000-805) — Participant’s Guide Page 5Before departing from Albany and Atlanta, theinvestigators had to make many decisions andpreparations, which could be grouped under theheadings of Epidemiologic / Scientific Issues;Supplies and Equipment; Investigative TeamComposition, Roles, Responsibilities; andAdministrative Issues.Question 5: Before departing, what preparations and decisions should be made regardinga. the epidemiologic / scientific aspects of the investigation?b. supplies and equipment?c. investigative team composition, role, responsibilities?d. administrative issues?

CDC EIS: No Fair Warning (000-805) — Participant’s Guide Page 6Over the next several days, more and morecases of diarrhea and bloody diarrhea werereported. While the earliest cases were inchildren, cases occurred among all age groups.The case-patients did not appear to have anyrestaurant, food, or home or work water supply incommon. However, almost all casesacknowledged attending the Washington CountyFair. The investigators, therefore, feltcomfortable focusing on the Washington CountyFair as the source of the outbreak.Question 6a: What is a case definition?Question 6b: Develop a case definition for this outbreak.Question 7:How might you look for additional cases?

CDC EIS: No Fair Warning (000-805) — Participant’s Guide Page 7PART IIStool samples provided by many ill personsyielded E. coli O157:H7. Investigators decided touse the following case definitions:Confirmed case: self-described diarrhea in aperson occurring less than 10 days after havingattended the Washington County Fair, andisolation from stool of E. coli O157:H7.Suspected case: self-described diarrhea in aperson after having attended the WashingtonCounty Fair, without isolation from stool of E. coliO157:H7.Within a couple of weeks, 921 persons had beenidentified with reported diarrhea after attendingthe Washington County Fair. Stool cultures from129 persons yielded E. coli O157:H7.The investigators decided that the next stepshould be to perform descriptive epidemiology,that is, characterize the cases by person, time,and place.The median age of cases was 28 years; 58%were female. Among the cases, 65 had beenhospitalized. Eleven children had developedhemolytic-uremic syndrome (HUS). Two personsdied — a 3-year-old girl with HUS and a 79-yearoldman with HUS and thromboticthrombocytopenic purpura.For the 761 of the 921 cases with documenteddates of onset, those dates are shown in thetable below.To characterize the outbreak by time,investigators decided to construct an epidemiccurve.Table 1. Cases of diarrheal illness by dates of onset, Washington County Fair, NY, 1999Onset # confirmed and suspect casesDate Total Confirmed SuspectAugust20 0 0 021 0 0 022 0 0 023 0 0 024 2 0 225 4 0 426 8 0 827 13 0 1328 13 3 1029 41 6 3530 84 11 7331 100 24 76September1 118 42 762 88 30 583 68 15 53Onset # confirmed and suspect casesDate Total Confirmed SuspectSeptember4 61 9 525 44 8 366 36 2 347 26 0 268 19 3 169 8 0 810 10 1 911 3 1 212 8 2 613 4 2 214 2 1 115 0 0 016 0 0 017 1 1 018 0 0 019 0 0 0Total 761 161 600

CDC EIS: No Fair Warning (000-805) — Participant’s Guide Page 8Question 8:What is an epidemic curve?Question 9:Using the graph paper provided, graph the cases by time of onset of illness (includeappropriate labels and title). Mark the dates of the fair on the epidemic curve.Question 10:Based on the epidemic curve and what you know about the incubation period, estimatewhen peak exposure likely occurred. What can you conclude from the epi curve andthe likely exposure period?Question 11:What is the value of an epidemic curve?

CDC EIS: No Fair Warning (000-805) — Participant’s Guide Page 9

CDC EIS: No Fair Warning (000-805) — Participant’s Guide Page 10The Washington County Fair is the major annualevent for this rural area east of the AdirondackMountains and near the Vermont border. It isNew York’s largest agricultural fair, wherehundreds of farm animals and poultry areexhibited. More than 108,000 total visits wererecorded during that week.Question 12:What hypotheses would you want to evaluate?Investigators learned that since the fairgroundsoperated only 42 days each year, its watersupply was not considered a public water systemand therefore was not regulated as such byeither federal or state law. The fairgrounds weresupplied by shallow wells approximately 20 feetdeep.Environmental investigation of the fairgroundsrevealed that much of the fair was supplied bychlorinated water. However, in one area of thefairgrounds, a shallow well providedunchlorinated water to several vendors who usedthe water to make beverages and ice.Question 13:What epidemiologic approach might you use to evaluate your hypotheses?

CDC EIS: No Fair Warning (000-805) — Participant’s Guide Page 11Part IIIInvestigators decided to conduct a case-controlstudy. The case group consisted of residents ofWashington County who developed diarrheaafter attending the fair and who had stoolcultures positive for E. coli.Question 14:Whom might you get for controls?Controls were residents of Washington Countyrandomly selected from the telephone book who,upon telephone interview, reported that they hadattended the fair. They were selected to have asimilar age distribution as the cases.The study included 32 cases and 84 controls. Allof the cases had attended the fair during its lastfour days, including those who had attended thefair only once.Based on analysis of information collected fromthe cases and controls about a wide range ofexposures at the fair, the investigators were ableto focus on two exposures, as depicted in Tables2 and 3. The data in these tables are limited tothe 32 cases and 57 controls who attended thefair at least once during its final four days.Table 2. Exposure to water from Well #6, Washington County Fair outbreak, 1999Exposure toWell #6 waterCases Controls TotalYes 26 9 35No 6 48 54Total 32 57 89Table 3. Exposure to chicken from Vendor A, Washington County Fair outbreak, 1999Exposure toVendor A chickenCases Controls TotalYes 14 5 19No 18 52 70Total 32 57 89

CDC EIS: No Fair Warning (000-805) — Participant’s Guide Page 12Question 15:What is the appropriate measure of association for a case-control study? Calculate thatmeasure for the data in Tables 2 and 3.On multivariate analysis, only one factorremained statistically significantly associatedwith infection — drinking beverages purchasedfrom vendors supplied by water from Well #6.Question 16:If your supervisor wanted you to conduct a retrospective cohort study, how might youdo it? What is the appropriate measure of association?

CDC EIS: No Fair Warning (000-805) — Participant’s Guide Page 13As part of the environmental investigation,samples of water were tested from Well #6, fromthe distribution pipe to the vendor area, and fromthe outlet pipe at the vendor area. Although E.coli is infrequently isolated from water supplies,all three sites yielded E. coli O157:H7.Pulse-field gel electrophoresis was done on thehuman and environmental isolates of O157. TheWell # 6 E. coli pattern was indistinguishablefrom the patients’ pattern.Investigators looked for sources that might havecontaminated the well. They identified two areasof concern — a nearby cattle barn with a manurestorage area, and a dormitory which housedapproximately 80 people who cared for farmanimals during the fair. The dorm had bathroomfacilities and shower drains that connected to aseptic tank and then to a seepage pit. Theseepage pit was only 36 feet from Well #6.A thunderstorm had drenched the fairgrounds onthe afternoon of Thursday, August 26.Three weeks after the fair ended, red fluorescentdye was flushed down a hole in the manurestorage area but was never detected in waterpumped from Well #6. Green fluorescent dyewas flushed down a toilet in the dormitory.Within hours, green dye was detected in theseptic tank, the seepage pit, and Well #6 water.Investigators speculated that, given the muddyconditions following the heavy rains onThursday, cattle manure on boots or clothing ofthe workers who slept in the dormitory couldhave washed down the floor shower or sinkdrains and entered Well #6. However, theinvestigators could not rule out Well 6 wascontaminated directly by the nearby manurestorage site — while the fluorescent dye test didnot demonstrate a connection during dryweather, the groundwater table may have beenhigher and more easily contaminated followingthe heavy rains.Question 17:What control / prevention measures might you suggest?Question 18:Who needs to know? Why? How might you communicate these findings?

CDC EIS: No Fair Warning (000-805) — Participant’s Guide Page 14ConclusionThis was one of the largest outbreaks of E. coliO157:H7 in U.S. history. Based onextrapolations from a household survey ofWashington County residents, perhaps up to5,000 fair attendees may have become ill.The source was an unchlorinated shallow wellthat was not regulated as a public water supply.To prevent similar outbreaks from occurring inthe future, New York State issued summaryorders to prohibit the use of untreated water atcertain agricultural fairs and to require dailytesting and disinfection of water supplies duringpublic events. However, infrequently used watersupplies continue to be unregulated in manyother states.ReferencesWashington County Fair OutbreakCDC. Outbreak of Escherichia coli O157:H7 and Campylobacter among attendees of the WashingtonCounty Fair — New York, 1999. MMWR 1999;48(36):803-804.Ackelsberg J, Wallace B, Limberger R, et al. A large waterborne outbreak of Escherichia coli O157:H7and Campylobacter jejuni gastroenteritis at a county fair. Submitted.Outbreak Investigation, GeneralCDC. Principles of Epidemiology, 2 nd edition. Atlanta: Centers for Disease Control and Prevention,1992.Gregg MB. Conducting a Field Investigation. In: Gregg MB, ed., Field Epidemiology, 2 nd edition. NewYork: Oxford University Press, 2002.E. coliWeb site: http://www.cfsan.fda.gov/~mow/intro.htmlWeb site: http://www.cdc.gov/ncidod/dbmd/diseaseinfo/escherichiacoli_g.htm