Centers for Disease Control and PreventionEpidemiology Program Office
Case Studies in Applied EpidemiologyNo. 401-303
Oswego – An Outbreakof Gastrointestinal Illness
Following a Church SupperStudent's Guide
Learning ObjectivesAfter completing this case study, the participant should be able to:
G Define the terms “cluster,” “outbreak,” and “epidemic;”
G List the steps in the investigation of an outbreak;
G Draw, interpret, and describe the value of an epidemic curve;
G Calculate and compare food-specific attack rates to identify possible vehicles;
G List reasons for investigating an outbreak that has apparently ended.
This case study is based on an investigation conducted by the New York State Department ofPublic Health Division. The case study was developed by Wendell Ames, MD, StaffordWheeler, MD, and Alexander Langmuir, MD in the early 1940s. It has been substantiallyupdated and edited since then by Philip Brachman, Michael Gregg, and Richard Dicker, withinput from the many instructors who have reviewed and taught “Oswego” as part of the EISSummer Course each year.
U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICESPublic Health Service
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PART I – BackgroundOn April 19, 1940, the local health officer in thevillage of Lycoming, Oswego County, New York,reported the occurrence of an outbreak of acutegastrointestinal illness to the District HealthOfficer in Syracuse. Dr. A. M. Rubin,epidemiologist-in-training, was assigned toconduct an investigation.
When Dr. Rubin arrived in the field, he learnedfrom the health officer that all persons known tobe ill had attended a church supper held on the
previous evening, April 18. Family memberswho did not attend the church supper did notbecome ill. Accordingly, Dr. Rubin focused theinvestigation on the supper. He completedInterviews with 75 of the 80 persons known tohave attended, collecting information about theoccurrence and time of onset of symptoms, andfoods consumed. Of the 75 personsinterviewed, 46 persons reportedgastrointestinal illness.
Question 1: Would you call this an epidemic? Would you call it an outbreak?
Question 2: Review the steps of an outbreak investigation.
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Clinical Description
The onset of illness in all cases was acute,characterized chiefly by nausea, vomiting,diarrhea, and abdominal pain. None of the illpersons reported having an elevated
temperature; all recovered within 24 to 30 hours.Approximately 20% of the ill persons visitedphysicians. No fecal specimens were obtainedfor bacteriologic examination.
Question 3: List the broad categories of diseases that must be considered in the differentialdiagnosis of an outbreak of gastrointestinal illness.
The investigators suspected that this was a vehicle-borne outbreak, with food as the vehicle.
Question 4: In epidemiologic parlance, what is a vehicle? What is a vector? What are other modesof transmission?
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Question 5: If you were to administer a questionnaire to the church supper participants, whatinformation would you collect? Group the information into categories.
Dr. Rubin put his data into a line listing.
Question 6: What is a line listing? What is the value of a line listing?
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PART IIDescription of the Supper
The supper was held in the basement of thevillage church. Foods were contributed bynumerous members of the congregation. Thesupper began at 6:00 p.m. and continued until11:00 p.m. Food was spread out on a table andconsumed over a period of several hours.
Data regarding onset of illness and food eatenor water drunk by each of the 75 personsinterviewed are provided in the attached linelisting. The approximate time of eating supperwas collected for only about half the personswho had gastrointestinal illness.
Question 7: What is the value of an epidemic curve?
Question 8: Using the graph paper provided, graph the cases by time of onset of illness (includeappropriate labels and title). What does this graph tell you?
Question 9: Are there any cases for which the times of onset are inconsistent with the generalexperience? How might they be explained?
Question 10: How could the data in the line listing be better presented?
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Line listing from investigation of outbreak of gastroenteritis,Oswego, New York, 1940
TIME DATE OF TIME OFID AGE SEX OF MEAL ILL ONSET ONSET 1 11 M unk N N N N N N N N N N N N N Y N2 52 F 8:00 PM Y 4/19 12:30 AM Y Y Y N N Y N N Y N N Y N N3 65 M 6:30 PM Y 4/19 12:30 AM Y Y Y Y N N N N Y N N Y Y N4 59 F 6:30 PM Y 4/19 12:30 AM Y Y N N N N N N Y N Y Y Y N5 13 F unk N N N N N N N N N N N N N Y N6 63 F 7:30 PM Y 4/18 10:30 PM Y Y N Y Y N N N N Y N Y N N7 70 M 7:30 PM Y 4/18 10:30 PM Y Y Y N Y Y Y N Y Y N Y N N8 40 F 7:30 PM Y 4/19 2:00 AM N N N N N N N N N N N Y Y N9 15 F 10:00 PM Y 4/19 1:00 AM N N N N N N N N N N Y N Y N
10 33 F 7:00 PM Y 4/18 11:00 PM Y Y Y N N Y Y N N Y N Y Y N
11 65 M unk N Y Y Y N Y Y N N N N N Y N N12 38 F unk N Y Y Y N N Y N N Y N N Y Y Y13 62 F unk N Y Y N Y Y Y Y N N Y N N N N14 10 M 7:30 PM Y 4/19 2:00 AM N N N N N N N N N N N Y Y N15 25 M unk N Y Y Y Y Y Y Y Y Y Y Y Y N N16 32 F unk Y 4/19 10:30 AM Y Y N N N Y N N Y N Y Y Y N17 62 F unk Y 4/19 12:30 AM N N N N N N N N N N N Y N N18 36 M unk Y 4/18 10:15 PM Y Y N Y N Y Y N N N N Y N N19 11 M unk N Y Y ? Y N Y N N N Y N N Y N20 33 F unk Y 4/18 10:00 PM Y Y Y Y Y Y N N Y Y Y Y Y N
21 13 F 10:00 PM Y 4/19 1:00 AM N N N N N N N N N N Y Y N N22 7 M unk Y 4/18 11:00 PM Y Y Y Y Y Y Y N N Y Y Y Y N23 64 M unk N N N N N N N N N N N N Y N N24 3 M unk Y 4/18 9:45 PM N Y Y N N Y N N N Y Y Y N N25 65 F unk N Y Y Y Y Y N Y N Y N Y Y Y N26 59 F unk Y 4/18 9:45 PM N Y Y Y N Y Y N N Y Y Y N N27 15 F 10:00 PM Y 4/19 1:00 AM N N N N N N N N N N Y Y Y N28 62 M unk N Y Y N Y N Y Y N Y Y Y N Y N29 37 F unk Y 4/18 11:00 PM Y Y Y N Y Y Y N Y N Y Y N N30 17 M 10:00 PM N N N N N N N N N N N Y Y Y N
31 35 M unk Y 4/18 9:00 PM Y Y Y N Y Y Y N Y N Y Y N Y32 15 M 10:00 PM Y 4/19 1:00 AM N N N N N N N N N N Y Y N N33 50 F 10:00 PM Y 4/19 1:00 AM N N N N N N N N N N N Y N N34 40 M unk N Y Y N N N Y Y N Y Y Y N Y Y35 35 F unk N Y Y Y N N Y Y N Y Y N N Y N36 35 F unk Y 4/18 9:15 PM Y Y Y Y N Y Y N Y N N Y N N37 36 M unk N Y N Y Y N Y Y N Y N N N Y N38 57 F unk Y 4/18 11:30 PM Y Y N Y Y Y Y N Y N Y Y Y N39 16 F 10:00 PM Y 4/19 1:00 AM N N N N N N N N N N Y N Y N40 68 M unk Y 4/18 9:30 PM Y N Y Y N N Y N Y N N Y N N
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Line listing from investigation of outbreak of gastroenteritis,Oswego, New York, 1940
TIME DATE OF TIME OFID AGE SEX OF MEAL ILL ONSET ONSET 41 54 F unk N Y Y Y N N Y N N Y N Y N Y N42 77 M unk Y 4/19 2:30 AM N N N N N N N N N N N Y N Y43 72 F unk Y 4/19 2:00 AM Y Y N Y Y N Y N Y N Y Y Y N44 58 M unk Y 4/18 9:30 PM Y Y Y N N N Y Y Y N N Y ? Y45 20 M 10:00 PM N N N N N N N N N N N Y Y Y N46 17 M unk N Y Y Y N N Y N N N Y N Y Y N47 62 F unk Y 4/19 12:30 AM Y Y N N N Y N N N Y N Y N N48 20 F 7:00 PM Y 4/19 1:00 AM N N N N N N N N N N N Y N N49 52 F unk Y 4/18 10:30 PM Y Y Y Y N Y N N Y N N Y Y N50 9 F unk N N N N N N N N N N N Y N Y N
51 50 M unk N Y Y Y Y Y Y Y Y Y Y Y N Y N52 8 M 11:00 AM Y 4/18 3:00 PM N N N N N N N N N N N Y Y N53 35 F unk N N N N N N N N N N N N Y Y N54 48 F unk Y 4/19 12:00 AM* Y Y Y Y Y Y Y Y Y N Y Y Y N55 25 M unk Y 4/18 11:00 PM Y N Y N N Y Y N N Y Y Y Y N56 11 F unk N N N N N N N N N N N N N Y N57 74 M unk Y 4/18 10:30 PM Y Y Y Y Y Y Y N Y N Y Y N N58 12 F 10:00 PM Y 4/19 1:00 AM N N N N N N N N N N Y Y Y N59 44 F 7:30 PM Y 4/19 2:30 AM Y Y Y N N Y N N N Y Y N Y N60 53 F 7:30 PM Y 4/18 11:30 PM Y Y Y Y Y N Y N Y Y Y Y Y N
61 37 M unk N N N N N N N N N N N N N Y N62 24 F unk N Y Y Y N N Y N N Y N N N N N63 69 F unk N N Y Y N Y N Y N N Y Y N Y N64 7 M unk N Y Y Y Y Y Y N N N Y Y N Y N65 17 F 10:00 PM Y 4/19 1:00 AM N N N N N N N N N N Y Y Y N66 8 F unk Y 4/19 12:30 AM Y N Y Y Y N N N N N Y Y Y N67 11 F 7:30 PM N Y Y Y Y N Y N N Y Y N N Y N68 17 M 7:30 PM N Y Y Y Y N Y N N Y N Y Y N N69 36 F unk N N N N N N N N N N N N N Y N70 21 F unk Y 4/19 12:30 AM Y N N Y Y N N N N N N Y Y N
71 60 M 7:30 PM Y 4/19 1:00 AM N N N N N N N N N N Y Y N N72 18 F 7:30 PM Y 4/19 12:00 AM* Y Y Y Y Y N N N N Y Y Y Y N73 14 F 10:00 PM N N N N N N N N N N N Y Y N N74 52 M unk Y 4/19 2:15 AM Y N Y N Y Y Y N Y Y Y Y Y N75 45 F unk Y 4/18 11:00 PM Y Y Y Y Y Y Y N Y N Y Y N Y
* Midnight between 4/18 and 4/19
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PART IIIAttached is the line listing sorted by illness status (ill or well), and by time of onset.
Question 11: Where possible, using the new line listing, calculate incubation periods and illustratetheir distribution with an appropriate graph.
Question 12: Determine the range and median of the incubation period.
Question 13: How does the information on incubation period, combined with the data on clinicalsymptoms, help in the differential diagnosis of the illness? (If necessary, refer toattached Compendium of Acute Foodborne Gastrointestinal Disease).
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Question 14: Using the data in the attached line listing, complete the table below. Which food is themost likely vehicle of infection?
Food Items Served Number of persons who ATEspecified food
Number of persons did NOT eatspecified food
Attack RateRatioIll Not Ill Total
Percent Ill(Attack rate) Ill Not Ill Total
Percent Ill(Attack rate)
Baked ham
Spinach
Mashed potato
Cabbage salad
Jello
Rolls
Brown bread
Milk
Coffee
Water
Cakes
Ice cream, vanilla
Ice cream, chocolate
Fruit salad
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Question 15: Outline further investigations that should be pursued.
Question 16: What control measures would you suggest?
Question 17: Why was it important to work up this outbreak?
Question 18: Refer to the steps of an outbreak investigation you listed in Question 2. How does thisinvestigation fit that outline?
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PART IV – CONCLUSION
The following is quoted verbatim from the reportprepared by Dr. Rubin:
“The ice cream was prepared by the Petriesisters as follows:
“On the afternoon of April 17 raw milk from thePetrie farm at Lycoming was brought to boil overa water bath, sugar and eggs were then addedand a little flour to add body to the mix. Thechocolate and vanilla ice cream were preparedseparately. Hershey's chocolate was necessarilyadded to the chocolate mix. At 6 p.m. the twomixes were taken in covered containers to thechurch basement and allowed to stand overnight. They were presumably not touched by anyoneduring this period.
“On the morning of April 18, Mr. Coe added fiveounces of vanilla and two cans of condensed milkto the vanilla mix, and three ounces of vanilla andone can of condensed milk to the chocolate mix. Then the vanilla ice cream was transferred to afreezing can and placed in an electrical freezerfor 20 minutes, after which the vanilla ice creamwas removed from the freezer can and packedinto another can which had been previouslywashed with boiling water. Then the chocolatemix was put into the freezer can which had beenrinsed out with tap water and allowed to freezefor 20 minutes. At the conclusion of this bothcans were covered and placed in large woodenreceptacles which were packed with ice. Asnoted, the chocolate ice cream remained in theone freezer can.
“All handlers of the ice cream were examined. No external lesions or upper respiratoryinfections were noted. Nose and throat cultureswere taken from two individuals who preparedthe ice cream.
“Bacteriological examinations were made by theDivision of Laboratories and Research, Albany,on both ice creams. Their report is as follows: 'Large numbers of Staphylococcus aureus andalbus were found in the specimen of vanilla icecream. Only a few staphylococci weredemonstrated in the chocolate ice cream.'
“Report of the nose and throat cultures of thePetries who prepared the ice cream read as
follows: 'Staphylococcus aureus and hemolyticstreptococci were isolated from nose culture andStaphylococcus albus from throat culture ofGrace Petrie. Staphylococcus albus wasisolated from the nose culture of Marian Petrie. The hemolytic streptococci were not of the typeusually associated with infections in man.'
“Discussion as to Source: The source ofbacterial contamination of the vanilla ice creamis not clear. Whatever the method of theintroduction of the staphylococci, it appearsreasonable to assume it must have occurredbetween the evening of April 17 and the morningof April 18. No reason for contaminationpeculiar to the vanilla ice cream is known.
“In dispensing the ice creams, the same scooperwas used. It is therefore not unlikely to assumethat some contamination to the chocolate icecream occurred in this way. This would appearto be the most plausible explanation for theillness in the three individuals who did not eatthe vanilla ice cream.
“Control Measures: On May 19, all remainingice cream was condemned. All other food at thechurch supper had been consumed.
“Conclusions: An attack of gastroenteritisoccurred following a church supper at Lycoming. The cause of the outbreak was contaminatedvanilla ice cream. The method of contaminationof ice cream is not clearly understood. Whetherthe positive Staphylococcus nose and throatcultures occurring in the Petrie family hadanything to do with the contamination is a matterof conjecture.”
Note: Patient #52 was a child who whilewatching the freezing procedure was given adish of vanilla ice cream at 11:00 a.m. on April18.
Addendum:Certain laboratory techniques not available atthe time of this investigation might prove veryuseful in the analysis of a similar epidemictoday. These are phage typing, which can bedone at CDC, and identification ofstaphylococcal enterotoxin in food byimmuno-diffusion or by enzyme-linked
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immunosorbent assay (ELISA), which is availablethrough the Food and Drug Administration (FDA).
One would expect the phage types ofstaphylococci isolated from Grace Petrie's noseand the vanilla ice cream and vomitus or stoolsamples from ill persons associated with thechurch supper to be identical had she been thesource of contamination. Distinctly differentphage types would mitigate against her as thesource (although differences might be observedas a chance phenomenon of sampling error) andsuggest the need for further investigation, such
as cultures of others who might have been incontact with the ice cream in preparation orconsideration of the possibility thatcontamination occurred from using a cow withmastitis and that the only milk boiled was thatused to prepare chocolate ice cream. If thecontaminated food had been heated sufficientlyto destroy staphylococcal organisms but nottoxin, analysis for toxin (with the addition ofurea) would still permit detection of the cause ofthe epidemic. A Gram stain might also detectthe presence of nonviable staphylococci incontaminated food.
ReferenceGross MB. Oswego County revisited. Public Health Reports 1976;91:160-70.
