Yoon 1
Oswego – An Outbreak of Gastrointestinal Illness Following a Church Supper
Case Study Responses– Week 3
Esther Yoon
PART I
1. Would you call this an epidemic? Would you call this an outbreak?
This would be an outbreak. It was not spreading rapidly throughout the community, and the
case study said, “occurrence of an outbreak of acute gastrointestinal illness.”
2. Review the steps of an outbreak investigation.
1. Detect possible outbreak
2. Define and find cases
3. Generate hypotheses about likely sources
4. Test the hypothesis
5. Find the point of contamination and source of the food
6. Control the outbreak
7. Decide if the outbreak is over
3. List the broad categories of disease that must be considered in the differential diagnosis of an
outbreak of gastrointestinal illness.
It could be staphylococcus, bacillus cereus, consumption of heavy metals, clostridium
perfringens, vibrio parahemolyticus, salmonella (but no high temperature), rotavirus, e. coli
enterotoxigenic, vibrio cholera O1 or 0139, Yersinia enterocolitiea, giardia lamblia, poisonous
mushrooms, or ciguatoxin.
4. In epidemiologic parlance, what is a vehicle? What is a vector? What are other modes of
transmission?
Vehicle: a nonliving intermediary in the indirect transmission of an agent that carries the agent
from a reservoir to the susceptible host.
Vector: a living intermediary in the indirect transmission of an agent carrying the agent to a host
from a reservoir.
Other modes of transmission = direct contact, droplets in the air, through secretions or saliva,
sexual contact, fecal-oral transmission, food borne, bioweapons.
5. If you were to administer a questionnaire to the church supper participants, what information
would you collect? Group the information into categories.
Food: What food was eaten? Who prepared the food? Did anyone see anything raw? How long
did the food stay out? Were there utensils used just for main dishes?
Other people: Did anyone seem ill? Was the dinner inside or outside?
Building: Has the building been checked for radon, mold, etc.?
6. What is a line listing? What is the value of a line listing?
Line listing is when data is shown in columns. Having it laid out this way allows for an easier
interpretation of individual data.
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PART II
7. What is the value of an epidemic curve?
An epidemic curve shows the number of cases by the onset for an illness. Graphing it as a curve
provides information on the pattern of the speed, magnitude, any outliers, and the time trend of the
disease, which can be used to start making a control plan.
Reference: University of North Carolina School of Public Health. Epidemic Curves Ahead. Retrieved from
cphp.sph.unc.edu/focus/vol1/issue5/1-5EpiCurves_slides.ppt
8. Using the graph paper, graph the cases by time of onset of illness (include appropriate labels and
title). What does this graph tell you?
2:30
2:15
2:00
1:45
1:30
1:15
1:00
0:45
0:30
0:15
0:00
23:45
23:30
23:15
23:00
22:45
22:30
22:15
22:00
21:45
21:30
21:15
12
10
8
6
4
2
0
15:00
CASES
Epidemic Curve for Oswego Outbreak
18-Apr 18- 18- 18- 18- 18- 18- 18- 18- 18- 18- 18- 19- 19- 19- 19- 19- 19- 19- 19- 19- 19- 19Apr Apr Apr Apr Apr Apr Apr Apr Apr Apr Apr Apr Apr Apr Apr Apr Apr Apr Apr Apr Apr Apr
From this graph, it is clear that most people experienced symptoms late at night on the 18th and into the
early morning. There were two peaks that came up which could be intermittent exposure or the wave of
people that came into the church to eat. Most people were ill between 12:30am and 1:00am on the
19th.
9. Are there any cases for which the times of inset are inconsistent with the general experience? How
might they be explained?
There is an outlier in the graph. There is one person who started feeling ill at 3PM. However, when
looking at the line listing, he consumed both ice creams at 11am, much earlier than the rest. There was
also a small spike from 2am till 2:30am. Most of those who were sick later ate later. However, there was
one individual who ate at 7pm but did not get sick until 1am. It is difficult to make assumptions because
there were quite a few people that did not know when they ate. Then we cannot figure out the time it
took for symptoms to appear after eating.
10. How could the data in the line listing be better presented?
It was very difficult to read, especially the food consumed section. When creating the chart for
#14, it was hard and very time consuming. Also, I wished that it was sorted by either time of meal,
illness, or time and date of onset. If it was organized by one, or more, of the sections, it would have
been clearer of when the onset was. If there was a simpler way to lay out the food, that would have
been very nice.
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PART III
11. Where possible, using the new line listing, calculate incubation periods and illustrate their
distribution with an appropriate graph.
Incubation Period Distribution Graph
12
10
Cases
8
6
4
2
0
3
3.5
4
4.5
5
5.5
6
6.5
7
Hours between Exposure and Symptoms
12. Determine the range and median of the incubation period.
Incubation Period Range: 3, 7
Incubation Period Median: 4.
(3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4.5, 4.5, 5.5, 5.5, 6, 6, 6, 6.5, 7)
13. How does the information on incubation period, combined with the data on clinical symptoms,
help in the differential diagnosis of the illness? (If necessary, refer to attached Compendium of Acute
Foodborne GI Disease).
With the clinical symptoms, having the incubation periods have helped to narrow down the possible
diagnosis for this case. It could be staphylococcus aureus, even though it is missing some of the
symptoms, or bacillus cereus, which is more commonly in rice even though there was no rice at the
meal. When looking beyond the clinical symptoms and incubation periods, it seems like staphylococcus
aureus is the more likely cause of this illness. Characteristic foods are cream fillings. As you can see from
the table below, it is the vanilla ice cream that has the highest attack rate ratio. There is cream in the
making of the ice cream.
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14. Complete the table below. Which food is the most likely vehicle of infection?
The Vanilla Ice Cream!!!
# persons who ATE specified food
Food Items Served
Ill
Not Ill
Total
Baked ham
29
17
46
Spinach
Mashed potato
Cabbage salad
Jello
Rolls
Brown bread
Milk
Coffee
Water
Cakes
Ice cream, Vanilla
Ice cream,
chocolate
Fruit salad
26
23
20
16
21
18
2
19
13
27
43
17
14
8
7
17
9
2
12
11
14
11
43
37
28
23
38
27
4
31
24
41
54
% Ill
(Attack
Rate)
29/46 =
63.0%
60.5%
62.2%
71.4%
69.6%
55.3%
66.7%
50.0%
61.3%
54.2%
65.9%
79.6%
25
22
47
4
2
6
# persons did NOT eat specified
food
% Ill (Attack
Rate)
Attack
Rate
Ratio
Ill
Not Ill
Total
17
12
29
19
23
28
30
25
28
44
27
33
19
3
13
14
19
22
13
20
27
17
18
16
18
32
37
47
52
38
48
71
44
51
35
21
17/29 =
58.6%
59.3%
62.2%
59.6%
57.7%
65.8%
58.3%
62.0%
61.4%
64.7%
54.3%
14.3%
53.2%
20
7
27
74.1%
0.72
66.7%
42
27
69
60.9%
1.10
1.07
1.02
1.00
1.20
1.21
0.84
1.14
0.81
1.00
0.84
1.21
5.57
15. Outline further investigations that should be pursued.
With strong evidence that the vanilla ice cream could be the agent, it should be tested to see if bacteria
is present. It would be interesting to test all the food to see if anything else had traces of the bacteria
which could be why those who did not consume the vanilla ice cream (three people were ill without
eating it) still became ill. Like the compendium states, other specimens, besides the food, would be the
handlers and then the cases. If handlers had and skin lesions or culture nares, that could be how the
bacteria ended up in the food. Also, the culture stool would tell if it is the cause and if the two are
linked.
16. What control measures would you suggest?
I would make sure to get samples from those who did not consume the vanilla ice cream and did not get
ill from the dinner. This way, a baseline of the possible bacteria could be created if it was naturally
present in the body. It might help to even have someone who was not present at the dinner provide a
sample.
Another control would be to measure all the food for bacteria. If there was contamination, the vanilla
ice cream would not be the only food to blame for causing this outbreak.
17. Why was it important to work up this outbreak?
These steps have shown how to narrow down the many possibilities. Having the data provided more
weight in the diagnosis. This assignment has shown how there are many different things that must be
considered prior to making big decisions. Without having the incubation periods, it would not have
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allowed for a precise diagnosis. Knowing what the type of food was the most likely vehicle in this
situation allowed for more details to narrow down the options to only staphylococcus aureus.
18. Refer to the steps of an outbreak investigation you listed in Question 2. How does this
investigation fit that outline?
This investigation followed the outline very well. There was a possible outbreak detected by the local
health officer and the District Health Officer assigned an epidemiologist to investigate. Step two is to
define and find the cases, which was done by figuring out that all of those ill were at a church dinner.
Looking at all of the patients’ symptoms, it was clear that this was some type of gastrointestinal illness.
Since most gastrointestinal illnesses come from consumption of food and all those ill were at a dinner
eating, the hypothesis for a likely source was immediately figured as some type of food. This hypothesis
was tested by going through the line listing, plotting graphs, and calculating different ratios. This led to
finding the point of contamination and the source of the bacteria. Once it was known what the cause
was, the ice cream was thrown out and those ill would be over their symptoms soon. There was no
mention in the case study about when the outbreak was declared over, but staphylococcus aureus
usually does not last for long, so once those infected with the bacteria have gotten it out of their system,
the outbreak would be declared over.