Simon Thornley
Field epidemiology: effect measures, and a cohort study of
a food borne outbreak
Overview• Introduce how epidemiology can be
used to help inform what caused a food borne outbreak
• Re-inforce ideas of cohort study design and analysis
• Consider how other evidence can inform the results of epidemiological study.
Basic study design
Participants
UnexposedDisease
Unexposed No disease
ExposedDisease
Exposed no disease
Exposed
Unexposed
Participants Exposure Outcomes
ParticipantsOutcomesExposure
Simple Outbreak• 6 October 2007• 16 people reported having diarrhoea to
North Shore City Council following ‘prize giving’ at North Shore Event Centre
• Prize giving was the final event of a week long, international soccer tournament of the South African/Indian, diaspora
First steps: what to do?• Outbreak group• Track down participants• Track down food• Microbiological samples• Assess risk• Immediate action required?• Media/ risk communication?
Hypothesis• Was this outbreak caused by food
served at function? Which food?• Epidemiological analysis one tool. • Clinical and microbiological
analysis provide complementary
The suspects...Lamb biryani
Vege biryani
Dahl
The premises
Consider raw food
Participants
UnexposedDisease
Unexposed No disease
ExposedDisease
Exposed no disease
Exposed
Unexposed
Participants Exposure Outcomes
RETROSPECTIVE COHORT
Buffers’ dance Dal/ biryani Diarrhoea?
Steps in assessing dataWhat are the
facts?Facts vs
Artifacts (bias)
Explanations?Chance
ConfoundingCausal
Is extra information required?
Symptom onset (outbreak curve)
01234567
11:3
0:00
p.m
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1:30
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a.m
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3:30
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a.m
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5:30
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a.m
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7:30
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a.m
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9:30
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a.m
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11:3
0:00
a.m
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1:30
:00
p.m
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3:30
:00
p.m
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5:30
:00
p.m
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7:30
:00
p.m
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9:30
:00
p.m
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11:3
0:00
p.m
.
1:30
:00
a.m
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3:30
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a.m
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Time of diarrhoea onset
frequ
ency
Common epidemic curves
Symptoms?Symptom Number Proportion
(%)Diarrhoea 51 92.7%Bloody
diarrhoea 3 5.5%
Abdominal cramps
43 78.2%
Nausea 13 23.6%Fever 3 5.5%Vomiting 5 9.1%Headache 15 27.3%
Incubation periodIllness Incubation,
hours (n=51)*
Duration, hours (n=43)*
Median 12’30” 21’00”
Mean 11’36” 21’44”
Range 3’30”-33’00” 2’00”-50’00”
What are the facts?• Facts vs artifacts?• Bias: selection, information• Selection: was the sample
representative of the total group?• Would New Zealand cases be
more/less likely to be sick?• Case definition?
Diagrams helpful
~750 people attended
List supplied134 patrons
54
ill
80Not ill
51
Diarrhoea
3No Diarrhoea
Contactdetails
~616No contactdetails
“Frequentist view” Ground hog day• Imagine the event
repeated over and over!
• Assume random probability
Error distribution• Assume outcome is
like flipping a biased coin (probability theory)
• Consider long run probability associated with biased coin (prob =attack rate)
• Bias = attack rate in exposed or unexposed groups
Crude associations (univariate or crude)
Eaten? Yes Eaten? No
Food Ill Well Attackrate (%)
Ill Well Attackrate (%)
Odds P
Ratio (95% CI)Dahl 46 60 43 5 23 18 3.5 (1.3, 10.0) 0.02
Lamb Biryani
47 76 38 1 6 14 4.6 (0.6, 38.6) 0.15
VegetableBiryani
5 19 21 38 62 38 0.3 (0.1, 0.98) 0.04
Salad 29 47 38 18 35 34 1.1 (0.5, 2.2) 0.61
In pictures (dal)
Dahl eaten
Diarrhoea
If unrelated; chance of diarrhoea after eating dahl =chance of diarrhoea (regardless of meal selection)
ActualNo effect
In pictures (dal)Diarrhoea
Actual
Dahl exposure
Odds Ratio
=0.77/0.22=3.5
In pictures (dahl)Diarrhoea
No effect
Dahl exposure
Odds Ratio
=0.61/0.65≈1 (no effect)
In pictures (lamb)
Lamb eaten
DiarrhoeaActual No effect
In pictures (vege)Diarrhoea
Actual No effect
Just consider first exposure!• Is dal the likely
culprit?• What information
does the p-value give you?
• What information does the confidence interval convey?
• What is the null hypothesis?
Risk of disease in dal eatersHistogram of x
Risk of disease after eating dal
Freq
uenc
y
0.3 0.4 0.5 0.6
050
010
0015
00
Odds of disease in dal eatersHistogram of ox
Odds of disease after eating dal
Freq
uenc
y
0.5 1.0 1.5
050
010
0015
0020
0025
0030
00
Risk of disease in non-dal eatersHistogram of y
Risk of disease after not eating dal
Freq
uenc
y
0.0 0.1 0.2 0.3 0.4 0.5
050
010
0015
0020
0025
0030
00
Odds of disease in non dal eatersHistogram of oy
Odds of disease without eating dal
Freq
uenc
y
0.0 0.2 0.4 0.6 0.8 1.0
050
010
0015
0020
00
Distribution of risk ratioHistogram of x/y
Risk ratio
Freq
uenc
y
0 5 10 15
010
0020
0030
00
Upper bound of risk ratio is reciprocal of risk in unexposed (28/5 or 5.6)
Distribution of odds ratioHistogram of or
Risk ratio
Freq
uenc
y
2 4 6 8 10
020
040
060
080
010
0012
00
What if we’d managed to get 10x participants?
Histogram of or
Risk ratio
Freq
uenc
y
2 3 4 5 6 7
050
010
0015
0020
0025
0030
00
What do these results mean?
• What food caused the outbreak?• What does the 95% confidence interval
mean?• Could these results be explained by
chance? • Are the populations comparable?• Was some other exposure accounting
for this difference?
How do we know differences are real (not due to chance)?
We don’t – but Statistics helps us put a number on the uncertainty!
Confounding: Are the populations similar?Variable Cases
(n=51)Non-cases
(n=83)P-value
Gender 0.5
Male, n (%) 24 (47%) 33 (40%) (chi-square)
Age (years)
Range 18-66 12-72 0.08Mean 45 41 (unpaired t-
test)
Multivariable ResultsExposure Crude Odds
ratio (95% CI)
Adjusted Odds ratio (95% CI)
Dal 3.5 (1.3 to 10.0) 3.7 (1.3 to 10.9)
Vegetable Biryani
0.4 (0.1 to 1.0) 0.5 (0.2 to 1.6)
Lamb Biryani
4.6 (0.6 to 38.6) 0.8 (0.1 to 5.3)
Microbiology• Stool (2/18 received)
• Clostridium perfringens (1.0 x 104 CFU/g)• Bacillus cereus • No toxin, no norovirus
• Food• Lentils ok• Spices
• Turmeric Bacillus cereus (1.0 x104 CFU/g, with faecal coliforms (4.0 MPN/g)
Reject null hypothesis• dal likely to cause outbreak• Relationship between illness and
dal confounded by lamb biryani
Putting it all together….• Turmeric seeded lentil soup with Bacillus
cereus• Time and temperature abuse – endotoxin
associated diarrhoeal syndrome.• Vegetable biryani protective, few
consumed, more likely to be heated in bain-marie.
• Likely temperature labile toxin.
Room set up
Lamb biry
ani and dal
Vege biry
ani and dal
Don’t forget the Public Health Action
• Is routinely contaminated turmeric ok?
• What should we do with the cook?
• Legal action justified?
Summary• Epidemiology- distribution and
determinants of health• Study design – take into account
factors such as cost, speed, causation, ethical issues
• Statistics P-value yes/no cf. 95% confidence interval
• Put results together with other analysis• Unexpected results!