field epidemiology: effect measures, and a cohort study of a food borne outbreak
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Field epidemiology: effect measures, and a cohort study of a food borne outbreak. Simon Thornley. 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 - PowerPoint PPT PresentationTRANSCRIPT
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Simon Thornley
Field epidemiology: effect measures, and a cohort study of
a food borne outbreak
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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.
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Basic study design
Participants
UnexposedDisease
Unexposed No disease
ExposedDisease
Exposed no disease
Exposed
Unexposed
Participants Exposure Outcomes
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ParticipantsOutcomesExposure
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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
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First steps: what to do?• Outbreak group• Track down participants• Track down food• Microbiological samples• Assess risk• Immediate action required?• Media/ risk communication?
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Hypothesis• Was this outbreak caused by food
served at function? Which food?• Epidemiological analysis one tool. • Clinical and microbiological
analysis provide complementary
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The suspects...Lamb biryani
Vege biryani
Dahl
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The premises
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Consider raw food
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Participants
UnexposedDisease
Unexposed No disease
ExposedDisease
Exposed no disease
Exposed
Unexposed
Participants Exposure Outcomes
RETROSPECTIVE COHORT
Buffers’ dance Dal/ biryani Diarrhoea?
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Steps in assessing dataWhat are the
facts?Facts vs
Artifacts (bias)
Explanations?Chance
ConfoundingCausal
Is extra information required?
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Symptom onset (outbreak curve)
01234567
11:3
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1:30
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5:30
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11:3
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1:30
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3:30
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5:30
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11:3
0:00
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1:30
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3:30
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Time of diarrhoea onset
frequ
ency
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Common epidemic curves
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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%
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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”
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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?
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Diagrams helpful
~750 people attended
List supplied134 patrons
54
ill
80Not ill
51
Diarrhoea
3No Diarrhoea
Contactdetails
~616No contactdetails
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“Frequentist view” Ground hog day• Imagine the event
repeated over and over!
• Assume random probability
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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
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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
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In pictures (dal)
Dahl eaten
Diarrhoea
If unrelated; chance of diarrhoea after eating dahl =chance of diarrhoea (regardless of meal selection)
ActualNo effect
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In pictures (dal)Diarrhoea
Actual
Dahl exposure
Odds Ratio
=0.77/0.22=3.5
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In pictures (dahl)Diarrhoea
No effect
Dahl exposure
Odds Ratio
=0.61/0.65≈1 (no effect)
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In pictures (lamb)
Lamb eaten
DiarrhoeaActual No effect
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In pictures (vege)Diarrhoea
Actual No effect
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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?
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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
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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
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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
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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
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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)
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Distribution of odds ratioHistogram of or
Risk ratio
Freq
uenc
y
2 4 6 8 10
020
040
060
080
010
0012
00
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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
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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?
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How do we know differences are real (not due to chance)?
We don’t – but Statistics helps us put a number on the uncertainty!
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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)
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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)
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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)
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Reject null hypothesis• dal likely to cause outbreak• Relationship between illness and
dal confounded by lamb biryani
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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.
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Room set up
Lamb biry
ani and dal
Vege biry
ani and dal
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Don’t forget the Public Health Action
• Is routinely contaminated turmeric ok?
• What should we do with the cook?
• Legal action justified?
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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!