Transcript Slide 1

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
• Consider how other evidence can inform the
results of epidemiological study.
Basic study design
Participants
Exposure
Exposed
Outcomes
Exposed
Disease
Exposed
no disease
Participants
Unexposed
Unexposed Unexposed
Disease
No disease
Exposure
Outcomes
Participants
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
Dahl
The suspects...
Lamb biryani
Vege biryani
The premises
Consider raw food
RETROSPECTIVE COHORT
Participants
Exposure
Exposed
Outcomes
Exposed
Disease
Exposed
no disease
Participants
Unexposed
Buffers’
dance
Unexposed Unexposed
Disease
No disease
Dal/ biryani
Diarrhoea?
Steps in assessing data
What are the
facts?
Facts vs
Artifacts (bias)
Explanations?
Chance
Confounding
Causal
Is extra
information
required?
frequency
7
6
5
4
3
2
1
0
Time of diarrhoea onset
3:30:00 a.m.
1:30:00 a.m.
11:30:00 p.m.
9:30:00 p.m.
7:30:00 p.m.
5:30:00 p.m.
3:30:00 p.m.
1:30:00 p.m.
11:30:00 a.m.
9:30:00 a.m.
7:30:00 a.m.
5:30:00 a.m.
3:30:00 a.m.
1:30:00 a.m.
11:30:00 p.m.
Symptom onset (outbreak curve)
Common epidemic curves
Symptoms?
Symptom
Number
Proportion (%)
Diarrhoea
51
92.7%
Bloody
diarrhoea
Abdominal
cramps
Nausea
3
5.5%
43
78.2%
13
3
5
15
23.6%
5.5%
9.1%
27.3%
Fever
Vomiting
Headache
Incubation period
Illness
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
No contact
details
~616
Not ill
80
No
Diarrhoea
3
Contact
details
List supplied
134 patrons
ill
54
Diarrhoea
51
“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
Food
Ill
Well
Eaten? No
Attack
rate (%)
Ill
Well
Attack
rate (%)
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
Vegetable
Biryani
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)
If unrelated; chance of diarrhoea after eating dahl =chance of diarrhoea (regardless of
meal selection)
Actual
Diarrhoea
No effect
Dahl eaten
In pictures (dal)
Actual
Diarrhoea
Odds Ratio
=0.77/0.22
=3.5
Dahl exposure
In pictures (dahl)
No effect
Diarrhoea
Odds Ratio
=0.61/0.65
≈1 (no effect)
Dahl exposure
In pictures (lamb)
Actual
Diarrhoea
No effect
Lamb eaten
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?
0
500
Frequency
1000
1500
Histogram of x
Risk of disease in dal eaters
0.3
0.4
0.5
Risk of disease after eating dal
0.6
1500
1000
500
0
Frequency
2000
2500
3000
Histogram of ox
Odds of disease in dal eaters
0.5
1.0
Odds of disease after eating dal
1.5
2000
1500
1000
500
0
Frequency
2500
3000
Risk of disease in non-dal
eaters
Histogram
of y
0.0
0.1
0.2
0.3
Risk of disease after not eating dal
0.4
0.5
Odds of disease in non dal eaters
1000
500
0
Frequency
1500
2000
Histogram of oy
0.0
0.2
0.4
0.6
Odds of disease without eating dal
0.8
1.0
Distribution of risk ratio
2000
1000
Upper bound of risk ratio is
reciprocal of risk in unexposed
(28/5 or 5.6)
0
Frequency
3000
Histogram of x/y
0
5
10
Risk ratio
15
600
400
200
0
Frequency
800
1000
1200
Distribution of odds ratio Histogram of or
2
4
6
Risk ratio
8
10
What if we’d managed to get 10x
participants?
1500
1000
500
0
Frequency
2000
2500
3000
Histogram of or
2
3
4
5
Risk ratio
6
7
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)
Gender
Male, n (%)
P-value
0.5
24 (47%)
33 (40%)
Range
18-66
12-72
Mean
45
41
(chi-square)
Age (years)
0.08
(unpaired ttest)
Multivariable Results
Exposure
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
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!