Field epidemiology: effect measures, and a cohort study of a food borne outbreak

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)

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3:30

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5:30

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7:30

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11:3

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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!