Outbreak investigations

Biagio Pedalino & Domenico Martinelli

24th October 2012

Objectives for this session

• Describe – the principles of outbreak

investigation– the steps in outbreak investigation

• Using practical example– Foodborne outbreak during a

birthday party in a pub

What do you normally do on Sunday afternooon?

Roberto, 41Pub owner

Something funny.We had lunch at sibilla’s. We were 13 and now 7 are vomiting…

Excellent. Do you need me to be there for the “plasil”? Have you eaten something with cream?

Excellent. Do you need me to be there for the “plasil”? Have you eaten something with cream?

What is it?

• Friendly call• Need of medical assistance?• Coincidence?• What else?...

• Outbreak?

What is an outbreak ?

“The occurrence in a community or region of cases of an illness (or an outbreak) with a frequency clearly in excess of normal expectancy”

[Heymann DL, CDC Manual – 19th Ed]

Outbreak investigation: tasks• Confirm outbreak and diagnosis• Form Outbreak Control Team• Define a case (case definition)• Identify cases and obtain information• Describe data by time, place, person• Develop hypothesis• Test hypothesis: analytical studies • Additional studies • Communicate results:

– outbreak report, publication

• Implement control measures

Co

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Co

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ication

Confirm outbreakIs this an outbreak?• More cases than expected?• Surveillance data• Surveys: hospitals, labs, physicians

Caution!• Seasonal variations• Notification artefacts• Diagnostic bias (new technique)• Diagnostic errors (pseudo-outbreaks)

Confirm diagnosis

• Laboratory confirmation – serology– isolates, typing of isolates– toxic agents

• Meet attending physicians• Examine some cases• Contact (visit) the laboratories

Not always necessary to confirm all the casesbut confirm a proportion throughout the outbreak

Outbreak confirmed

Immediate control measures?

- prophylaxis- exclusion / isolation- public warning- hygienic measures- others

Alert !!!• Informal alert from the pub

– 16:43: Alert• 7 participants were sick with vomit

• Formal intervention: a simple phone call !

Phone call 4:45 pm

• Talk to the pub owner– How many people (list with contact

details)– Food items – Symptoms/treatment

• “Impose” preventive closure of the pub

Outbreak confirmed

Immediate control measures?

Further investigation?

- prophylaxis- exclusion / isolation- public warning- hygienic measures- others

- etiological agent- mode of transmission- vehicle of transmission- source of contamination- population at risk- exposure causing illness

Steps of an outbreak investigation

• Confirm outbreak and diagnosis• Form Outbreak Control Team• Define a case • Identify cases and obtain information• Describe data by time, place, person• Develop hypothesis• Test hypothesis: analytical studies • Additional studies• Communicate results:

– outbreak report, publication

• Implement control measures

Control m

easur es

My outbreak control team

Ale, Preventive medicine school student - II yr

Fra, PHD student - I yr

Case definition

A person presenting with vomit or nausea or diarrhoea, within 24 hours after participating at the meal in the pub (Sunday January 22nd; 2pm)

Case definition

• Standard set of criteria for deciding if a person should be classified as suffering from the disease under investigation

• Criteria

– clinical and/or biological criteria

– time

– place

– person

Case definition

• Simple, practical, objective

• Sensitive?

• Specific?

• Multiple case definitions– confirmed

– probable

– possible

• CD can be adjusted, if new information becomes available

Steps of an outbreak investigation

• Confirm outbreak and diagnosis• Form Outbreak Control Team• Define a case • Identify cases and obtain information• Describe data by time, place, person• Develop hypothesis• Test hypothesis: analytical studies • Additional studies• Communicate results:

– outbreak report, publication

• Implement control measures

Control m

easur es

Descriptive epidemiology

- When did they become ill? (time)

- Where do they live? (place)

- Who are the cases? (person)

Identify & count cases

Obtain information

Identifying information

Demographic information

Clinical details

Exposures and known risk factors

notificationshospitals, GPslaboratoriesschoolsworkplace, etc

Monday 8 am

• Contact the participants:– Symptoms– Food consumption– Stool specimen collection (n=5)

In the pub• Food samples collection • Info on food preparation

Information collected: symptoms

• 14:00: lunch (13 participants: birthday party)– 12 were sick

• Vomit (n= 10)• Nausea (n=9)• Diarrohea (n=5)• Abdominal pain (n=6)• Fever (<38°C; n=2)

Information collected: food consumption (n=13)

• Basmati rice: 92% (12)• Ragu’ : 77% (10)• Red rice: 69% (9) • Apple cake: 61% (8)

Possible cause of the outbreak?

• Virus ?

• Bacteria ?

• Toxins ?

• Parasite ?

Organize information: Line list

• Names• Date of birth• Addresse• Onset of symptoms• Treating physician• Hospital stay• Laboratory results

Line List

Case

No. Name

Date

of birthAddresse

Date of onset

Lab

results

1

2

3

4

5

6

XY

AB

CD

…

…

…

Identify & count cases

Obtain information

Descriptive studyDescribe in

- time

- place

- person

Time: Epidemic Curve

• Histogram

• Distribution of cases by time of onset of symptoms, diagnosis or identification– time interval depends on incubation period

Cases

0

1

2

3

4

5

6

7

8

9

10

1 2 3 4 5 6 7 8 9 10 11 12

Days

Epidemic curve

• Describe– start, end, duration

– peak

– importance

– atypical cases

• Helps to develop hypotheses– incubation period– etiological agent– type of source– type of transmission– time of exposure

0

1

2

3

4

5

6

7

8

9

10

1 2 3 4 5 6 7 8 9 10 11 12

Cases

Days

Examples of Epidemic curves

0

1

2

3

4

5

6

1 2 3 4 5 6 7 8

0

1

2

3

4

5

6

7

8

9

10

1 2 3 4 5 6 7 8 9 10 11 12

0

2

4

6

8

10

12

1 2 3 4 5 6 7 8 9 10 11 12 13

hours

cases

cases

cases

days

weeks

Common point source Common persistent source

Propagated source

0

1

2

3

4

5

6

7

8

9

10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

cases

days

Common intermittent source

Hepatitis A by date of onsetOgemaw county, Michigan, April - May 1968

2 8 14 20 26 2 8 14 20 26 1 7

Days

0

5

10

15

Number of cases

one case

30 days

15 days

50 days

Exposure

Hypothesis on the moment of infection unknown pathogen and point source

0

5

15

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29Time

1 median

50% 50%3Possible moment of infection

2~ median incubation period

(= duration of the epidemic)

Epi curve Incubation period:•Median = 2 hrs•Range = 1-5 hrs

Incubation period:•Median = 2 hrs•Range = 1-5 hrs

Place

• Place of residence• Place of possible exposure

– work– meals– travel routes, – day-care– leisure activities

Maps– identify an area at risk

Person

• Distribution of cases – age– sex– occupation, etc

• Distribution of these variables in population

• Attack rates

Roberto41 yrs oldRoberto41 yrs old

Yasemin35 yrs oldYasemin35 yrs old

Marygiò42 yrs oldMarygiò42 yrs oldMassimo

40 yrs oldMassimo

40 yrs old

Barbara34 yrs oldBarbara34 yrs old

The grandmother, 70 yrs oldThe grandmother, 70 yrs old

Steps of an outbreak investigation

• Confirm outbreak and diagnosis• Form Outbreak Control Team• Define a case • Identify cases and obtain information• Describe data by time, place, person• Develop hypothesis• Test hypothesis: analytical studies • Additional studies• Communicate results:

– outbreak report, publication

• Implement control measures

Control m

easur es

Develop hypotheses

- Who is at risk of becoming ill?

- What is the disease?

- What is the source and the vehicle?

- What is the mode of transmission?

Hyp in the pub: summary

• Close community • 12 out of 13 were sick• Mainly vomit• Basmati rice mostly consumed food• Basmati rice left out of the fridge for >

24hrs

Compare hypotheses with facts

Test specific hypotheses

Analytical studies- cohort studies- case-control studies

Testing hypothesis

• Cohort - attack rate exposed group- attack rate unexposed group

• Case control- proportion of cases exposed- proportion of controls exposed

Cohort study

• Among participants• Telephone interviews

Description of the cohort• 13 persons

– 54% F– Median age: 42 (15 – 70)

• Overall Attack Rate (AR): 92%• Food specific AR:

– Basmati rice: 100%– Apple cake: 100%– Ragu’: 91%– Red rice: 90%

Steps of an outbreak investigation

• Confirm outbreak and diagnosis• Form Outbreak Control Team• Define a case • Identify cases and obtain information• Describe data by time, place, person• Develop hypothesis• Test hypothesis: analytical studies • Additional studies• Communicate results:

– outbreak report, publication

• Implement control measures

Control m

easur es

Verify hypothesisAdditional investigations

• Microbiological investigation of food samples

• Environmental investigation

• Veterinarian investigation

• Molecular Typing

• Trace back investigations (origin of foods)

• Meteorological data

• Entomological investigations

Tuesday 10 am: laboratory results

Tuesday 10 am: laboratory results

2/5 faecal specimens 2/5 faecal

specimens

Steps of an outbreak investigation

• Confirm outbreak and diagnosis• Form Outbreak Control Team• Define a case • Identify cases and obtain information• Describe data by time, place, person• Develop hypothesis• Test hypothesis: analytical studies • Additional studies• Communicate results:

– outbreak report, publication• Implement control measures

Co

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Outbreak report

• Regular updates during the investigation

• Detailed report at the end

– communicate public health messages

– influence public health policy

– evaluate performance

– training tool

– legal proceedings

Actions & Recommendations

• Pub closed for one day• Appropriate food preparation (cold

chain, storage, etc)• Timely alert for prompt intervention

• … To follow grandmother’s wisedom… « no thanks, no rice for me… »

The reality….

Outbreak suspected

time

Confirmation

Form Outbreak Control Team

Confirm Diagnosis

Site visit

Case definition

Line list

Organize Data

Descripitve Epidemiology

Control measures

AnalyticalEpidemiology

Recommendations

ReportPublication

Communication

Why investigate outbreaks?

• Stop the outbreak• Understand what happened and why• Prevent future outbreaks• Improve our knowledge• Improve surveillance and outbreak detection • Training

Acknowledgements

• Pub owner• Participants of the party (special

thanks to the grandmother)

Questions ???