INVESTIGATION OF AN EPIDEMIC

Dr. Amna Rehana Siddiqui Department of Family and Community MedicineFebruary 28, 2011

OBJECTIVES List and explain the steps of investigation of

an epidemic Draw & interpret the epidemic curve Compare food specific attack rates to identify

possible vehicles List reasons for investigating an epidemic

WHAT IS AN EPIDEMIC ?

The occurrence of more cases in aThe occurrence of more cases in aplace (or population) and time place (or population) and time than expected than expected (can be 1 case if endemicity was 0)

The terms outbreak and epidemic areused interchangeably

It is due to breaks in a system that needs to be identified & corrected

Types of Epidemics/Outbreaks

Propagated Indicative of person to person

transmission e.g. MeaslesPoint-source

Indicative of a common exposure to a contaminated vehicle or reservoir e.g. food poisoning

ENDEMIC SITUATION

One that is usually present in a given geographical area of a population

group in comparison with other areas or populations

FROM ENDEMIC TO EPIDEMIC ?

Observation Increase in disease events Increase in risk factors

Surveillance (ongoing) reports

Predefined Threshold value

WHY INVESTIGATE? Identify source of infection/s urgently Detect weakness in existing system To prevent mortality and morbidity To highlight public health needs Could result in policy changes To improve public health To be well informed about agent,

host, and environment.

During an outbreak u need to devise a best strategy utilizing all steps e.g. u will also be controlling an outbreak (e.g. advice to take precautionary measures) while you r still defining cases; collecting data and at the same time communicating each day to those who need to know ; e.g. newspapers; medical director, ministry of health etc.

HOW TO INVESTIGATE ?Example 1:An undiagnosed case of Crimean Congo hemorrhagic fever(CCHF) infects a hospital worker who gets moderately sick

Example 2: You are requested to investigate food poisoning cases after a gathering at a local community hall

Identify potential investigation team

Research the disease Make administrative arrangements Clarify your and team members’ role Identify & arrange for resources Develop communication strategies for

team, hospital employees, and public

STEP 1. Prepare for fieldwork

STEP 2: Establish the existence of epidemic

Does the observed number of cases exceed the existing number ?

Literature search Consult the experts / Government Surveillance records Previous reports Is there a confirmed case ? Are there additional cases?

Speak directly with the persons who are affected

Clinical criteria Laboratory methods available ? Arrange to send samples to relevant

laboratories Initial reports correct ? Possible modes of spread

STEP 3 Verify the diagnosis

PS: STEPS ARE NOT IN ANY ORDER

Step 3 Continued Possible Exposures

Possible Cause of illnessTry best to know your agent/disease

Clinical picturePathogenesisMode of transmissionNatural ReservoirCommon Vehicle or Vector

Step 3 Continued

Consider Chain of Infection

STEP 4 Define and identify cases

Develop a case definition, identify and count cases

Develop a line listing for all Create a working case definition for CCHF

For hospital workers/Contacts Any one with fever &/ sore throat For Cases Anyone with undiagnosed fever, or has

any type of bleeding

PS: STEPS ARE NOT IN ANY ORDER during outbreak

Step 4a: Establishing a Case Definitionfinition

A case definition is a standard set of criteria for deciding whether an individual should be classified as having the health condition of interest.

A case definition includes clinical criteria and--particularly in the setting of an outbreak investigation--restrictions by time, place and person.

Apply them consistently and without bias to all persons under investigation.

To be classified as confirmed, a case usually must have laboratory verification. A case classified as probable usually has typical clinical features of the disease without laboratory confirmation. A case classified as possible usually has fewer of the typical clinical features.

CASE DEFINITION

Early in an investigation, investigators often use a WORKING case definition which includes confirmed, probable, and even possible cases.

Later on, when hypotheses have come into sharper focus, the investigator may “tighten” the case definition ACCORDINGLY /by dropping the possible category.

DEFINING CASES

Step 4 continuedCase finding and Line listing /Spread Sheet

Each row represents data for a subject and columns represent following

Identification Symptom onset date and time Symptoms present Suspected exposure date if known Residence Age Gender Laboratory tests Confirmed case

Line Listing: example

ID Date Surg

Sym

onset

age sex C/S Case defn

OT # Surg Type

Surg

001 Jan 1 Fev 33 M Await poss/prob

1 clean Hern

002 Jan 1 Pain 25 M Staph CONFIRM

2 clean Hern

003 Jan 1 Fev 28 F E coli ?2nd

await

1 clean Biop

004 Jan 1 Red 24 M Staph CONFIRM

2 clean Hern

Each row represents data for a subjectChange order by any column and evaluate

STEP 5 Descriptive epidemiology

Find cases systematically Epidemic / Outbreak curve Map Identify demographic and other

characteristics of persons at risk Tabulate and orient the data in terms of

Person Place Time

PS: STEPS ARE NOT IN ANY ORDER

STEP 5 Continued

Epidemic Curve

Graph showing number of cases on ‘Y’ axis and time line on ‘X’ axis

Indicates the magnitude of epidemic over time

Can distinguish epidemic from an endemic situation

Indicates pattern of spread Outliers

EPIDEMIC CURVE for CCHF 2000 Propagated Outbreak

0

1

2

3

Sep1-7

Sep15-21

Sep29-5

Oct13-19

Oct27-2

Nov10-16

Nov24-30

TIME IN WEEKS

Onset of Gastrointestinal Illness

0

1

2

3

4

5

6

7

2/27/0118:00

2/28/01 0:00 2/28/01 6:00 Noon Feb 28 2/28/0118:00

3/1/01 0:00 3/1/01 6:00 Noon Mar 1 3/1/01 18:00 3/2/01 0:00

Date and Time

Nu

mb

er

of

Ca

se

s

EPIDEMIC CURVE Point Source outbreak

STEP 5 Continued

Person: who is getting sick ?

Age SexRace/EthnicitySocio-Economic StatusBehavior related

STEP 5 Continued Place: where are the cases coming from ?

Geographic Distribution

HomesWorkSchoolOrphanagesHospitals/Clinics

STEP 5 Continued Time: when are they getting sick?

Onset of symptoms Incubation Period Infectious Period Seasonality Baseline vs. epidemic

Following things help Open ended and wide ranging

interviews/history of travel/foods eaten Descriptive epidemiology

Occupation/residence/raw meat use

Line listing of all subjects Epidemic curve Existing knowledge of disease

STEP 6DEVELOP HYPOTHESES

Step 6: Developing Hypotheses

All aspects of the investigation should be addressed:- source of the agent,

- mode of transmission, (vehicle/vector), - exposures that caused the disease, - any additional time, person, & place factors

e.g. CCHF transmitted by blood and body fluids what other means as there were patients from occupations who were not exposed to blood and body fluids?

First, consider what you know about the disease itself:

What is the agent’s usual reservoir? How is it usually transmitted? What vehicles are commonly

implicated? What are the known risk factors?

DO EXTENSIVE Literature Search; Be familiar with the disease

STEP 7 EVALUATE HYPOTHESES

Further analyze existing data Compare ill populations with not ill

populations Compare attack rates Conduct statistical tests Recommend Control Measures

Step 8 Execute additional Studies

Refine hypothesis Calculate & compare attack rates (AR)* Compare AR in Occupational exposure to

blood/body fluids with occupations not exposed to blood/body fluids; e.g. HCW vs Non HCW; butchers vs non butchers

2. Which food item caused poisoning? Compare attack rates (e.g. RR) in those who ate meat and in those who did not eat meat

AR (a) in meat eaters=# who got sick / Total # who ate meatAR (b) in not meat eaters=# who got sick / Total # who did not eat meat

Step 8 Execute additional Studies

Environmental & Laboratory tests (e.g. home visit; left over food samples)

Do additional epidemiologic studies;

Case control; Retrospective cohort studies; Follow up to evaluate control measures;

Basic measure of infectivity

Attack rate =

# of new cases of specific disease in a time

population at risk during the same period

Define Population

Retrospective cohortFood

Items at dinner

Ate food item

(exposed)

Did not eat (unexposed)

RR= a/b

Ill well AR

% (a)

Ill well AR

% (b)

RR

meat 29 17 63 17 12 59

spinach 26 17 60 20 12 62

potato 23 14 62 23 14 62

salad 13 11 54 28 19 60

Ice cream 43 11 80 3 18 14

Retrospective cohortFood

Items at dinner

Ate food item

(exposed)

Did not eat (unexposed)

RR= a/b

Ill well AR

% (a)

Ill well AR

% (b)

RR

meat 29 17 63 17 12 59 1.07

spinach 26 17 60 20 12 62 0.97

potato 23 14 62 23 14 62 1.00

salad 13 11 54 28 19 60 0.90

Ice cream 43 11 80 3 18 14 5.71

Case control

Exposure Case Control Total

Ate at A

restaurant

Yes 30 36 66

No 10 70 80

Total 40 106 146

Calculate the ORCalculate the OR

Case control

Exposure Case Control Total

Ate at A

restaurant

Yes 30 36 66

No 10 70 80

Total 40 106 146

OR= (30 x 70) / (36 x 10) = 5.83OR= (30 x 70) / (36 x 10) = 5.83

Example of Surgical Wound Infections Investigating Post operative Surgical Wound

Infections Confirm; case definition, Determine outbreak

period, Epidemic curve, Line listing for risk factors

Risk factors; OT, Doctors, Staff, Anesthetists, type of surgery, ASA status, Duration of surgery, aseptic conditions, etc.

Calculate attack rate for all Evaluate hypothesis

RR of illness for the Theatre

DiseaseExposure

SWI

Yes

SWI

No

Attack Rate

OT 1 9 7

(9/16)

56%

OT 2 7 11

(7/18)

38%

Relative Risk= 56/38=1.4 (95% CI 0.70 – 2.98); (Chi sq) p=0.50

Risk Ratio: Attack rate in OT1 = 56/38 = 1.4 Attack rate in OT2

RR of illness for Surgery Duration

DiseaseExposure

SWI

Yes

SWI

No

Attack Rate

Duration

> 2 hrs 14 6

(14/20)

70%

Duration

<=2 hrs 5 12

(5/12)

29%

Relative Risk= 70/29 = 2.4, 95% CI 1.08-5.25; p value 0.03

Risk Ratio: Attack rate Duration > 2 hrs = 70/29 = 2.4 Attack rate Duration <= 2 hrs

STEP 9: Take Immediate Control Measures

Surveillance for hospital employees Surveillance for cases Health education Reduce panic How to avoid exposures/Contacts How and where to report Appropriate quarantine and isolation

measures Cohorting

PS: STEPS ARE NOT IN ANY ORDER

An outbreak might be controlled by :

- destroying contaminated foods,

- sterilizing contaminated water, or

- destroying mosquito

breeding sites - vaccinating animals/tick prevention -

An infectious food handler could be removed from the job and treated.

-Cancelling restaurant permit and reviewing inspecting rules

Control Measures

reducing host susceptibility ,

- Immunization ,

- chemoprophylaxis for travelers

STEP 10 COMMUNICATE FINDINGS

Summarize investigation for requesting authority

Prepare written report Prevention of future similar outbreaks

The report should include:

Introducing the identification of epidemic Methods; data collection, analyses, and

interpretations for environmental, laboratory, and epidemiological methods

Results; descriptive as well as analytic Implemented preventive and control measures Effectiveness of control measures. Impacts relevant to prevention and control. Make recommendations regarding future

surveillance and control. Distribute report to others in disease control

programs as well as publish by doing studies.

The report:The report: A record of performance and a document

for potential legal issues.

Reference if the health department encounters a similar situation in the future.

Broader purpose for contributing to the knowledge base of epidemiology and public health.

STEP 10 FOLLOW UP

May execute further additional studies Maintain surveillance for a pre-defined

period of timeEvaluate interventions and control

measures Formulate recommendations

Figure . Meningococcal disease during the 2000 Hajj: Jeddah, Mecca, and Medina, January 24–June 5, 2000.

The number of cases of sero group specific meningococcal disease is shown by date. The duration of the 2000 Hajj is indicated.

Cholera epidemic: evolution of Epidemiology John Snow and the Broad Street Pump, 1854

Snows dot map of London