investigation of epidemic presentation

Dr. Moustapha RamadanFellow of Community Medicine Department

Faculty of MedcineAlexandria University

Definitions of ( cluster/outbreak/pseudo outbreak/ epidemic)

Healthcare settings infections Reasons to investigate How to recognize outbreak/epidemic Goals of investigations Steps of investigations

Cluster : refers to a group of cases in a specific time and place

Outbreak: An increase in the incidence of a disease above what is normally expected in localized area over a specific period of time

Pseudo outbreak:Increase in detection of true cases that does not

represent an outbreak eg: Change in local reporting practices, increased interest in certain diseases because of local or national awareness, changes in diagnostic methods

or increase in incidence not related to true increase in cases eg: misdiagnosis, laboratory contamination

Epidemic : The occurrence in a defined community or region of cases of illness with a frequency clearly in excess of normal expectancy…endemo epidemic

A single case of a communicable disease long absent from a population or the first invasion by a disease not previously recognized…exotic epidemic

Significant increase in the rate of HAIs above the facility’s background rate.

5% of HAIs occur in outbreaks

Most outbreaks go unrecognized

Localized or systemic condition that results from adverse reaction due to the presence of an infectious agent(s) or its toxin(s) that was not present or incubating at the time of admission to the health facility.

Examples : UTI, VAP ,SSI, BSI, Clostridium difficile etc…

Because every report by itself is very much influenced by the local situation. Thus, only a systematic evaluation of a large number of outbreak reports will provide a less biased assessment of data.

May represent breakdowns in public health measures.

Evaluate existing prevention strategies, e.g., vaccines

Describe new diseases Teach (and learn) epidemiology of infectious

diseases Address public concern about the outbreak

Recently WHO recommends global alert and response system (GAR).

Early warning which is timely surveillance systems that collect information on epidemic-prone diseases in order to trigger prompt public health interventions.

Relies on an in-depth review done by epidemiologists of the data coming in.

Surveillance system

Reporting : Healthcare settings, laboratories, pharmacies

Surveillance is the collection, analysis, interpretation and dissemination of information about a selected health event. This information is important to plan, implement and evaluate a health program.

Uses of Surveillance

I) Identify the disease trend so that planning of preventive and control programs can be adjusted to meet the new situation.

2) Identify, investigate and help the control of outbreaks or epidemics.

3) Identify the population at risk for certain disease or death.

4) Identify new emerging disease.5) Evaluation of preventive and control measures of

the disease under study.

Identify the etiologic agents

Identify the reservoir(s)

Identify the mode of transmission

Apply control and prevention measures

Eliminate the reservoir(s) and transmission

Prevent future outbreaks

Prepare for field work

Establish the existence of an epidemic

Verify the diagnosis

Define and identify cases

Descriptive epidemiology and epidemic curves.

Develop hypotheses

Evaluate hypotheses

Refine hypotheses and carry out additional studies

Implement control and prevention measures

Communicate findings

Develop an outbreak team.

Determine the role of each member in the

investigation

Research the disease and gather the supplies and

equipment you will need

Make necessary administrative and personal

arrangements for such things

Review existing information to determine the number of potential cases, location, and severity of the problem

surveillance recordslaboratory recordspatient medical recordsradiologyPharmacy

Compare rates of infection for both the background period and the outbreak period

Goal is to rule out pseudo-outbreaks.

Requires to : Review the clinical findings Laboratory findings Examination a sample of affected persons

A. Define cases ( Establish case definition):

Case definition should be broad enough to include most, if not all, of the actual cases. (sensitive not specific)

Case definition must not include an exposure or risk factor you want to test

Case definition must be equally applied and without bias to all persons under the investigation


Usually includes four components: clinical information about the disease, characteristics about the people who are

affected, information about the location or place, and a specification of time during which the

outbreak occurred


Possible (suspected)

New or Worsening of cough, Fever >38, nasal

discharge, sore throat

Probable

Symptoms +Radiology evidence

Definite (confirmed)

Laboratory confirmed


Example :A patient hospitalized in the ICU from 24th March

2013, with new or worsening of cough, Fever >38, with suggestive X-ray changes and cultures identify a respiratory microorganism.

B. Identify cases (line listing)

Identifying information

e.g. Hospital admission number, unit, name, address, phone.

Demographics

e.g. Age, sex, date of admission, date of surgery.

Risk factor informatione.g. Type of surgery, comorbidity, catheters, implants

Clinical datae.g. Onset of symptoms and signs, frequency and duration of clinical features, treatments, devices, etc

A. Person:

Determine what population at risk

Define population by host characteristics or exposure

Use attack rate to identify high-risk groups

Numerator = number of cases

Denominator = number of people at risk

Number of patients affected divided by the total

number of population at risk

Number of infections divided by the total number

of population at risk

Number of adverse outcomes divided by the total

number of population at risk

Probability of disease

ba

c d

a+b

c+d

D D

E

E

Pr (D E( =a

a+b

Probability of disease

ab

c d

a+b

c+d

D D

E

E

Pr (D E( =c

c+d

Ratio of the risk of disease among exposed to the risk among the non

exposed

Pr (D E(

Pr (D E(=

aa+bc

c+d

B. Place:

Geographic extent of problem

Clusters or patterns providing important etiologic

clues

Spot maps

Where cases live, work or may have been exposed

C. Time: (Epidemic curve)

Histogram of the number of cases by their date of onset

Magnitude of the outbreak and time trend

Where are we in the time course of the outbreak?

Future course?

Probable time period of exposure

Examples of Examples of Epidemic CurvesEpidemic Curves

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5

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15

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25

1 3 5 7 9 11 13 15 17 19

Point sourcePoint source

0

5

10

15

20

1 4 7 10 13 16 19

Common source- PropagetedCommon source- Propageted

0

5

10

15

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1 3 5 7 9 11 13 15 17 19

Multiple waves –IntermittentMultiple waves –Intermittent

Hypotheses should address

Type of exposure

The agent and its reservoir

Mode of transmission

Risk factors that caused disease

Evaluate the credibility of your hypotheses

Use analytic epidemiology to quantify

relationships and explore the role of chance

Case control studies

Cohort studies

Compares patients with HAI’s to a group of controls (individuals without the infection).

Two or three controls/case

Are the cases more likely to have been exposed to a certain risk factor than the controls?

Case-control studies start with a disease (infected patients) and go back to exposures.

Strength of the association between the infection and the risk factor is measured by the Odds Ratio

Compares the rate of infection among those with a certain risk factor to the rate among those without the risk factor

Asks: of those exposed to the risk factor under study, how many will go on to develop the infection? (compared to those not exposed to the risk factor)

Cohort studies start with an exposure and go forward to diseases (infected patients)

Quantifies the extent to which exposure increases the risk of the disease

Relative risk

Factors that should also be considered when evaluating possible causality:

Testing statistical significance Consistency with other studies Temporality. Exposure to the factor precedes onset

of disease. Biologic plausibility. Does the association make

sense biologically?

When analytic epidemiological studies do not confirm hypotheses, we need to reconsider hypotheses and look for new vehicles or modes of transmission.

Sometimes you will need to refine your hypotheses to obtain more specific exposure histories or a more specific control group.

Laboratory and environmental studies

It should start as soon as possible

May be aimed at agent, source, or reservoir

Short or long term (reducing susceptibility)

Type of transmission suspected

Suggested Actions

Cross- transmission (transmission between persons)

Patient isolation and barrier precautions determined by infectious agent

Hand transmission Promotion of hand hygiene

Airborne agent Isolation; negative pressure

Foodborne Elimination of the food causing infection

0

10

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40

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1 4 7 10 13 16 19 22 25 28 31 34 37 40

DAY

CASES

LabConfirmationResponse

Opportunity for control

Detection/Reporting

First Case

0

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1 4 7 10 13 16 19 22 25 28 31 34 37 40

DAY

CASES

First CaseDetection/

ReportingLab

Confirmation Response

Opportunity for control

Orally (Feedback/Debriefing)

Local health authorities and persons responsible for

implementation of control and prevention measures

Written reports follows the usual scientific format of

introduction, background, methods, results, discussion,

and recommendations.

Also it is a record of performance, legal issues, reference,

adding to knowledge base.