Outbreak Management

Learning Objectives

1. Describe the occurrence of disease in a population.

2. Discuss how an epidemic curve can help in managing an outbreak.

3. Outline the steps in outbreak investigation.

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Time involved

• 60 minutes

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Purposes of Outbreak Investigation

• To Identify the causal agent• To find the source of infection by studying

the occurrence of the disease among persons, place, or time, as well as determining specific attack rates

• Formulate recommendations to prevent further transmission

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Definitions - 1

• Outbreak or epidemic• An excess over the expected (usual) level of a disease

within a geographic area• However, one case of an unusual disease may

constitute an epidemic• E.g., postsurgical group A streptococcus infection

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Definitions - 2

• Endemic• The usual level of a disease within a geographic area

• E.g., a hospital

• These ‘sporadic’ infections represent most preventable healthcare-associated infections

• Baseline incidence

• Pandemic• An epidemic that spreads in several countries• Usually affecting many people

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Endemic and Epidemic

“Endemic”

“Epidemic”

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Num

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Healthcare-associated infections

• Majority of healthcare-associated infections are endemic

• Outbreak of healthcare-associated infections occur typically in a specific group of patients

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Definitions - 3

• Relative risk• A measure of association between a disease or

condition and a factor under study• Calculation

• Divide the incidence rate of those exposed to the factor by the incidence rate of those not exposed

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Interpretation of Relative Risk

RR =1 Incidence in the exposed group is the same as in the non-exposed

There is no association between exposure and disease

RR >1 Denotes a larger incidence in the exposed than in the non-exposed

Exposure seems to increase the probability of developing the disease

RR <1 Denotes a smaller incidence in the exposed than in the non-exposed

Exposure seems to decrease the probability of developing the disease

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Types of Outbreaks

• Community-acquired• Food-borne infections, measles

• Healthcare-associated• When two or more cases of infection

appear to be epidemiologically related

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Steps of Outbreak Investigation

1. Verify existence of the outbreak2. Determine if there were changes in case

finding or diagnostics 3. Establish diagnosis of reported cases 4. Case finding5. Characterise cases 6. Formulate hypothesis7. Test the hypothesis 8. Institute control measures and follow up 12

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Verify existence of an outbreak

• Compare the number of current cases with the usual baseline incidence

• from previous months or years • If local data are not available, compare to

information from national surveillance systems or the literature • these data may not be applicable to the local situation

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Determine if there were changes in case finding or diagnostics • New techniques or laboratory tests may increase

identification when historically cases would not have been identified

• Provides a new ‘baseline’ of disease

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Define cases

• Typically involves identifying an agent• Define cases based on the following common

factors:• Population risk factors

• Age, race, sex, socioeconomic status

• Clinical data• Onset of signs and symptoms• Frequency and duration of clinical features associated with

the outbreak

• Laboratory results

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Case Definition

• The inclusion and exclusion criteria for cases must be precisely identified

• The definition should differentiate between infection and colonisation

• Includes a unit of time and place and specific biological and/or clinical criteria

• A graded definition (definite, probable, or possible) often helps

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Case Finding

Collect critical data and specimen information from:

• Laboratory reports • Medical records• Patient charts• Physicians and nursing staff• Public health data

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Characterise cases

• Assemble and organise available information for analysis

• From this information, the population at risk can be accurately described

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Characterise cases - Time• The exact period of the outbreak• The probable period of exposure• Date of onset of illness for cases; draw an

epidemic curve• Is the outbreak common source or

propagated?

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Characterise cases – Place/TimePlace

• Service, ward, operating room• Clustering of cases

Person• Patient characteristics (age, sex, underlying disease)• Possible exposures (surgery, nursing and medical

staff , infected patients)• Therapy (invasive procedures, medications,

antibiotics)

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Calculate rates

• Incidence rate• The number of new cases occurring in the

population during a specified time / number of persons exposed to the risk of developing the disease during that time

• Attack rate• The cumulative incidence rate of infection in a

group over a period of an epidemic• Number of people at risk who are infected / Total

number of people at risk

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Source of Infection • Common source (single-point source)

• Same origin

• Propagated or continuing source (on-going transmission)• Infections are transmitted from person to person

• Both common and propagated source• Intermittent exposure to a common source • Produces an epidemic curve with irregularly

spaced peaks

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Epidemic curve - 1

• Also called a histogram• Determines the character of an epidemic • Cases are plotted by date of onset of illness• Time intervals (on the X axis) are based on

the incubation or latency period of the disease and the length of the period over which cases are distributed

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Epidemic curve - 2The reasons for constructing an epidemic curve include:

To determine whether the source of infection was common, propagated, or both

To identify the probable time of exposure of the cases to the source(s) of infection

To identify the probable incubation period To determine if the problem is ongoing

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A . Propagated source: single exposure, no secondary cases (e.g., measles).B. Propagated source: secondary and tertiary cases (e.g., hepatitis A).C. Common source: point exposure . D. Common source: Intermittent exposure.

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Epidemic curve

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Helps to develop hypotheses• Incubation period• Etiological agent• Type of source• Mode of transmission• Propagated source• Common source• Time of exposure

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Common Source Outbreak

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An explosive epidemic of short duration generally indicates• A common source of infection

• Exposure over a short period of time

• A large numbers of susceptible exposed

• A high attack rate

• An incubation period with little variation

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Propagated Source Outbreak

• Cases occur over a long period• Explosive epidemics due to person-to-person

transmission may occur• E.g., chickenpox

• If secondary and tertiary cases occur, intervals between peaks usually approximate to the average incubation period

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Propagated source - 1

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Cases

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Single exposure, no secondary cases (e.g. measles)

Propagated source - 2

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Secondary and tertiary cases (e.g., hepatitis A)

Determine Exposure Period

• Using the mean or median incubation period• Identify the peak of the epidemic or the date of onset

of the median case• Count back into one incubation period

• Using minimum and maximum incubation periods• Start with the first case identified and count back in

time the minimum incubation period• Then using the last case, count back in time the

maximum incubation period

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Exposure Period

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Formulate hypothesis

• Make a best guess to explain the observations• The hypothesis should explain most cases

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Test the Hypothesis

• Many investigations do not reach this stage• Investigation may end with descriptive

epidemiology and then the problem goes away without intervention or does not require a special study

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Institute Control Measures

• Control measures are determined by the results of the initial analysis in consultation with appropriate professionals

• They will vary depending on the agent, the mode of transmission, and observations

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Control Measures

Interventions commonly used to control an outbreak are as follows:

• Control the source of the pathogen • Remove the source of contamination, e.g., discard

contaminated food

• Remove persons from exposure• Keep people from being exposed to mosquito bites to

prevent encephalitis

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Healthcare-Associated Infections – Control Measures

Type of transmission suspected Suggested action

Contact- Cross-transmission (transmission between individuals)

Patient isolation and barrier precautions determined by infectious agent(s)

Contact- Hand transmission Improvements in hand hygiene (e.g., washing, disinfection, glove use)

Airborne agent Patient isolation with appropriate ventilation

Waterborne agent Checking of water supply and all liquid containers Use of disposable devices

Foodborne agent Elimination of the at-risk food

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Evaluate Efficacy of Control Measures

• Cases cease to occur or return to endemic level

• No change (re-evaluate cases)

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Communicate and Write a Final Report • During the investigation, up-to-date information is

communicated to administration and appropriate authorities

• A final report should be prepared describing the outbreak, interventions, and effectiveness, and summarising the contribution of each team member participating in the investigation• It should include recommendations to prevent any future

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Summary

• Outbreaks of infection should be clearly defined, identified, and promptly investigated because of their importance in terms of morbidity, cost, improvement of patient care, and institutional image

• Proper steps and effective techniques should be used to investigate a suspected outbreak

• Clear recommendations should be formulated to prevent further transmission and/or outbreaks

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References• Flora IH, Manuel B. Disease Outbreak Investigation. The Young

Epidemiology Scholars Program (YES) supported by The Robert Wood Johnson Foundation and administered by the College Board, 2005. http://www.collegeboard.com/yes/ft/iu/home.html

• Reingold AL. Outbreak Investigations—A Perspective. National Center for Infectious Diseases, Centers for Disease Control and Prevention. Emerg Infect Diseases 1998; 4 (1). http://www.cdc.gov/ncidod/eid/vol4no1/reingold.htm

• Susan P. Outbreak Investigation. The University of Texas, Harris county psychiatric center, 2010. http://www.uth.tmc.edu/uth_orgs/hcpc/procedures/volume1/chapter6/infection_control-51.htm

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Web Resource

• How to Investigate an Outbreak• http://

www.cdc.gov/excite/classroom/outbreak/index.htm • Communicable Disease Control: Outbreak Investigation

• http://www.healthknowledge.org.uk/public-health-textbook/disease-causation-diagnostic/2g-communicable-disease/outbreak-investigation

• Epidemiological Investigation of Outbreaks• http://

www.gov.mb.ca/health/publichealth/cdc/protocol/investigation.pdf

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Quiz1. Outbreak investigations are performed primarily to:

a. Gather clinical information about a diseaseb. Find the source of infectionsc. Make sure patients are treatedd. All of the above

2. Epidemic curves help determine if the problem is on-going. T/F

3. Control measures for healthcare-associated infections include all except:

a. Patient isolationb. Use of disposablesc. Hand hygiened. Immunisation

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International Federation of Infection Control• IFIC’s mission is to facilitate international networking in

order to improve the prevention and control of healthcare associated infections worldwide. It is an umbrella organisation of societies and associations of healthcare professionals in infection control and related fields across the globe .

• The goal of IFIC is to minimise the risk of infection within healthcare settings through development of a network of infection control organisations for communication, consensus building, education and sharing expertise.

• For more information go to http://theific.org/

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