Investigating Outbreaks

Lecture overview

• Importance of investigating reported Importance of investigating reported outbreaksoutbreaks

• Steps in the investigation of an Steps in the investigation of an outbreakoutbreak

• Describe epidemic curvesDescribe epidemic curves

“Outbreak investigations, an important and Outbreak investigations, an important and challenging component of epidemiology and challenging component of epidemiology and public health, public health, can help identify the source of can help identify the source of ongoing outbreaksongoing outbreaks and and prevent additional prevent additional casescases. Even when an outbreak is over, a . Even when an outbreak is over, a thorough epidemiologic and environmental thorough epidemiologic and environmental investigation often can investigation often can increase our knowledgeincrease our knowledge of a given disease and of a given disease and prevent future prevent future outbreaksoutbreaks. Finally, outbreak investigations . Finally, outbreak investigations provide provide epidemiologic trainingepidemiologic training and foster and foster cooperation cooperation between the clinical and public between the clinical and public health communities.”health communities.”

Outbreak Investigations—A Perspective Arthur L. Reingold University of California, BerkeleyEmerging Infectious Diseases, Vol. 4 , No. 1

Why Investigate Why Investigate Possible OutbreaksPossible Outbreaks? ?

• Control/preventionControl/prevention

• Research opportunitiesResearch opportunities

• TrainingTraining

• Public, political, or legal concernsPublic, political, or legal concerns

http://www.phppo.cdc.gov/phtn/catalog/pdf-file/LESSON6.pdf

• “Once a decision is made to investigate an outbreak, three types of activities are generally involved

• The epidemiologic investigation.

• The environmental investigation.

• The interaction with the public, the press, and, in many instances, the legal system.”

Outbreak Investigations—A Perspective Arthur L. Reingold University of California, Berkeley, California, USAEmerging Infectious Diseases, Vol. 4 , No. 1

Step 1: Step 1: Prepare for FieldworkPrepare for Fieldwork

• InvestigationInvestigation

• AdministrationAdministration

• ConsultationConsultation

Step 2: Step 2: Establishing the ExistenceEstablishing the Existence

of an Outbreak of an Outbreak

• Is this an epidemic or cluster of Is this an epidemic or cluster of cases?cases?

• Does the observed number Does the observed number exceed the expected number of exceed the expected number of cases?cases?

• For a notifiable disease use health department surveillance records.

• For other diseases and conditions, find existing data locally—hospital discharge records, mortality statistics, cancer or birth defect registries.

• If local data are not available, apply rates from neighboring cities or national data, or, alternatively, conduct a telephone survey of physicians to determine whether they have seen more cases of the disease than usual.

• Finally, conduct a survey of the community to establish the background or historical level of disease.

Step 3: Step 3: Verifying the DiagnosisVerifying the Diagnosis

• to ensure that the problem has been properly diagnosed

• to rule out laboratory error as the basis for the increase in diagnosed cases.

• summarize the clinical findings with frequency distributions

• should visit several patients with the disease.

Why?Why?

• Diseases can be misdiagnosed.

• Case may not be actual case, but rather suspected case.

• Information from non-cases must be excluded from the case information used to confirm the presence or absence of an epidemic.

Step 4a: Step 4a: Establishing a Case DefinitionEstablishing a Case Definition

• 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.

• Early in an investigation, investigators often use a sensitive or “loose” 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 by dropping the possible category.

Step 4b: Step 4b: Identifying and Counting CasesIdentifying and Counting Cases

• Direct case finding at health care facilities where the diagnosis is likely to be made: physicians’ offices, clinics, hospitals, and laboratories.

• In some outbreaks, public health officials may decide to alert the public directly, usually through the local media.

• Ask case-patients if they know anyone else with the same condition.

• Conduct a survey of the entire population.

• Regardless of the particular disease being investigated, collect the following

types of information about every case:

• identifying information

• demographic information

• clinical information

• risk factor information

• reporter information

Develop a line listing:

Step 5: Step 5: Performing Descriptive Performing Descriptive

EpidemiologyEpidemiology

• Characterize an outbreak by time, place, and person.

Why?Why?

• By looking at the data carefully, you become familiar with them

• Provide a comprehensive description of an outbreak by portraying its trend over time, its geographic extent (place), and the populations (persons) affected by the disease.

• Assess description of the outbreak in light of what is known about the disease (usual source, mode of transmission, risk factors and populations affected, etc.) to develop causal hypotheses.

• To identify the likely period of exposure from To identify the likely period of exposure from an epidemic curve,an epidemic curve,

• 1. Look up the average and minimum 1. Look up the average and minimum incubation periods of the disease. This incubation periods of the disease. This information can be found in information can be found in Control of Control of Communicable Diseases in ManCommunicable Diseases in Man ..

• 2. Identify the peak of the outbreak or the 2. Identify the peak of the outbreak or the median casemedian case and count back on the and count back on the xx-axis -axis one average incubation period. Note the date.one average incubation period. Note the date.

• 3. Start at the earliest case of the epidemic 3. Start at the earliest case of the epidemic and count back the minimum incubation and count back the minimum incubation period, and note this date as well.period, and note this date as well.

TimeTime

• What is the exact period of the outbreak?What is the exact period of the outbreak?

• What is the probable period of exposure?What is the probable period of exposure?

• Is the outbreak likely common source or Is the outbreak likely common source or propagated?propagated?

PlacePlace

• What is the most significant What is the most significant geographic distribution of cases? geographic distribution of cases?

Place of residence? Workplace?Place of residence? Workplace?

• What are the attack rates?What are the attack rates?

PersonPerson

• What were the age and gender specific What were the age and gender specific attack rates?attack rates?

• What age and gender groups are at What age and gender groups are at highest and lowest risk if illness?highest and lowest risk if illness?

• In what other ways do the In what other ways do the characteristics of the cases differ characteristics of the cases differ significantly from those of the general significantly from those of the general population?population?

Interpreting an epidemic curveInterpreting an epidemic curve

• Interpretation of the epidemic curve can prove Interpretation of the epidemic curve can prove to be very helpful in determining the source of to be very helpful in determining the source of the outbreak.  the outbreak. 

Through review of the different patterns Through review of the different patterns illustrated in an epidemic curve, it is possible illustrated in an epidemic curve, it is possible to hypothesize:to hypothesize:

• how an epidemic spread throughout a how an epidemic spread throughout a population population

• at what point you are in an epidemic at what point you are in an epidemic • the diagnosis of the disease by establishing the the diagnosis of the disease by establishing the

potential incubation periodpotential incubation period

• When analyzing an epidemic curve, it is When analyzing an epidemic curve, it is important to consider the following factorsimportant to consider the following factors

to assist in interpreting an outbreak.to assist in interpreting an outbreak.

• the overall pattern of the epidemic the overall pattern of the epidemic

• the time period when the persons were the time period when the persons were exposed exposed

• if there any outliersif there any outliers

Point SourcePoint Source

• In a In a point source epidemicpoint source epidemic, persons , persons are exposed to the same exposure are exposed to the same exposure over a limited, defined period of time, over a limited, defined period of time, usually usually within one incubation periodwithin one incubation period.  .  The shape of this curve commonly The shape of this curve commonly rises rapidly and contains a rises rapidly and contains a definite definite peakpeak at the top, followed by a gradual at the top, followed by a gradual decline. decline.

The graph above illustrates an outbreak of gastrointestinal illness from a The graph above illustrates an outbreak of gastrointestinal illness from a single exposure. While there are outliers to this dataset, it is clear that single exposure. While there are outliers to this dataset, it is clear that there is an outbreak over a limited period of time, and the shape of the there is an outbreak over a limited period of time, and the shape of the curve is characteristic of one source of exposurecurve is characteristic of one source of exposure

Continuous Common SourceContinuous Common Source

• In a In a continuous common sourcecontinuous common source epidemic, exposure to the source is epidemic, exposure to the source is prolongedprolonged over an extended period of over an extended period of time and may occur over time and may occur over more than one more than one incubation period.incubation period. The down slope of the  The down slope of the curve may be curve may be very sharpvery sharp if the common if the common source is removed source is removed or gradualor gradual if the if the outbreak is allowed to exhaust itself.outbreak is allowed to exhaust itself.

The data above is from the well-known outbreak of cholera in London that The data above is from the well-known outbreak of cholera in London that was investigated by the "father of epidemiology," John Snow. Cholera was investigated by the "father of epidemiology," John Snow. Cholera spread from a water source for an extended period of time. Note that the spread from a water source for an extended period of time. Note that the typical incubation period for cholera is 1--3 days that the duration of this typical incubation period for cholera is 1--3 days that the duration of this outbreak was more than 1 month.outbreak was more than 1 month.

Propagated Propagated (Progressive Source)(Progressive Source)

• A A propagated (progressive source) epidemicpropagated (progressive source) epidemic occurs when occurs when a case of disease serves as a a case of disease serves as a source of infection for subsequent casessource of infection for subsequent cases and and those subsequent cases, in turn, serve as those subsequent cases, in turn, serve as sources for later cases. The shape of the curve sources for later cases. The shape of the curve usually contains a usually contains a series of successively series of successively larger peakslarger peaks, reflective of the increasing , reflective of the increasing number of cases caused by person-to-person number of cases caused by person-to-person contact, until the pool of susceptible is contact, until the pool of susceptible is exhausted or control measures are exhausted or control measures are implemented.implemented.

The graph above illustrates an outbreak of measles. The graph shows a The graph above illustrates an outbreak of measles. The graph shows a single common source (the index case), and the cases appear to increase single common source (the index case), and the cases appear to increase exponentially. Measles is caused by person-to-person contact. Its exponentially. Measles is caused by person-to-person contact. Its incubation period is typically 10 days but may be 7--18 days.incubation period is typically 10 days but may be 7--18 days.

Common Source Common Source QuestionsQuestions

• Is the outbreak from a single source or Is the outbreak from a single source or common source?common source?

• Is the disease spread from person to Is the disease spread from person to person?person?

• Is there continued exposure to a single Is there continued exposure to a single source?source?

Adapted from: Timmreck: An Introduction to Epidemiology

Propagated Source Propagated Source QuestionsQuestions:

• Is the outbreak from multiple sources or Is the outbreak from multiple sources or exposures?exposures?

• Is the outbreak airborne, behaviorally or Is the outbreak airborne, behaviorally or chemically caused and does it involve chemically caused and does it involve multiple events?multiple events?

• Are the sources of infection from Are the sources of infection from inapparent sources?inapparent sources?

• Is there a vector or animal reservoir Is there a vector or animal reservoir involved? involved?

Adapted from: Timmreck: An Introduction to Epidemiology

Duration of an epidemicDuration of an epidemic

• Is determined by:Is determined by:

– The number of susceptible persons who are The number of susceptible persons who are exposed to a source of infection and become exposed to a source of infection and become infected.infected.

– Period of time over which susceptible persons are Period of time over which susceptible persons are exposed to the source.exposed to the source.

– Minimum and maximum incubation periods for Minimum and maximum incubation periods for the disease.the disease.

Step 6: Step 6: Developing HypothesesDeveloping Hypotheses

• Should address all aspects of the Should address all aspects of the investigation: the investigation: the source source of the agent, of the agent, the the mode mode (and vehicle or vector) of (and vehicle or vector) of transmission, and the transmission, and the exposuresexposures that that caused the disease and any additional caused the disease and any additional time, person, and place factorstime, person, and place factors..

• First, consider what you know about the disease First, consider what you know about the disease itself: itself:

• What is the agent’s usual reservoir?What is the agent’s usual reservoir?• How is it usually transmitted?How is it usually transmitted?• What vehicles are commonly implicated?What vehicles are commonly implicated?• What are the known risk factors?What are the known risk factors?

In other words, simply by becoming familiar with In other words, simply by becoming familiar with the disease, you can, at the very least, “round the disease, you can, at the very least, “round up the usual suspects.”up the usual suspects.”

• Another useful way you can generate Another useful way you can generate hypotheses is to talk to a few of the case-hypotheses is to talk to a few of the case-patients,patients,

• In addition, investigators have sometimes In addition, investigators have sometimes found it useful to visit the homes of case-found it useful to visit the homes of case-patients and look through their patients and look through their refrigerators and shelves for clues.refrigerators and shelves for clues.

Step 7: Step 7: Evaluating HypothesesEvaluating Hypotheses

• Either by comparing the hypotheses Either by comparing the hypotheses with the established facts, or by using with the established facts, or by using analytic epidemiologyanalytic epidemiology to quantify to quantify relationships and explore the role of relationships and explore the role of chance.chance.

• Use the first method when the clinical, Use the first method when the clinical, laboratory, environmental, and/or laboratory, environmental, and/or epidemiologic evidence so obviously epidemiologic evidence so obviously supports the hypotheses that formal supports the hypotheses that formal hypothesis testing is unnecessarilyhypothesis testing is unnecessarily..

• In many other settings, however, In many other settings, however, the circumstances are not as the circumstances are not as straightforward. In those straightforward. In those instances, you should use analytic instances, you should use analytic epidemiology to test your epidemiology to test your hypotheses. The key feature of hypotheses. The key feature of analytic epidemiology is a analytic epidemiology is a comparison groupcomparison group..

Step 8: Step 8: Refining Hypotheses Refining Hypotheses

and Executing and Executing Additional StudiesAdditional Studies

• Epidemiologic studiesEpidemiologic studies

• Laboratory and environmental studiesLaboratory and environmental studies

Retrospective cohort

Food ate Did not eat

Ill well AR Ill well AR 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 restaurant A

Yes 30 36 66

No 10 70 80

Total 40 106 146

OR= 5.83

Step 9: Step 9: Implementing Control Implementing Control

and Prevention Measuresand Prevention Measures

• Aim control measures at the weak Aim control measures at the weak link or links in the chain of link or links in the chain of infectioninfection..

Source• Destroying contaminated foods, estroying contaminated foods, • sterilizing contaminated water, sterilizing contaminated water, • Destroying mosquito breeding sites. Destroying mosquito breeding sites. • Remove an infectious food handlerRemove an infectious food handler

Interrupting transmission or exposureInterrupting transmission or exposure • having nursing home residents with a particular having nursing home residents with a particular

infection “cohorted,” or put together in a separate infection “cohorted,” or put together in a separate area to prevent transmission to others. area to prevent transmission to others.

• wear insect repellent and protective clothing.wear insect repellent and protective clothing.

Reducing the susceptibility of the hostReducing the susceptibility of the host..• Immunization against rubella and malaria Immunization against rubella and malaria

chemoprophylaxis for travelerschemoprophylaxis for travelers

Step 10: Step 10: Communicating the FindingsCommunicating the Findings

• An oral briefing for local authorities An oral briefing for local authorities and and

• A written reportA written report.

Included in the report:

• Summarize relevant data, methods of collection, Summarize relevant data, methods of collection, analyses performed and interpretations.analyses performed and interpretations.

• Describe preventive and control measures Describe preventive and control measures implementedimplemented

• Describe effectiveness of control measuresDescribe effectiveness of control measures• Describe other impacts relevant to prevention and Describe other impacts relevant to prevention and

control.control.• Make recommendations regarding future Make recommendations regarding future

surveillance and control.surveillance and control.• Distribute report to others in disease control Distribute report to others in disease control

programs.programs.

The report:The report:

• blueprint for action.• 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 of contributing to the

knowledge base of epidemiology and public health.