Outbreak Investigation

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Outbreak Investigation
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Objectives

• Determine if an outbreak is occurring
• Characterise the outbreak
• Identify additional cases
• Identify causative agent
• Identify the source
• Initiate steps to terminate transmission
• Learn for the future

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Surveillance for disease

• Certain infectious diseases pose significant
  threats to the health of the public
• It is important that public health know about
  them
• States, federal and international health
  authorities develop lists of reportable disease.

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Surveillance

• Physicians, hospitals and clinical laboratories
  are required to report, usually within a
  specified time period
• Serious diseases are often categorized separately
• Basic functions of communicable disease control
  at the state and local levels is the gathering
  and analysis of reportable disease data

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Examples of Reportable Diseases in Michigan

• Enteric diseases
• Salmonellosis, shigellosis, Campylbacter,
  hepatitis A
• Sexually Transmitted Diseases
• Syphilis, gonorrhea, HIV
• CNS diseases
• Bacterial and viral meningitis, Arboviral WNV,
  EEE
• Vaccine Preventable Diseases
• Measles, mumps, diphtheria, polio, hepatitis B

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Collection and analysis of data

• LHD receives reports
• LHD logs and reviews data
• Number of cases within a specific time period e.g
  a week
• Geographic distribution of cases
• Determination if the cases need further
  investigation
• LHD send s reports to MDCH via LHDSurv (soon to
  be replaced by MDSS)
• MDCH sends reports to CDC via NETSS

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Data analysis

• Need to know background level
• Track weekly occurrence of disease over a number
  of years to establish an average number of cases
  per week for that particular week
• Need to determine when the number of cases is
  above the background

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Establish when an outbreak is occurring

• For some diseases, a single case is an outbreak
  e.g measles, smallpox
• In others, use the background as a threshold, but
  take into account other variables
• e.g. is the gender or age frequency similar to
  previous years,
• are the cases clustered geographically

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• Sometimes the illness is reported before a
  disease is identified
• Many calls from physicians and the public about
  persons becoming ill with diarrhea, vomiting
• Using symptoms try to ascertain the likely
  disease
• Get specimens to send to the laboratory if not
  already obtained

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Investigation

• Characterize the data that you have
• Try to identify other similar cases occurring
• How would you do this?
• Fax to ERs
• Notices on electronic boards

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Case definition

• What would you need in a case definition
• Time
• Place
• Symptoms
• E.g. All persons who experienced vomiting,
  diarrhea, nausea or abdominal cramping and who
  ate at Joes Greasy Spoon between Dec 10 and Dec
  14.

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Data Analysis

• Determine which cases fit the case definition
• Analyse by variables
• Age
• Gender
• Race
• Geography
• Risk factor
• Food
• Drink
• Behavior e.g. smoking, outdoor activity, attends
  day care

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Data analysis

• Frequencies
• Incubation periods
• Laboratory tests
• Epidemic curve
• Attack rates
• By person
• By place
• By risk factor

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Hypothesis

• Using the data analysis, formulate hypotheses
• Select the hypothesis which fits the picture
• Hypothesis should address
• Source of the disease
• Etiologic agent
• Method of transmission
• Control methods

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Hypothesis generation

• Hypotheses can be generated at any time during
  the investigation, and refined as more data
  becomes available. But beware of making false
  assumptions
• But dont leave it too late, as hypothesis
  generation should lead to control measures

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Hypothesis testing

• Laboratory results can confirm the etiologic
  agent
• Prevent further consumption of implicated food
  can confirm the transmission/source
• Data analysis
• Case control study using matched or unmatched
  controls.

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Control methods

• Initiate control methods based upon hypothesis

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Report of findings

• So others can learn from our experience