Feldmand. Module 6 - Outbreak investigation Flashcards
What is an outbreak?
- More cases of a particular disease than expected in a given area, or among a specific population, over a particular period of time
- The number of expected cases varies
Why investigate outbreaks?
• Protect the community’s health
– Remove the source of infection
– Provide prevention guidance
– Address concerns in the community
• Increased knowledge base
– Completing the clinical picture
– Identify new etiologic agents
– Identify new routes of transmission
Steps in an Outbreak Investigation
- Prepare for field work
- Establish the existence of an outbreak
- Verify the diagnosis
- Define and identify cases
- Describe and orient the data in terms of time, place, and subject
- Develop hypotheses
- Evaluate hypotheses
- Refine hypotheses and carry out additional studies
- Implement control and prevention measures
- Communicate findings
- Prepare for field work
An Outbreak in the Clinic…
• May be limited in scope and impact • Preparation includes – Coordination with clinic staff – Data collection equipment assembled: • Computer • Paper • Pencil • Calculator – Records pulled
- Prepare for field work
An Outbreak in the Community…
• Notification of people who “need to know” • Coordination with investigation partners • Assembly of equipment – Data collection – Communications – Clothing – Laboratory • Travel preparation
- Prepare for field work
Probable Partners
- Clinicians
- Epidemiologists
- Laboratory scientists
- Environmental scientists
- Infection control practitioners
- Decision makers
- Communications experts
- Law enforcement?
- Others depending on context
- Establish the existence of an outbreak
• Need to determine (quickly) if current rate exceeds background
• Many infectious diseases are reportable
• Beware of artifactual changes
– Change in reporting practices
– New diagnostic tests
– Increased clinician awareness
- In the clinic, pull records for same period last year and compare
- Survey clinicians
- Acquire absentee records
- May need to proceed with investigation because of local pressures even if unsure outbreak exists
- Verify the diagnosis
Confirm the clinical diagnosis
• Confirm by standard laboratory techniques
• If possible, visit the laboratory and verify in person
• Do not need confirmation on every patient
• Sometimes new agent
– Proceed despite lack of diagnosis
- Define and identify cases
Case Definition
• Imperative to be systematic
• Consists of clinical criteria and other criteria to specify subject, place, and time
• Use simplest and most objective criteria
• Apply definition equally and without bias
• Degrees of certainty may be included (e.g., confirmed, probable, suspect)
- Describe and orient the data in terms of
time, place, and subject
Orient the Data in Terms of TIME
• Epidemic “epi” curve: Graphic plotting of distribution of cases (y axis) by time of onset (x axis), using an appropriate time interval
• Can assess:
– Magnitude of the outbreak
– Possible mode of spread
– Duration of the epidemic
• A time interval of 1⁄4 to 1⁄2 an incubation period works best
Types of epidemic curves
Point Source Epidemic
Continuous Common Source
Propagated Epidemic
Steps in Creating an Epidemic Curve
- Identify date of onset for first case
- Set the time interval
- Create X-axis lead and end periods
- Assign area equal to one case to Y-axis • Plot cases
- Mark critical events in the outbreak
- Add labels
Orient the Data in Terms of PLACE
• Mapping can provide clues regarding – Source of agent
– Nature of exposure
• Dissemination of pathogens and who is at risk may be affected by
– Water supply
– Milk distribution routes
– Sewage disposal outflows – Prevailing wind currents
– Et cetera
Orient the Data in Terms of SUBJECT
• Knowing
– Number of cases
– When they became ill – Where they became ill – General characteristics
will help in generating hypotheses and targeting prevention
- Develop hypotheses
Hypothesis Generation
• Probably have good sense of source by this point in investigation
• Hypothesis generating questionnaires
– For foodborne outbreaks will ask about food consumption patterns in general
• Consider “new” sources of infection but don’t forget the “usual” sources
Types of Epidemiologic Investigations
• Case-control studies – Identify cases – Identify appropriate control group – Collect exposure data on both groups – Compare
• Cohort studies
– Identify a group (cohort) based on exposure
– In that group, collect information on exposure and case status
– Compare
In both types of studies, the same information is collected for both sick and well subjects
Statistical tests are used to compare the two populations with respect to various exposures
Interpreting Measures of Association
- TheORortheRRwillbe>1.0whenthe exposure being assessed is a RISK FACTOR
- TheORortheRRwillbe<1.0whenthe exposure being assessed is a PROTECTIVE FACTOR
- WhatdoesORorRR=1mean?
Disseminate Results
• Written report – Document for action – Record of performance – Document for potential medical / legal issues – Publication in peer-reviewed journal to target audience • Community presentations • Presentations to policy makers
Challenges in Outbreak Investigations
- Completeness and consistency of various data sources
- Small sample size
- Collection of samples “after the fact”
- Publicity
- Reluctance to participate
- Conflicting pressures
- Jurisdictional issues
- Logistical issues
