OUTBREAK INVESTIGATION

1. OUTBREAK INVESTIGATION

2. Definition • Occurrence of more cases of disease than expected • Nosocomial outbreak-any group of illnesses of common etiology occurring in patients of a medical care facility acquired by exposure of those patients to the disease agent while confined in such a facility.

3. Reasons to investigate • Control/prevention • Research opportunities • Training • Public, political, or legal concerns • Program considerations

4. Control/prevention • Where are we in the outbreak? • Goals will be different depending on answer(s) • Cases continuing to occur • Goal: prevent further cases • Assess population at risk, implement control measures • Outbreak appears to be coming to an end • Goal: prevent future outbreaks • Identify factors contributing to outbreak, implement measures to prevent similar events in the future

5. Research opportunities • Gain additional knowledge • Opportunity to study natural history of the disease • Newly recognized disease • Define natural history • Characterize the population at risk • Well recognized diseases • Assess impact of control measures • Usefulness of new epidemiology and laboratory techniques

6. Training • Requirements of an epidemiologist! • Diplomacy • Logical thinking • Problem solving ability and quantitative skills • Epidemiologic know-how • Judgment • Pair a new epidemiologist with an experienced epidemiologist

7. Public, political, or legal concerns • Sometimes override scientific concerns • Health department needs to be responsible and responsive to public concerns • Even if the concern has little scientific basis

8. Program considerations • Outbreak of disease targeted by a public health program • Reveal a weakness and opportunity to change or strengthen the program • Identify population that may have been overlooked • Failure of intervention strategy • Changes in the agent • Events beyond scope of the program

9. Steps of an Outbreak Investigation

10. 1. Preparation • Investigation • Scientific knowledge • Review literature • Consult experts • Sample questionnaires • Supplies • Consult with laboratory • Equipment • Laptop, camera etc.

11. Preparation cont. • Administration-assure personnel resources, funding • Travel arrangements (orders) • Approval • Personal matters • Consultation-make sure you know your role and its parameters • Lead investigator or just lending a hand? • Know who to contact when you arrive

12. 2. Establish existence of an outbreak • Is an outbreak truly occurring? • True outbreak • Sporadic and unrelated cases of same disease • Unrelated cases of similar unrelated disease • Determine the expected number of cases beforedeciding whether the observed number exceeds the expected number

13. Establish existence of an outbreak cont. • Comparing observed with expected • through surveillance records for notifiable diseases • hospital discharge data, registries, mortality statistics • data from other facilities, states, surveys of health care providers • community survey

14. 3. Verify the Diagnosis • Ensure proper diagnosis and rule out lab error as the bias for increased diagnosis • Review clinical findings, lab results • Summarize clinical findings with frequency distributions • Characterize spectrum of disease • Verify diagnosis • Develop case definition

15. Verify the Diagnosis cont. • See and talk with patients if at all possible • Better understand clinical features • Mental image of disease and the patients affected • Gather critical information • Source of exposure • What they think caused illness • Knowledge of others with similar illness • Common denominators • Helpful in generating ideas for hypothesis about etiology and spread

16. 4a. Establish a case definition • Case definition • Standard set of criteria for deciding whether an individual should be classified as having the health condition of interest • Includes clinical criteria and restrictions by time, place and person • Must be applied consistently and without bias to all persons under investigation • Must not contain an exposure or risk factor you want to test

17. 4a. Establish a case definition cont. • Classification • Definite (confirmed) • Laboratory confirmed • Probable • Typical clinical features without lab confirmation • Possible (suspected) • Fewer of the typical clinical features

18. 4a. Establish a case definition cont. • Early in investigation may use a “loose” case definition • Better to collect more than necessary so you don’t need to make repeat visits • Identify extent of problem and population affected • Generating hypotheses • Later when hypotheses are sharpened investigator may “tighten” case definition

19. 4b. Identify and count cases • Target health care facilities where diagnosis likely to be made • Enhanced passive surveillance e.g. letter describing situation and asking for reports • Active surveillance e.g. phone or visit facility to collect information • Alerting the public • Media alert to avoid contaminated food product and seek medical attention if symptoms arise

20. 4b. Identify and count cases cont. • OB population restricted and large proportion of cases are unlikely to be diagnosed e.g. on a cruise ship • Survey entire population • Always ask case-patients if they know of any others ill with the same symptoms

21. 4b. Identify and count cases cont. • Information to be collected about every case • Identifying information • Re-contact if additional questions come up • Notification of lab results and outcomes of investigation • Check for duplicate records • Map geographic extent • Demographics • Provide “person” characteristics for defining population at risk

22. 4b. Identify and count cases cont. • Information to be collected about every case cont. • Clinical findings • Verify case definition met • Chart time course • Supplemental date e.g. deaths • Risk factor information • Tailored to specific disease in question • Reporter information • Id of person making report

23. 4b. Identify and count cases cont. • Collection forms • Standard case report form • Questionnaire • Data abstraction form • Line listing • Abstraction of selected critical items from above forms • Contains key information

24. 5. Perform Descriptive Epidemiology • After collection of data characterize the outbreak by: • Time • Place • Person

25. Time • Epidemic curve • Histogram of the number of cases by their date of onset • Visual display of the outbreak’s magnitude and time trend • Where you are in the time course of the outbreak • Future course? • Probable time period of exposure • Helps in development of questionnaire focusing on that time period • Common source vs. Propagated

26. Epidemic Curve

27. Place • Geographic extent of problem • Clusters or patterns providing important etiologic clues • Spot maps • Where cases live, work or may have been exposed

28. Person • Determine what population at risk • Usually define population by host characteristics or exposure • Use rates to identify high-risk groups • Numerator = number of case • Denominator = number of people at risk

29. 6. Develop Hypotheses • Hypotheses should address • Source of the agent • Mode of transmission • Vector or vehicle • Exposure that caused disease

30. Develop Hypotheses • Generating the hypothesis • What do you know about the disease? • Reservoir, transmission, common vehicles and known risk factors • Talk to several case-patients • Use open ended questions • Ask lots of questions • Talk to local health department staff • Use descriptive epidemiology e.g. epi curve

31. 7. Test Hypotheses • Evaluate the credibility of your hypotheses • Compare with established facts • When clinical, lab, environmental and/or epi data undoubtedly support hypothesis • Use analytic epidemiology to quantify relationships and explore the role of chance • Cohort studies • Case control studies

32. Cohort studies start with an exposure and go forward to diseases. Drawing by: Nick Thorkelson

33. 7. Test Hypotheses cont. • Cohort • Small, well defined population • Contact each attendee and ask a series of questions • Ill Vs not ill • Look for source exposure • Attack rate is high among those exposed • Attack rate is low among those not exposed • Most of the cases were exposed, so that the exposure could explain most, if not all, of the cases • Relative risk = measure of association between exposure and disease

34. Case-control studies start with a disease and go back to exposures. Drawing by: Nick Thorkelson

35. 7. Test Hypotheses cont. • Case-control • Population not well defined • Case patients and comparison group (controls) questioned about exposure(s) • Compute measure of association = Odds Ratio • Quantify relationship between exposure and disease

36. 8. Refine hypotheses and do additional studies • Epidemiologic • When analytical epi unrevealing need to reconsider your hypotheses • Go back and gather more information • Conduct different studies • Laboratory • Additional tests • Environmental studies

37. 9. Implement Control /Prevention Measures • Implement control measures as soon as possible • May be aimed at agent, source, or reservoir • Short or long term

38. 10. Communicate the Findings • Orally within facility/community • Local health authorities and persons responsible for implementation of control and prevention measures • Written reports (consider publication) for planning, record of performance, legal issues, reference, adding to knowledge base