Outbreak Investigations: Sources of Information An Introduction • “Steps of an Outbreak Investigation,” U.S. Centers for Disease Control and Prevention, www.cdc.gov/excite/classroom/outbreak_steps_htm Angela Faust-Becker, MPH • “Investigating an Outbreak,” P. Pontones, Indiana Division of Epidemiology State Department of Health, http://www.state.in.us/isdh/bioterrorism/epidemiology.htm Alabama Department of Public Health • L. Gordis, Epidemiology, Second Edition, Philadelphia: W.B. Saunders, 2000.

Disease Outbreaks: A Combined Effort Recent Examples • Epidemiologists • State health • Laboratory departments • Measles workers • Federal agencies • Shigella • Environmental • Public information health specialists officers • E. coli O157:H7 • Healthcare providers • The media • West Nile Virus • Local health departments

The Ten Steps in an The Ten Steps in an Outbreak Investigation Outbreak Investigation 1. Prepare for fieldwork 6. Develop hypotheses 2. Establish the existence of an outbreak 7. Evaluate hypotheses 3. Verify the diagnosis 8. Refine hypotheses 4. Define and identify cases and carry out additional studies 9. Implement control & prevention 5. Describe and orient the data measures in terms of time, place, person 10. Communicate findings

1 Establish the Existence Review Clinical Data and of an Outbreak Establish Case Definition • Review signs and symptoms, onset • More cases than normally expected dates, common exposures, obtain in the population at a given time lab results, if any • Work with area healthcare • Case definition = criteria to decide practitioners, pharmacies, local whether person is part of outbreak health department, etc. to determine – time, place, person if observed number is greater than – clinical criteria expected • Can change during investigation as more information becomes available

Case Classification Identify/Count Cases and Identify Population At Risk • Confirmed: laboratory • Initiate active surveillance verification • Use questionnaires to survey • Probable: clinical criteria present hospitals, ERs, clinics, but w/o lab verification laboratories and known cases –Administer as soon as possible • Suspected: only some clinical –Both cases and controls criteria present • Review surveillance data

Describe & Analyze Outbreak Three Types of Outbreak

• Describe outbreak by • Point source – time – place • Continuous common source – person • Person-to-person • Plot number of cases by onset date • Index case: first case of outbreak • Determine time course and future course, exposure period

2 Point Source Outbreaks Continuous Common Source

• Single location with all exposed at one time • Rather than single point, source is common (e.g., water) • All cases occur within one incubation period • May begin suddenly or gradually • Outbreak stops unless secondary • Curve has gradual or steep spread upslope, plateau, trickling down • Epidemic curve has steep upslope, followed by gradual down slope slope, and may repeat

Person-to-Person Outbreaks Key Epidemiological Tools: Spot Maps • Disease is transmitted from person to person • One spot = case • Secondary cases appear one incubation period after peak of • Population first wave density not reflected • Taller successive waves of cases

Analytic Epidemiology Cohort Studies

• Quantify relationships between • Defined population exposures and disease • Can contact all in timely manner • Approaches • Calculate attack rate –Cohort study –Case-control study • Calculate risk ratio to determine risk of contracting illness from exposure

3 Attack Rates Risk Ratios

Calculate for those ill and exposed and those ill and not exposed attack rate of ill and exposed number of people at risk who attack rate of ill, not exposed develop illness >1.0 = increased risk for exposed total number of people x100 at risk 1.0 = equal risk <1.0 = decreased risk for exposed

Case-Control Studies Odds Ratios

• Population not defined • Cross multiply and divide • Looks at diseased and non- a b ad diseased c d bc • Calculate odds ratio to determine likelihood of contracting illness >1.0 = increased likelihood from exposure 1.0 = same likelihood <1.0 = decreased likelihood

Conclusion Selected Control Measures • Ten key steps in an outbreak investigation – Prepare for fieldwork • Get the word out to the public – Establish existence of outbreak – Verify diagnosis • Post-exposure prophylaxis/treatment – Define and identify cases – Describe and orient data – Develop hypotheses • Recalling/destroying contaminated – Evaluate hypotheses food – Refine hypotheses – Implement control & prevention measures • Cleaning or closing an establishment – Communicate findings • Education • A combined effort involving many professions, fields and disciplines

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