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Comprehensive Guide to Outbreak Investigations

Learn the 10 essential steps of outbreak investigation from preparation to communication of findings, including case identification, hypothesis development, and control measures implementation.

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Comprehensive Guide to Outbreak Investigations

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  1. Approach to Outbreak Investigations Danae Bixler, MD, MPH Infectious Disease Epidemiology Program

  2. 10 Steps of Outbreak Investigation (CDC) 1. Prepare for fieldwork • Research the disease • Make administrative arrangements • Clarify your role 2. Establish the existence of an outbreak • Does the observed number of cases exceed the expected number? WVDHHR/BPH/OEHP/DSDC/IDEP

  3. 10 Steps of Outbreak Investigation (CDC) 3. Verify the diagnosis • Speak directly with persons who are affected 4. Define and identify cases • Establish a case definition • Identify and count cases • Line listing WVDHHR/BPH/OEHP/DSDC/IDEP

  4. 10 Steps of Outbreak Investigation (CDC) 5. Describe and orient the data in terms of time, place and person • Outbreak curve • Map • Identify demographic and other characteristics of persons at risk 6. Develop hypotheses • Open-ended and wide-ranging interviews with a few people WVDHHR/BPH/OEHP/DSDC/IDEP

  5. 10 Steps of Outbreak Investigation (CDC) 7. Evaluate hypotheses • Comparison: hypotheses with established facts • Analytic epidemiology • Cohort studies (RR; 95% CI) • Case-control studies (OR; 95% CI) 8. Refine hypotheses and carry out additional studies WVDHHR/BPH/OEHP/DSDC/IDEP

  6. 10 Steps of Outbreak Investigation (CDC) 9. Implement control and prevention measures • Should occur as soon as information is available 10. Communicate findings WVDHHR/BPH/OEHP/DSDC/IDEP

  7. 3/1/2001: Illness in 28 of 60 staff of a Family Medicine Clinic • Predominant symptom = vomiting • Onsets: late PM of February 28 and early AM of March 1, 2001 • Physicians, nurses, residents • Staff had eaten three meals in common: • Mon: catered meal of Heavenly Ham • Tue: Mardi-Gras pot-luck • Wed: food from Subway WVDHHR/BPH/OEHP/DSDC/IDEP

  8. Step 2: Establish the existence of an outbreak • Occurrence of more cases of disease than expected in a given area or among a specific group of people over a particular period of time. WVDHHR/BPH/OEHP/DSDC/IDEP

  9. Step 1: Prepare for Fieldwork • Investigation: • Appropriate scientific knowledge, supplies, equipment WVDHHR/BPH/OEHP/DSDC/IDEP

  10. Vomiting as a Chief Complaint • Viral gastroenteritis • Rotavirus (infant) • Norovirus (older child / adult) • Food poisoning due to pre-formed toxin • Staphylococcus aureus • Bacillus cereus • Non-infectious (Sb, As, Cd, Cu, Fl, Zn, etc.) WVDHHR/BPH/OEHP/DSDC/IDEP

  11. Incubation Periods for Suspect Infectious Agents WVDHHR/BPH/OEHP/DSDC/IDEP

  12. Step 1 & 9: Prepare for Fieldwork and Implement Control Measures • Administration • Make travel and coverage arrangements • Consultation (roles) • Collaboration on all steps (state / regional epi / LHD) • Work restriction for ill health care workers • ICP involvement • LHO involvement • IDEP (consultative role) WVDHHR/BPH/OEHP/DSDC/IDEP

  13. Step 3: Verify the Diagnosis • Through effort of the Regional Epidemiologist: • Routine stool cultures submitted through the hospital • Stool for Norovirus submitted to the CDC WVDHHR/BPH/OEHP/DSDC/IDEP

  14. Step 4: Establish a case definition and identify and count cases • 4a) Establish a case definition: • Initial case definition: persons employed by or assigned to the Family Medicine Clinic who called in sick on March 1, 2001 • 4b) Identify and count cases: • Twenty-eight individuals were identified. WVDHHR/BPH/OEHP/DSDC/IDEP

  15. Step 3, 5 and 6 Verify diagnosis. Do descriptive epidemiology and develop hypotheses.

  16. Open-Ended Interviews (3/1/01) N=10 persons who called in sick • Verify diagnosis: • Symptoms: • Sudden onset of profuse vomiting and diarrhea • Systemic symptoms, including headache, arthralgias, myalgias, weakness • Recovery (or near recovery):12 hours WVDHHR/BPH/OEHP/DSDC/IDEP

  17. Open-Ended Interviews (3/1/01) N=10 persons who called in sick • Descriptive Epidemiology: • Onset: late on 2/28; early AM and morning of 3/1 WVDHHR/BPH/OEHP/DSDC/IDEP

  18. Open-Ended Interviews (3/1/01) N=10 persons who called in sick • Hypothesis generation: • No common events outside of work • Attendance at: • Monday luncheon (2/26) – 3 (30%) • Mardi Gras pot luck (2/27) – 10 (100%) • Wednesday lunch (2/28) – 7 (70%) WVDHHR/BPH/OEHP/DSDC/IDEP

  19. Step 7: Evaluate hypotheses • Regional epidemiologist obtained the menu for the Mardi Gras luncheon • Questionnaire constructed (state) • Interview of a convenience sample: • Recovered / well individuals on-site • Local/ regional public health personnel • Ill individuals by phone • State staff WVDHHR/BPH/OEHP/DSDC/IDEP

  20. Back to Step 4: • Case: individual in attendance at the Mardi Gras luncheon (2/27/01) with illness characterized by vomiting or two or more episodes of diarrhea, and onset on or after February 28, 2001 • Control: individual in attendance at the Mardi Gras luncheon with no symptoms of illness the week of 2/26/02. WVDHHR/BPH/OEHP/DSDC/IDEP

  21. Study Population • 39 interviews • Exclusions: • 1 person ill, but did not meet the case definition • 1 did not attend the dinner • 3 had onset prior to 2/28/02 • Final population: N = 34 • 16 cases • 18 controls WVDHHR/BPH/OEHP/DSDC/IDEP

  22. And Back to Steps 3 and 5: Verify the diagnosis and perform descriptive epidemiology • Interviews allow refinement of • Descriptive epidemiology: outbreak curve (time) • Diagnosis WVDHHR/BPH/OEHP/DSDC/IDEP

  23. WVDHHR/BPH/OEHP/DSDC/IDEP

  24. Aches 10 (62%) Chills 10 (62%) Cramps 12 (75%) Diarrhea 13 (72%) Avg. 5.25 episodes Headache 11 (69%) Nausea 13 (81%) Vomiting 13 (83%) Avg. 5.3 episodes Fever 4 (25%) Characteristics of Illness (N=16) WVDHHR/BPH/OEHP/DSDC/IDEP

  25. Step 7: RR of illness for the exposure candied sweet potatoes = 0.69; (95% CI 0.13 to 3.56); p=1.0) WVDHHR/BPH/OEHP/DSDC/IDEP

  26. Step 7: RR of illness for the exposure chocolate cake = 0.97; 95% CI 0.46 to 2.03); p=0.78 WVDHHR/BPH/OEHP/DSDC/IDEP

  27. Step 7: RR of illness for the exposure seafood jambalaya = 1.45; 95% CI 0.70 to 2.98); p=0.50 WVDHHR/BPH/OEHP/DSDC/IDEP

  28. Step 7: RR of illness for the exposure Mardi Gras punch = 4.9; 95% CI 1.32 to 18.25); p=0.004) WVDHHR/BPH/OEHP/DSDC/IDEP

  29. Step 8: Refine the hypothesis How could the punch have become contaminated?

  30. Other data • Nursing home outbreak (same week): • Onsets consistent with person-to-person spread • Background illness • Community • Family Practice Center WVDHHR/BPH/OEHP/DSDC/IDEP

  31. March 3, 2001: How was Mardi Gras punch made? • Bottled grape juice • Unsweetened canned pineapple juice • Sprite • Homemade ice rings • Water • Sliced fruit • Doubloons • Sliced fruit • Mixed in bowl ‘found on top of refrigerator’ WVDHHR/BPH/OEHP/DSDC/IDEP

  32. Ice ring –household A City water Person who made it had ‘GI distress’ the day of the event Ice ring –household B Well water All members of family of this household sequentially had similar illness over the previous month Homemade Ice Rings WVDHHR/BPH/OEHP/DSDC/IDEP

  33. Step 9,10: Control Measures / Communication • Contacted Regional Epidemiologist / Clinic Director March 3, 10:30 AM • Preliminary results of analysis suggest Mardi Gras Punch is the most likely culprit • No evidence for contamination of commercial food product • Recommend: exclusion of ill persons and good handwashing WVDHHR/BPH/OEHP/DSDC/IDEP

  34. Timeline • Thursday, March 1, 2001 • Notification; approximately 3:00 PM • Open-ended interviews • Study design • Friday, March 2, 2001 • Interviews using a standard questionnaire • Data entry • Analysis completed 10:30 PM • Saturday, March 3, 2001 • Phone interview of persons who made the punch 8:00 AM • Preliminary results shared with the regional epidemiologist and clinic director 10:30 AM WVDHHR/BPH/OEHP/DSDC/IDEP

  35. Step 8: The lab gets the last word • Environmental Specimen: • Water sample from kitchen tap of household B: • (+) total coliforms • (+) E coli WVDHHR/BPH/OEHP/DSDC/IDEP

  36. Step 8: The lab gets the last word • Human Specimens • 12 stool specimens • Negative for Salmonella, Shigella, Yersinia and Campylobacter in the clinical laboratory • 10 stool specimens • PCR positive for NLV at CDC • Identical nucleotide sequence WVDHHR/BPH/OEHP/DSDC/IDEP

  37. Conclusion • Mardi Gras punch was the source of an outbreak affecting approximately half the staff of a family medicine center • Contamination likely introduced by: • Fecally-contaminated well water, OR • Hands of one of the people who prepared the punch; OR • (possibly) residual environmental contamination in household B. WVDHHR/BPH/OEHP/DSDC/IDEP

  38. Limitations • Incomplete response rate on the cohort study WVDHHR/BPH/OEHP/DSDC/IDEP

  39. Step 9: Implement control and prevention measures The well was taken out of service. WVDHHR/BPH/OEHP/DSDC/IDEP

  40. Step 10: Communicate findings • Written outbreak report distributed with laboratory results approximately one month later to: • LHD • Clinic Director • OLS • Environmental Health • Regional Epidemiologist • CDC WVDHHR/BPH/OEHP/DSDC/IDEP

  41. Conclusions • NLV outbreaks are: • Good practice • Important to investigate because of the total burden of disease • Cause of significant disability and death, especially in vulnerable populations • Challenging to investigate because laboratory diagnosis is not readily available WVDHHR/BPH/OEHP/DSDC/IDEP

  42. Conclusions • 10 steps of outbreak investigation • Conceptual • Provide a logical progression for the investigation • Can / should be taken out of order (with caution) WVDHHR/BPH/OEHP/DSDC/IDEP

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