Approach to Outbreak Investigations

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Approach to Outbreak Investigations

• Danae Bixler, MD, MPH
• Infectious Disease Epidemiology Program

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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?
  

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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

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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

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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

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10 Steps of Outbreak Investigation (CDC)

• 9. Implement control and prevention measures
• Should occur as soon as information is available
• 10. Communicate findings

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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

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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.

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Step 1 Prepare for Fieldwork

• Investigation
• Appropriate scientific knowledge, supplies,
  equipment
  

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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.)

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Incubation Periods for Suspect Infectious Agents
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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)

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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

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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.

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Step 3, 5 and 6

• Verify diagnosis.
• Do descriptive epidemiology and develop
  hypotheses.

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Open-Ended Interviews (3/1/01) N10 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

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Open-Ended Interviews (3/1/01) N10 persons who
called in sick

• Descriptive Epidemiology
• Onset late on 2/28 early AM and morning of 3/1

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Open-Ended Interviews (3/1/01) N10 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)

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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

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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.

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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

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And Back to Steps 3 and 5 Verify the diagnosis
and perform descriptive epidemiology

• Interviews allow refinement of
• Descriptive epidemiology outbreak curve (time)
• Diagnosis

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(No Transcript)
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Characteristics of Illness (N16)

• Headache
• 11 (69)
• Nausea
• 13 (81)
• Vomiting
• 13 (83)
• Avg. 5.3 episodes
• Fever
• 4 (25)

• Aches
• 10 (62)
• Chills
• 10 (62)
• Cramps
• 12 (75)
• Diarrhea
• 13 (72)
• Avg. 5.25 episodes

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Step 7 RR of illness for the exposure candied
sweet potatoes 0.69 (95 CI 0.13 to 3.56)
p1.0)
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Step 7 RR of illness for the exposure chocolate
cake 0.97 95 CI 0.46 to 2.03) p0.78
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Step 7 RR of illness for the exposure seafood
jambalaya 1.45 95 CI 0.70 to 2.98) p0.50
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Step 7 RR of illness for the exposure Mardi
Gras punch 4.9 95 CI 1.32 to 18.25) p0.004)
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Step 8 Refine the hypothesis

• How could the punch have become contaminated?

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Other data

• Nursing home outbreak (same week)
• Onsets consistent with person-to-person spread
• Background illness
• Community
• Family Practice Center

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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

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Homemade Ice Rings

• 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

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Step 9,10 Control Measures / Communication

• Contacted Regional Epidemiologist / Clinic
  Director March 3, 1030 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

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Timeline

• Thursday, March 1, 2001
• Notification approximately 300 PM
• Open-ended interviews
• Study design
• Friday, March 2, 2001
• Interviews using a standard questionnaire
• Data entry
• Analysis completed 1030 PM
• Saturday, March 3, 2001
• Phone interview of persons who made the punch
  800 AM
  
• Preliminary results shared with the regional
  epidemiologist and clinic director 1030 AM
  

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Step 8 The lab gets the last word

• Environmental Specimen
• Water sample from kitchen tap of household B
• () total coliforms
• () E coli

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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

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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.

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Limitations

• Incomplete response rate on the cohort study

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Step 9 Implement control and prevention measures

• The well was taken out of service.

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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

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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
  

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Conclusions

• 10 steps of outbreak investigation
• Conceptual
• Provide a logical progression for the
  investigation
  
• Can / should be taken out of order (with
  caution)