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Pining for a Common Source

Pining for a Common Source. SELF-PACED EXERCISE. Developed by the New York State Integrated Food Safety Center of Excellence ( NYCoE ). Course Objectives:. After completing this case study, you will be able to: Explain what constitutes a foodborne illness cluster/outbreak

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Pining for a Common Source

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  1. Pining for a Common Source • SELF-PACED EXERCISE Developed by the New York State Integrated Food Safety Center of Excellence (NYCoE)

  2. Course Objectives: After completing this case study, you will be able to: • Explain what constitutes a foodborne illness cluster/outbreak • Describe the steps in a foodborne illness outbreak investigation • Identify the roles and responsibilities of the outbreak investigation team • Epidemiology, environmental health, laboratory and communications • Understand the importance of information sharing during an outbreak investigation • Draw and interpret an epidemic curve • Establish and refine an outbreak case definition • Describe the types of epidemiologic tools used during an outbreak investigation • Describe the purpose of conducting environmental assessments/traceback investigations • Describe the types of laboratory techniques used during an outbreak investigation • Explain how to summarize and communicate outbreak investigation findings


  4. Routine Disease Surveillance • During the first week of September, five laboratory reports of SalmonellaEnteritidis are reported to Empire County in Liberty (LB) State from local laboratories • In LB, physicians, hospitals and laboratories are required to report confirmed/suspected Salmonella infections to the local health jurisdiction where the patient resides • This number of laboratory reports of S. Enteritidis is not unusual within the given time period and not considered above baseline for the county

  5. Routine Case Follow-up • Epidemiologists at Empire County investigate the Salmonella cases following standard procedures • Each case is contacted to complete an initial interview using a standardized Salmonella questionnaire which collects information on exposures preceding their illness • All clinical, laboratory, and exposure data are entered into a secure LB State Health Department (LBHD) electronic surveillance database

  6. Routine Cluster Analysis • Completed interview questionnaires are reviewed and compared by Empire County epidemiologists to identify: • Commonalties (e.g., household contacts, attended same party or event) • High-risk exposure settings (e.g., cases that work/reside in a sensitive setting such as a restaurant, daycare or healthcare facility)

  7. Initial PFGE Report from the Laboratory • On Friday, September 16, an epidemiologist from the LBHD Communicable Disease Program was notified that the LB State Public Health Laboratory (LBPHL) had 2 S. Enteritidis infections (from the 5 cases reported in early September) with matching pulsed-field gel electrophoresis (PFGE) patterns in Empire County • The assigned XbaI pattern is JEGX01.0008

  8. Note: In LB, hospital and commercial laboratories are required to submit all Salmonella isolates to the LBPHL where confirmation, serotype determination, PFGE and whole genome sequencing (WGS) are completed

  9. Question 1: You are an epidemiologist at LBHD • What additional laboratory information would you request from LBPHL?

  10. Q.1 Answer • The epidemiologist places an inquiry to LB State Lab to determine if PFGE pattern JEGX01.0008 is common or rare • The epidemiologist will also request that a PulseNet search be performed to identify cases from other states that may have a matching PFGE pattern • LB State Lab normally reviews PulseNet data for the prior 60 days, but this may vary by state or disease

  11. Question 2: You are an epidemiologist at LBHD • What steps would you take next?

  12. Q.2 Answer • The epidemiologist decides as a first step to review both patient case reports to identify dates of onset illness and any commonalities or rare exposures among the 2 cases • The epidemiologist will also conduct a quick literature search to determine what products, if any, have been implicated in past outbreaks involving: • S. Enteritidis, in general • S. Enteritidis pattern JEGX01.0008, in particular

  13. Question 3: • What type of questionnaire do you use in your jurisdiction? • What type of information is gathered?

  14. Q.3 Answer • Options may include: • a lengthy, hypothesis generating questionnaire • a short questionnaire to assess attendance at high-risk settings • a short questionnaire that focuses on exposures commonly identified to be the source of Salmonella • Type of questionnaire used will dictate what information is obtained and how it can be used in an investigation

  15. Review of Initial Case Reports • Case 1 is a 5-year-old male from Empire County • Illness onset: 8/31 • Symptoms: vomiting, diarrhea and fever • Duration of illness: approximately 7 days • Case 2 is a 16-year-old female from Empire County • Illness onset: 9/9 • Symptoms: nausea, vomiting, diarrhea and headache • Duration of illness: approximately 5 days

  16. Comparisons and Commonalities • Cases were not household contacts • Cases did not attend the same school • Neither case attended large gatherings during their incubation period

  17. Comparisons and Commonalities • Both cases had exposure to: • Commonly consumed items (chicken, tomatoes, eggs) • Dining out (different locations) • Supermarket Chain W (different store locations) • Chain W is an extremely popular supermarket chain which has multiple locations in the region • Household pets

  18. Initial Investigation – Laboratory Findings • Late Friday afternoon, LBPHL verifies that there are no other matching isolates in other states at this time • LBPHL also reports that PFGE pattern JEGX01.0008 had never been previously reported in LB State

  19. Initial Investigation – Communications • The epidemiologist prepares a summary to send to the individuals on their investigation team to inform them of the cluster investigation and the action steps underway

  20. Question 4: • Who might be part of the investigation team? • In one sentence, describe the role of each team member

  21. Q.4 Answer • Laboratory Investigator • focuses on testing clinical specimens and environmental/food samples, and on analyzing/interpreting test results • Environmental Health Investigator • focuses on the contaminated food and/or food preparation site, water, and environmental sources • Epidemiology Investigator • focuses on cases, developing and testing hypotheses, and case finding to identify and describe clusters • Public Health Nurse • focuses on patient and public health education

  22. The Basic Investigation Team • At LBHD, epidemiologists and public health nurses often play overlapping roles • Members of this team may also be comprised of staff from various levels of government (county/local health department, state health department, etc.)

  23. Question 5: • Would you involve all these team members right now? • If not, who would you involve?

  24. Q5. Answer • The epidemiologist shares the cluster summary with the members on their investigation team from: • LB State Health Department (LBHD) epidemiology and environmental health staff • LB State Public Health Laboratory (LBPHL) • Empire County (epidemiologists/public health nurses) • Optionally: Empire County environmental health (if warranted, based on strength of food/event association)

  25. Question 6: • What are your next steps?

  26. Q.6 Answer • Right now, no commonalities, no other PFGE matching isolates – so, with a sigh of relief, the epidemiologist leaves their office at 5PM - very pleased to not have to stay late and spend the entire weekend following up on this investigation Probably nothing to worry about…!


  28. The Cluster Grows… • On Monday September 26th, LBHD is notified that LBPHL just confirmed 7 additional cases with PFGE-matching S. Enteritidis pattern JEGX01.0008 • 6 of the 7 new cases reside in Empire County • The 7th case resides in Apple County (in LB State)

  29. Case Review • The epidemiologist compares the interview forms for all 9 LB cases to date • 2 case interviews are pending • 6/7 cases reported shopping at various locations of Chain W in Empire and Apple Counties, where the chain is a popular place to shop

  30. Case Review • None are members of the same household • None work or reside in a sensitive setting • No common restaurants or event exposures • Food history exposures are extensively reviewed to identify the food items consumed by 50% or more of the cases • Chicken, tomatoes, eggs and grapes

  31. Case Review Note: Typically any exposure reported by 50% or more of the cases could be considered a possible hypothesis to explore further

  32. Question 7: • Describe how this situation could be defined as a cluster AND an outbreak? • Provide an explanation for your choice

  33. Q.7 Answer • At this point, the investigation would be defined as a cluster • To classify cases as part of an outbreak, there must be a common exposure among the cases, like dining at the same restaurant, attending the same catered event, or eating the same food item • What about shopping at Chain W as a commonality?

  34. Q.7 Answer • While we have identified multiple PFGE-matching cases, at this point we have not been able to identify a link among the cases other than shopping at Chain W • Some may consider this enough of a commonality to be an outbreak, however, the investigators in this cluster did not • Chain W was the most popular grocery store chain in the area where the cases were occurring

  35. Question 8: • Who would you notify about this growing investigation?

  36. Q.8 Answer • The epidemiologist notifies members of their investigation team, which includes: • LBHD – epidemiology, environmental health and leadership staff (Department Directors) • LBPHL • Epidemiologists/public health nurses and environmental health investigators at Empire County Health Department and Apple County Health Department

  37. Q.8 Answer • The epidemiologist decides it is too early in the investigation to communicate with: • The public (via a press release or public notification) • USDA or the FDA as a contaminated product has not been suspected or confirmed • The epidemiologist may or may not choose to notify CDC at this time • Because no other PFGE-matching cases have been identified outside of LB State, the epidemiologist decides not to notify the CDC to alert them about this ongoing investigation

  38. Cluster vs. Outbreak? • The epidemiologist shares the new case count and exposure summary with the investigation team, and classifies the cases as a cluster, not an outbreak • Although multiple PFGE-matching cases have been identified, no common associations among cases have been identified from review and comparisons of initial case questionnaires

  39. Epidemic Curve • The epidemiologist draws an epidemic curve of the cluster hoping to shed some light about how the Salmonella has spread and to provide a visual representation of the cluster • The shape of the epidemic curve can reveal the outbreak’s likely mode of spread

  40. Point Source Outbreak • All or the majority of cases occur within one incubation period • The short duration of the outbreak suggests a single “point” of exposure event • Cases rise rapidly to a peak and fall off gradually • Examples include a GI outbreak following a wedding reception https://www.med.uottawa.ca/sim/data/Pub_Infectious_e.htm#epi_curves

  41. Continuing Common Source Outbreak • Cases occur over a prolonged period of time, spanning multiple incubation periods • Cases are exposed to the same source but exposure is ongoing over a period of weeks (or more) • The curve tends to rise gradually and may plateau • Examples include consumption of a contaminated, commercially distributed shelf-stable food like dried cilantro https://www.med.uottawa.ca/sim/data/Pub_Infectious_e.htm#epi_curves

  42. Propagated/Progressive Source Outbreak • Person-to-person transmission, whereby a single infected case (index case) infects a group of others (initial wave), who then go on to infect others (secondary wave), etc. • The curve presents as a series of progressively taller peaks, with peaks typically occurring one incubation period apart https://www.med.uottawa.ca/sim/data/Pub_Infectious_e.htm#epi_curves

  43. Epidemic Curve • The epidemiologist creates a line list of illness onset dates for S. Enteritidis cases, pattern JEGX01.0008 Question 9: • Use this data to draw an epidemic curve

  44. Hints for Creating Epidemic Curves • The epidemic curve is a histogram showing the number of outbreak associated cases by their time of onset • Include a brief, descriptive title (including place and time) • The x/y axis should be clearly labeled: • The x-axis represents the date or time of illness onset among cases; the unit of time is usually 1/4 to 1/3 of median incubation period • The typical incubation period for Salmonella is 2-3 days • The y-axis shows the number of ill cases

  45. Q.9 Answer: Epidemic Curve Illness Onset for S. Enteritidis Cases, Pattern JEGX01.0008 during August - September 2017 Number of Cases Question 10: • What does the epidemic curve tell us about the likely mode of transmission? Illness Onset Date

  46. Q.10 Answer • The epidemic curve is most consistent with a continuous common source outbreak • Cases do not all occur within one incubation period, thus decreasing the likelihood of a point source outbreak • The epidemic curve suggests that our cases were exposed to the same source but exposure was prolonged over a period of days, weeks, or longer • Cases will continue to be identified until the source of the outbreak is removed from commerce or people’s homes

  47. Case Review • The epidemiologist examines demographic characteristics (e.g., gender, age, and race/ethnicity) of cases for possible clues as to what exposure is causing the illnesses • Strong demographic characteristics of a cluster may suggest a particular food vehicle • In accordance with religious practices, cultural traditions or preferences by age and/or gender

  48. Empire County Facts • Population: 220,000 • Racial makeup • 92% White • 4% Asian • 3% Black or African American • 1% other races • 2% of the population is Hispanic or Latino of any race • 23% of the population is under 18 years of age • Median age: 41 years

  49. Demographic Characteristics of the Initial 9 Cases in the Cluster Question 11: • Summarize the demographic information in the table • Are there any noteworthy characteristics?

  50. Q.11 Answer The epidemiologist doesn’t identify any noteworthy items • 89% (8/9) white • 100% non-Hispanic or Latino • 56% (5/9) female • Ages range is 5–94 years (median 29 years) • 78% (7/9) cases are adults • The epidemiologist wonders if the Apple County case travelled to Empire County during their incubation period • Empire and Apple Counties are neighboring counties

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