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Outbreak investigations

Outbreak investigations

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Outbreak investigations

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  1. Outbreak investigations Biagio Pedalino & Domenico Martinelli 24th October 2012

  2. Objectives for this session • Describe • the principles of outbreak investigation • the steps in outbreak investigation • Using practical example • Foodborne outbreak during a birthday party in a pub

  3. What do you normally do on Sunday afternooon?

  4. Something funny. We had lunch at sibilla’s. We were 13 and now 7 are vomiting… Roberto, 41 Pub owner Excellent. Do you need me to be there for the “plasil”? Have you eaten something with cream?

  5. What is it? • Friendly call • Need of medical assistance? • Coincidence? • What else?... • Outbreak?

  6. What is an outbreak ? “The occurrence in a community or region of cases of an illness (or an outbreak) with a frequency clearly in excess of normal expectancy” [Heymann DL, CDC Manual – 19th Ed]

  7. Outbreak investigation: tasks • Confirm outbreak and diagnosis • Form Outbreak Control Team • Define a case (case definition) • Identify cases and obtain information • Describe data by time, place, person • Develop hypothesis • Test hypothesis: analytical studies • Additional studies • Communicate results: • outbreak report, publication • Implement control measures Communication Control measures

  8. Confirm outbreak Is this an outbreak? • More cases than expected? • Surveillance data • Surveys: hospitals, labs, physicians Caution! • Seasonal variations • Notification artefacts • Diagnostic bias (new technique) • Diagnostic errors (pseudo-outbreaks)

  9. Confirm diagnosis • Laboratory confirmation • serology • isolates, typing of isolates • toxic agents • Meet attending physicians • Examine some cases • Contact (visit) the laboratories Not always necessary to confirm all the cases but confirm a proportion throughout the outbreak

  10. Immediate control measures? Outbreak confirmed  - prophylaxis - exclusion / isolation - public warning - hygienic measures - others

  11. Alert !!! • Informal alert from the pub • 16:43: Alert • 7 participants were sick with vomit • Formal intervention: a simple phone call !

  12. Phone call 4:45 pm • Talk to the pub owner • How many people (list with contact details) • Food items • Symptoms/treatment • “Impose” preventive closure of the pub

  13. Further investigation? Immediate control measures? Outbreak confirmed  - prophylaxis - exclusion / isolation - public warning - hygienic measures - others - etiological agent - mode of transmission - vehicle of transmission - source of contamination - population at risk - exposure causing illness

  14. Control measures Steps of an outbreak investigation • Confirm outbreak and diagnosis • Form Outbreak Control Team • Define a case • Identify cases and obtain information • Describe data by time, place, person • Develop hypothesis • Test hypothesis: analytical studies • Additional studies • Communicate results: • outbreak report, publication • Implement control measures

  15. My outbreak control team Ale, Preventive medicine school student - II yr Fra, PHD student - I yr

  16. Case definition A person presenting with vomit or nausea or diarrhoea, within 24 hours after participating at the meal in the pub (Sunday January 22nd; 2pm)

  17. Case definition • Standard set of criteria for deciding if a person should be classified as suffering from the disease under investigation • Criteria • clinical and/or biological criteria • time • place • person

  18. Case definition • Simple, practical, objective • Sensitive? • Specific? • Multiple case definitions • confirmed • probable • possible • CD can be adjusted, if new information becomes available

  19. Control measures Steps of an outbreak investigation • Confirm outbreak and diagnosis • Form Outbreak Control Team • Define a case • Identify cases and obtain information • Describe data by time, place, person • Develop hypothesis • Test hypothesis: analytical studies • Additional studies • Communicate results: • outbreak report, publication • Implement control measures

  20. Descriptive epidemiology • When did they become ill? (time) • - Where do they live? (place) • - Who are the cases? (person)

  21. notifications hospitals, GPs laboratories schools workplace, etc Identifying information Demographic information Clinical details Exposures and known risk factors Identify & count cases Obtain information

  22. Monday 8 am • Contact the participants: • Symptoms • Food consumption • Stool specimen collection (n=5) In the pub • Food samples collection • Info on food preparation

  23. Information collected: symptoms • 14:00: lunch (13 participants: birthday party) • 12 were sick • Vomit (n= 10) • Nausea (n=9) • Diarrohea (n=5) • Abdominal pain (n=6) • Fever (<38°C; n=2)

  24. Information collected: food consumption (n=13) • Basmati rice: 92% (12) • Ragu’ : 77% (10) • Red rice: 69% (9) • Apple cake: 61% (8)

  25. Possible cause of the outbreak? • Virus ? • Bacteria ? • Toxins ? • Parasite ?

  26. Organize information: Line list • Names • Date of birth • Addresse • Onset of symptoms • Treating physician • Hospital stay • Laboratory results

  27. Line List

  28. Describe in - time - place - person Identify & count cases Obtain information Descriptive study

  29. Time: Epidemic Curve • Histogram • Distribution of cases by time of onset of symptoms, diagnosis or identification • time interval depends on incubation period Cases Days

  30. Epidemic curve Cases • Describe • start, end, duration • peak • importance • atypical cases • Helps to develop hypotheses • incubation period • etiological agent • type of source • type of transmission • time of exposure Days

  31. Examples of Epidemic curves Common persistent source Common point source cases cases hours days Propagated source cases Common intermittent source cases weeks days

  32. Hepatitis A by date of onsetOgemaw county, Michigan, April - May 1968 Number of cases 15 one case 50 days 30 days 10 5 15 days 0 2 8 14 20 26 2 8 14 20 26 1 7 Exposure Days

  33. 2 ~ median incubation period (= duration of the epidemic) Possible moment of infection 3 50% 50% 1 median Hypothesis on the moment of infection unknown pathogen and point source 15 5 0 Time 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29

  34. Epi curve • Incubation period: • Median = 2 hrs • Range = 1-5 hrs

  35. Place • Place of residence • Place of possible exposure • work • meals • travel routes, • day-care • leisure activities Maps • identify an area at risk

  36. Person • Distribution of cases • age • sex • occupation, etc • Distribution of these variables in population • Attack rates

  37. Roberto 41 yrsold Marygiò 42 yrsold Massimo 40 yrsold Sick The grandmother, 70 yrs old Sick Not-sick Barbara 34 yrs old Yasemin 35 yrsold Sick Sick

  38. Control measures Steps of an outbreak investigation • Confirm outbreak and diagnosis • Form Outbreak Control Team • Define a case • Identify cases and obtain information • Describe data by time, place, person • Develop hypothesis • Test hypothesis: analytical studies • Additional studies • Communicate results: • outbreak report, publication • Implement control measures

  39. Develop hypotheses • - Who is at risk of becoming ill? • - What is the disease? • - What is the source and the vehicle? • - What is the mode of transmission?

  40. Hyp in the pub: summary • Close community • 12 out of 13 were sick • Mainly vomit • Basmati rice mostly consumed food • Basmati rice left out of the fridge for > 24hrs

  41. Test specific hypotheses • Analytical studies • - cohort studies • - case-control studies Compare hypotheses with facts

  42. Testing hypothesis • Cohort • attack rate exposed group • attack rate unexposed group • Case control • proportion of cases exposed • proportion of controls exposed

  43. Cohort study • Among participants • Telephone interviews

  44. Description of the cohort • 13 persons • 54% F • Median age: 42 (15 – 70) • Overall Attack Rate (AR): 92% • Food specific AR: • Basmati rice: 100% • Apple cake: 100% • Ragu’: 91% • Red rice: 90%

  45. Control measures Steps of an outbreak investigation • Confirm outbreak and diagnosis • Form Outbreak Control Team • Define a case • Identify cases and obtain information • Describe data by time, place, person • Develop hypothesis • Test hypothesis: analytical studies • Additional studies • Communicate results: • outbreak report, publication • Implement control measures