Field epidemiology: effect measures, and a cohort study of a food borne outbreak

1. Field epidemiology: effect measures, and a cohort study of a food borne outbreak Simon Thornley

2. Overview • Introduce how epidemiology can be used to help inform what caused a food borne outbreak • Re-inforce ideas of cohort study design and analysis • Consider how other evidence can inform the results of epidemiological study.

3. Basic study design

4. Exposure Participants Outcomes

5. Simple Outbreak • 6 October 2007 • 16 people reported having diarrhoea to North Shore City Council following ‘prize giving’ at North Shore Event Centre • Prize giving was the final event of a week long, international soccer tournament of the South African/Indian, diaspora

6. First steps: what to do? • Outbreak group • Track down participants • Track down food • Microbiological samples • Assess risk • Immediate action required? • Media/ risk communication?

7. Hypothesis • Was this outbreak caused by food served at function? Which food? • Epidemiological analysis one tool. • Clinical and microbiological analysis provide complementary

8. The suspects... Dahl Lamb biryani Vegebiryani

9. The premises

13. Consider raw food

14. Retrospective cohort Buffers’ dance Dal/ biryani Diarrhoea?

15. Steps in assessing data

16. 7 6 5 4 frequency 3 2 1 0 9:30:00 a.m. 1:30:00 p.m. 3:30:00 p.m. 5:30:00 p.m. 7:30:00 p.m. 1:30:00 a.m. 3:30:00 a.m. 1:30:00 a.m. 3:30:00 a.m. 5:30:00 a.m. 7:30:00 a.m. 9:30:00 p.m. 11:30:00 a.m. 11:30:00 p.m. 11:30:00 p.m. Time of diarrhoea onset Symptom onset (outbreak curve)

17. Common epidemic curves

18. Symptoms?

19. Incubation period

20. What are the facts? • Facts vsartifacts? • Bias: selection, information • Selection: was the sample representative of the total group? • Would New Zealand cases be more/less likely to be sick? • Case definition?

21. Diagrams helpful

22. “Frequentist view” Ground hog day Imagine the event repeated over and over! Assume random probability

23. Error distribution • Assume outcome is like flipping a biased coin (probability theory) • Consider long run probability associated with biased coin (prob =attack rate) • Bias = attack rate in exposed or unexposed groups

24. Crude associations (univariate or crude)

25. In pictures (dal) If unrelated; chance of diarrhoea after eating dahl =chance of diarrhoea (regardless of meal selection) Actual Diarrhoea No effect Dahl eaten

26. In pictures (dal) Actual Diarrhoea Odds Ratio =0.77/0.22 =3.5 Dahl exposure

27. In pictures (dahl) Diarrhoea No effect Odds Ratio =0.61/0.65 ≈1 (no effect) Dahl exposure

28. In pictures (lamb) Diarrhoea Actual No effect Lamb eaten

29. In pictures (vege) Diarrhoea No effect Actual

30. Just consider first exposure! Is dal the likely culprit? What information does the p-value give you? What information does the confidence interval convey? What is the null hypothesis?

31. Risk of disease in dal eaters

32. Odds of disease in dal eaters

33. Risk of disease in non-dal eaters

34. Odds of disease in non dal eaters

35. Distribution of risk ratio Upper bound of risk ratio is reciprocal of risk in unexposed (28/5 or 5.6)

36. Distribution of odds ratio

37. What if we’d managed to get 10x participants?

38. What do these results mean? • What food caused the outbreak? • What does the 95% confidence interval mean? • Could these results be explained by chance? • Are the populations comparable? • Was some other exposure accounting for this difference?

39. How do we know differences are real (not due to chance)? We don’t – but Statistics helps us put a number on the uncertainty!

40. Confounding: Are the populations similar?

41. Multivariable Results

42. Microbiology • Stool (2/18 received) • Clostridium perfringens (1.0 x 104 CFU/g) • Bacillus cereus • No toxin, no norovirus • Food • Lentils ok • Spices • Turmeric Bacillus cereus (1.0 x104 CFU/g, with faecal coliforms (4.0 MPN/g)

43. Reject null hypothesis • dallikely to cause outbreak • Relationship between illness and dal confounded by lamb biryani

44. Putting it all together…. • Turmeric seeded lentil soup with Bacillus cereus • Time and temperature abuse – endotoxin associated diarrhoeal syndrome. • Vegetable biryani protective, few consumed, more likely to be heated in bain-marie. • Likely temperature labile toxin.

45. Room set up Lamb biryani and dal Vegebiryani and dal

46. Don’t forget the Public Health Action • Is routinely contaminated turmeric ok? • What should we do with the cook? • Legal action justified?

47. Summary • Epidemiology- distribution and determinants of health • Study design – take into account factors such as cost, speed, causation, ethical issues • Statistics P-value yes/no cf. 95% confidence interval • Put results together with other analysis • Unexpected results!