Oswego: An outbreak of gastrointestinal illness following a church supper Question 1: Would you call this an epidemic? Yes. Epidemic can be defined as the occurrence of more cases in a place and time than expected. Of the 75 persons interviewed, 46 were ill with gastroenteritis during a 24-hour time period. This is clearly above the “expected” or background rate of gastroenteritis in a community. If the excess were not as obvious, one should compare the rate with some baseline data. For examples, we could compare the observed attack rate with National Health Interview Survey data of 2 episodes of diarrheal illness/person/year. The terms outbreak and epidemic are used interchangeably by most epidemiologists. The term outbreak is sometimes preferred, particularly when talking to the press or public, because it is not as frightening as epidemic. The term cluster may be defined as the occurrence of a group of cases in a circumscribed place and time. The number of cases may or may not be greater than expected. Though not covered in this exercise, an important step in the investigation of an outbreak is the development of a case definition. Question 2: Review the steps of an outbreak investigation. 1. Identify potential investigation team and resources / prepare for field work 2. Establish the existence of an epidemic 3. Verify the diagnosis 4. Construct a working case definition 5. Find cases systematically, develop line listing 6. Perform descriptive epidemiology 7. Develop hypotheses 8. Evaluate hypotheses 9. As necessary, reconsider/refine hypotheses and execute additional studies 10. Implement control and prevention measures (as early as possible) 11. Communicate findings Summarize investigation for requesting authority Prepare written report(s) 12. Maintain surveillance Oswego: An outbreak of gastrointestinal illness following a church supper Question 3: If you were administering a questionnaire to the church supper participants, what information would you collect? (a) Identifying information (b) Demographic information (c) Clinical information (signs/symptoms, duration, documented medical care (name & phone number if you need to contact the doctor) (d) Epidemiology information (exposures and contacts) including: 1. What was eaten at the picnic, how much, when 2. Foods eaten before and after the picnic (but before illness) 3. Contacts with ill persons Question 4: Graph the cases by time on onset of illness. What does this graph tell you? 1. The epidemic curve is a basic tool of epidemiologist to: a. Establish existence of epidemic vs. endemic illness b. Delineate time course and magnitude of epidemic c. Develop inferences about transmission: i. Common source ii. Person to person iii. Intermittent exposure d. Predict future course of the epidemic: i. When it will end ii. That a second wave is underway iii. That secondary cases are occurring 2. In a common source outbreak, the width of the curve is determined by incubation period, varying doses, and host variability. 3. Often a few cases don’t fit into the body of the curve. Such exceptions may be important – as index cases or other special situations. Cases of gastrointestinal illness by time of onset of symptoms. Oswego County, New York; APril 18-19, 1940 12 10 Frequency 8 6 4 2 0 1 3 5 7 9 1 3 5 7 9 11 11 Time Oswego: An outbreak of gastrointestinal illness following a church supper Question 5: How does the information on incubation period, combined with the data on clinical symptoms, help in the differential diagnosis of the illness? In general, short incubation periods (less than 6 hours) are typical of diseases caused by chemicals or preformed bacterial toxins, while longer incubation periods occur when the disease results from in vivo toxin production, microbial growth, or tissue invasions. Incubation period here is too short for viral agents or botulism; too long for heavy metals (usually act within 15-30 minutes), and in range for fish toxins or some of the toxic mushrooms, S. aureus enterotoxin, and the short-incubation syndrome caused by B. cereus. B. Cereus is almost exclusively transmitted by leftover rice. Are rice, fish, or mushrooms on the menu? (Could there have been muschroom in the cabbage salad). Ciguatera usually results in striking sensory symptoms, and scombroid in a bright red flushed face. The short incubation period limit the differential diagnosis considerably. The most likely cause is now S. aureus poisonong; B. cereus is less likely, and mushroom poisoning would be a possibility if mushrooms were eaten. Question 6: By appropriate tabulation for specific items of food, identify, if possible, any common vehicle(s) of infection. The appropriate analysis in this setting is a retrospective cohort analysis, because we have information on the entire population (almost), and we can calculate risks. Many students will want to analyze these data in case-control fashion; while this is not wrong, it is less desirable. As a general rule, if you can calculate risk or rate, you should do so. Using the retrospective cohort approach, we calculate food specific attack rates for each food. We then compare the attack rate womon those who ate the food to the attack rate among those who did not eat the food. Number of persons who ate Number of persons who did specific item not eat specific item Ill Well Total AR% Ill Well Total AR% RR AR Baked ham 29 17 46 63 17 12 29 59 1.07 0.04 Spinach 26 17 43 60 20 12 32 62 0.97 -0.02 Mashed Pots 23 14 37 62 23 14 27 62 1.00 0.00 CabbageSalad 18 10 28 64 28 19 46 60 1.07 0.04 Jello 16 7 23 70 30 22 52 58 1.21 0.12 Rolls 21 16 37 57 25 13 38 66 0.86 -0.09 Brown Bread 18 9 27 67 28 20 48 58 1.16 0.09 Milk 2 2 4 50 44 28 71 62 0.81 -0.12 Coffee 19 12 31 61 27 17 44 61 1.00 0.00 Water 13 11 24 54 33 18 51 65 0.83 -0.11 Cakes 27 13 40 67 19 16 35 54 1.24 0.13 Ice Cream (V) 43 11 54 80 3 18 21 14 5.71 0.66 Ice Cream (C) 25 22 47 53 20 7 27 74 0.72 -0.21 Fruit Salad 4 2 6 67 42 27 69 61 1.10 0.06 Oswego: An outbreak of gastrointestinal illness following a church supper For Vanilla Ice Cream: the RR is 5.71 Among persons who ate specified foods, the attack rate was highest for vanilla ice cream. Among non-eaters of specified foods, the attack rate was lowest for vanilla ice cream. Question 7: What control measures would you suggest? Ascertain whether a commercial product is involved. Prevent consumption of remaining vanilla ice cream. Throw it away after analyzing samples and be sure no one has taken any home. Prevent recurrence of similar events in future by educating food handlers in proper techniques, treating S. aureus skin lesions, stressing need for refrigeration. When applicable, eliminate contaminated sources of food. Question 8: Why was it important to work up this outbreak? A. Rule out contamination of a commercial product (Prompt intervention may prevent thousands of further cases). B. An outbreak represents a breakdown in the public health system. Prevent future outbreaks by identifying infected food handler, specific gap in education or food handling techniques. C. Public health officials need to be responsibly responsive, i.e., they need to respond to a community problem in order to maintain a cooperative relationship with local health department, private physicians, and the communities. D. An epidemiologically and biologically based explanation of the cause of the outbreak may allay community fears and concerns that the outbreak was caused by something else (e.g., terrorists, toxic waste). E. Every outbreak is an experiment of nature. The outbreak may provide opportunities for investigators to answer questions about the agent, host response, epidemiologic, and laboratory methods, etc.