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Oswego An outbreak of gastrointestinal illness following a church incubation period

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					      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.

				
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