Question 6A: Summarize the demographic and clinical information on these patient

Question

Question 6A: Summarize the demographic and clinical information on these patients and draw an epidemic curve.

Question 6B: When was the most likely period of exposure among these cases?

Question 7: What type(s) of study would you use to investigate this hypothesis? Why?

Staff from the local health department where the terminal stop of the bus route was located were invited to participate in the investigation. Physicians attending the cases of botulism were asked to provide demographic and clinical information on their patients. (Table 1) Characteristics of cases of botulism, Buenos Aires, January 1998.

Explanation / Answer

Answer:

6A. All patients were male and all were bus drivers from the same company working on the morning shift of the same route. The median age of cases was 42 years (range: 23-54). All patients had drooping eyelids (ptosis). Six patients had blurry vision; five had fatigue; four each had double vision (i.e., diplopia), upper extremity weakness, and respiratory difficulty; three had lower extremity weakness. Onset of neurologic symptoms occurred from January 5 through January 15. Cases did not peak at any particular point during the 11-day period.

Epidemic curve: The epidemic curve (histograms) can be drawn by plotting the number of cases on the X-axis and date of onset (January) on the Y-axis.

6B. As noted above, the average incubation period for botulism is 18-36 hours, but symptoms can occur as early as six hours or as late as 10 days after exposure. The following two methods can help determine the time of exposure if dates of onset of symptoms are known:

Method 1: If a point source is suspected and the organism and time of onset of illness are known,

1) identify the last known case in the outbreak and count back the maximum incubation period and

2) identify the first case and count back the minimum incubation period.

Ideally, the dates should be similar and represent the probable period of exposure. Using this technique, the period of exposure for this outbreak would be January 4-5. However, at this point, it is unknown if the exposure occurred on one day or over a more extended period of time. Therefore, one would consider a slightly longer period of exposure in exploring possible sources of the outbreak.

Method 2: If a point source is suspected and the organism and time of onset of illness are known, count back the median incubation period from the peak of the outbreak. Looking at the epidemic curve for this outbreak, cases do not cluster at any obvious point in time; therefore, it might be more difficult to use this technique to identify the exposure period.

7. To test the hypothesis, one would want to compare specific exposures among persons who became ill with those among persons who did not become ill. One could use either a cohort or case-control study to explore this hypothesis. Because the outbreak is confined to a well-defined and relatively small group of individuals (i.e., 27 bus drivers on a particular route and shift), a cohort study would seem most logical. A cohort study would also permit investigators to calculate attack rates.

In addition to epidemiologic studies, other studies may be of use (e.g., collection and testing of food samples, environmental investigations, or exploration of food preparation methods at the terminal home). Results from each of these studies could provide important information on the source of the outbreak and the necessary means to control it.

Should one undertake the epidemiologic study first or the other studies? The epidemiologic investigation can often focus subsequent investigations; however, on certain occasions, other studies must be initiated first or simultaneously (e.g., the collection of food specimens which might be discarded). Therefore, the order of various investigations will depend largely on the specifics of the outbreak.

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