Investigate food handling procedures

MANUAL OF NAVAL PREVENTIVE MEDICINE

history questionnaires on both ill and well and obtain clinical specimens

from ill persons. It may be appropriate to seek assistance, either

consultative support or on-site support, from the nearest NAVENPVNTMEDU.

h. Investigate food handling procedures. The investigation must

inquire into the source and method of preparation of each item of food or

drink served at a suspected meal. Although a standard inspection may be

conducted, an investigation focusing on high risk foods and their handling

may be more productive. A flow chart documenting the individual steps from

delivery, through preparation, to service of highly suspect items may be

helpful. Talk with the person in charge, shift supervisors and the watch

captains. Collect menus, recipes, and lists of personnel with their

assignments. Separately interview food service personnel involved in

handling the suspect item(s). Food service personnel should have a

physical examination and specimens should be collected (e.g., stool sample

or rectal swab), if appropriate.

i. Analyze the data. The organization and summary of data collected

from ill and well persons who ate or drank the suspect item or meal help to

classify the illness, identify involved groups, and identify a possible

vehicle for transmission.

(1) Plot an epidemic curve. Prepare a graph of the distribution of

cases (ill persons) by the time of onset of their symptoms (Figure 1-11.)

The period of time covered by the outbreak determines the unit of time used

on the graph. For staphylococcal food poisoning, use a scale of hours; for

a possible salmonella outbreak, use 6 or 12-hour periods; and for hepatitis

A, use days. A common source outbreak graph will show a sharp peak when

many cases developed their symptoms followed by a gradual tapering off of

cases. Figure 1-1 displays data for a common source outbreak of

staphylococcal food poisoning. An outbreak with person-to-person spread

(e.g., shigellosis) will show a slower rise to a less distinct peak or may

have no dominate peak.

(2) Identify the common symptoms and signs. Symptoms are felt by a

person, while signs are noted by an observer. Use data from ill persons to

prepare a chart showing the percentage of cases with specific symptoms

(e.g., nausea or headache) and signs (e.g., fever). The predominate signs

and symptoms, whether enteric, neurologic or generalized, help limit the

list of possible agents that caused the outbreak.

(3) Calculate incubation periods and determine a median incubation

period.

(a) The interval between ingestion of the suspect food and the

appearance of an initial symptom or sign of illness is the incubation

period. Knowledge of the median incubation period further limits the list

of possible causative agents for the outbreak. The median is used because

it is not affected by exceptionally long or short incubation periods, as is

the mean (average) value.

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