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
(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.