Environmental Engineering Reference
In-Depth Information
of physician records may be warranted against the possibility that certain patients
were overlooked.
Once individuals are identified as cases, personal history of each of the cases
must be obtained. In addition to the usual descriptors (e.g., name, age, sex, etc.),
personal information relevant to the case definition is needed. Accessory data
may be collected on cases (e.g., information about whereabouts and activities
leading up to the occurrence of disease symptoms). Such information is useful to
establish the incubation period for the disease and to compare the evaluation with
published incubation periods for suspected etiologic agents. The medical survey
should assist in developing a clinical picture to enable identification of the dis-
ease and its causative agent. Typical symptoms, date of onset of the first case,
date of onset of last case, range of incubation periods, number of cases, num-
ber hospitalized, number of deaths, and number exposed are usually determined
by the epidemiologist. To assemble this information and analyze it carefully, a
questionnaire should be completed, by trained personnel if possible, for each
person available or on a sufficient number of people to give reliable information
(see Figure 1.5).
The importance of animal reservoirs of infection should not be overlooked
where small-scale water systems are involved. Table 1.4 contains in condensed
form symptoms and incubation periods of many diseases that, when compared to
a typical clinical picture, may suggest the causative organism and the disease. A
high attack rate, 60 to 80 percent, for example, would suggest a virus (Norovirus)
as the cause of a foodborne outbreak.
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Finally, all data collected in the description phase of the investigation are
analyzed and charted in various ways to obtain a picture of the outbreak. Visual
aids will be areal maps, graphs displaying the chronology of case densities over
time with subplots according to age, sex, ethnicity, and so on. A simple bar graph,
with hours and days (possibly weeks) as the horizontal axis and number who are
ill each hour or other suitable interval plotted on the vertical axis, can be made
from the data. The time between exposure to or ingestion of water and illness or
first symptoms or between peaks represents the incubation period. The average
incubation period is the sum of the incubation periods of those ill (time elapsing
between the initial exposure and the clinical onset of a disease), divided by the
number of ill persons studied. The median, or middle, time may be preferable
when incubation periods vary widely. The shape of the curve is useful in revealing
the period of primary infection as may be due to point source infection vs.
person-to-person contact. Extended case-time plots may be biomodal, indicating
a point-source outbreak and a secondary person-to-person outbreak. Good data
presentation adds to the strength of the investigation and the location of “hot
spots” that may reveal points of interest in the drinking-water distribution system
subject to possible contamination.
Hyothesis Formulation
The data collected and analyzed in connection with
the “Description” are used to formulate hypotheses concerning the events respon-
sible for the outbreak and make preliminary recommendations for remedial con-
trol measures. More than one hypothesis is possible. The outbreak may be
