Environmental Engineering Reference
In-Depth Information
ill, age spread, symptoms, incubation period, and so on, will greatly assist the
laboratory in its work.
Epidemiology and Risk
In the foregoing discussion, a scheme for dealing with the orderly investigation
of a waterborne disease outbreak was presented. Central to the conduct of the
investigation is the team of workers appropriately trained to perform specific
roles. One such team member, if available, and a likely leader of the group, is
the epidemiologist. Epidemiology literally translated is “study of epidemics.” In
the broader sense, it is the science (with considerable art) of defining the causes
of disease distribution within a population and the causal factors that made the
disease possible. A causal factor is an event, condition, or characteristic that
increases the likelihood of a disease. 4
Environmental epidemiology is the study of environmental factors that influ-
ence the distribution and determinants of disease in human populations. 26 In the
context of a waterborne outbreak, the epidemiologist is interested in learning the
susceptibility of the population under the sphere of influence of a water trans-
mitted disease, what regions or groups of people in the population are at the
greatest risk, how the disease will manifest itself temporally and spatially in the
population, commonalities, and differences among the individuals listed as hav-
ing been symptomatically affected and not affected, and something of the risk to
the population under the conditions of exposure to water.
During the course of the investigation of a waterborne outbreak, a descriptive
epidemiologic study will be undertaken with the collection of data sets obtained
from laboratory, hospital and physician, environmental, and residential records
and field surveys. The emphasis will be put on establishing the veracity of the
outbreak, containing the spread of the disease through emergency measures, and
characterizing the event in support of formulating a hypothesis on the cause of
the outbreak. A follow-up to the descriptive epidemiologic study would be an
analytical epidemiologic exercise involving a case-control study to identify causal
factors to the outbreak. A case-control study is an observational study in which
a group of persons with a disease (cases) and a group of persons without the
disease (controls) are identified without knowledge of prior exposure history and
are compared with respect to exposure history. 140
If the selection of control participants is truly random, some of the subjects
selected to be controls may also have expressed the illness. Selection of indi-
viduals making up the control group is not a simple process and, as with the
convening of any sample of people intended to be representative of a particular
population, bias is inevitable. Bias impacts the strength of the study results. The
object of the exercise is to analyze the behaviors of both groups prior to the
outbreak so that a determination can be made about the importance of the water
as a condition to developing the disease. For this, a simple approximation of the
essentiality of the water to the infectious outcome is obtained by computing an
odds ratio. A 2
×
2 square is constructed by pairing the number of people that
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