studying this guide and answering the questions in the exercises, you will
be able to
the reasons that health agencies investigate reported outbreaks
the steps in an outbreak investigation
the terms "cluster," "outbreak," and
how to determine whether an epidemic actually exists
a "line listing" and describe what it is used for
the initial steps of an investigation and develop biologically
a traditional epidemic curve
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A Challenge for the Disease Detective
One of the
most exciting and challenging tasks facing an epidemiologistâ€”or
"disease detective"â€”working in a public health department is
investigating an outbreak. Frequently, the cause and source of the
outbreak are unknown. Sometimes large numbers of people are affected.
Often, residents are concerned because they fear more people, including
themselves, may become ill unless the cause is found quickly. There may be
hostility and defensiveness if an individual, product, or company has been
accused of being the source of the outbreak. Into this pressure-packed
setting comes the epidemiologist from the health department, who must
remain calm, professional, and scientifically objective. Fortunately,
epidemiology provides the scientific basis, the systematic approach, and
the focus on prevention and the population at large that are needed.
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What is an Outbreak?
An outbreak or an epidemic exists when there are more cases of a particular
disease than expected in a given area, or among a specific group of
people, over a particular period of time. An aggregation of cases in a
given area over a particular period, regardless of whether the number of
cases is more than expected, is a cluster. In an outbreak or
epidemic, we usually presume that the cases are related to one another or
that they have a common cause.
Many epidemiologists use the terms "outbreak" and
"epidemic" interchangeably; however, some restrict the use of
"epidemic" to situations involving large numbers of people over
a wide geographic area. The public is more likely to think that
"epidemic" implies a crisis situation.
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Health departments learn about most outbreaks in one of two ways. The first and
probably most common way is through calls from a doctor, some other health
care provider, or a citizen who knows of "several cases." In one
such instance in 1989, a nationwide epidemic of a severe illness,
eosinophilia-myalgia syndrome (EMS), was first detected when a physician
in New Mexico called a consultant in Minnesota and realized that,
together, they had seen three patients with highly unusual symptoms. All
three patients said they had used the dietary supplement L-tryptophan. The
local physician promptly called the New Mexico state health department,
setting into motion a chain of actions leading to the discovery of a large
nationwide epidemic and the recall of L-tryptophan.
The second means of identifying outbreaks is the routine analysis of public health
surveillance data. Through public health surveillance, data on
health are systematically collected, analyzed, interpreted, and
disseminated on an ongoing basis. This information, which is based on
reports sent in by doctors, laboratories, and other sources, allows
investigators to track patterns of disease in a community and to determine
how to control and prevent it. When, during routine analysis, the data
show an increase over the normal background level of reported cases of a
particular disease, an outbreak may be indicated.
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