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In talking about epidemiology, it is
important to review how outbreaks occur. First, we'll look at three
commonly used, and often misunderstood, terms: "epidemic,"
"outbreak," and "cluster." An epidemic
is the occurrence of more cases of disease than would normally be expected
in a specific place or group of people over a given period of time. To an
epidemiologist, "outbreak" means basically the
same thing. In the public's mind, however, "epidemic" has a far
more serious connotation than "outbreak." For this reason,
"outbreak" is often used to avoid sensationalism. The third
term, "cluster," is occasionally used,
incorrectly, in place of "epidemic" or "outbreak." A
cluster is a group of cases in a specific time and place that may or may
not be greater than the expected rate. Often the aim of investigating
clusters is to determine the baseline rate of disease for that time and
place. Two other terms you will come across are "endemic,"
meaning a high background rate of disease, and "pandemic,"
meaning very widespread, often global, disease.
For an outbreak, or epidemic, to occur,
the basic elements of disease causation and an adequate chain of
transmission must be present. Disease occurs when an outside agent
capable of causing the disease meets a host that is
vulnerable to the agent in an environment that allows the
agent and host to interact. Then, given a chain of transmission from one
host to another and a suitable mode of spread, an outbreak can develop.
These basic concepts help guide the selection of public health strategies
to prevent health problems. Depending on which approach might be most
effective, we might direct efforts at the specific agent (e.g., guinea
worm), host (e.g., immunization to prevent measles), or environment (e.g.,
sanitation improvements to prevent salmonella). We can also target a
specific point in the chain of transmission. This was the response in the
E.coli outbreak in Washington State in the early 1990s, when health
officials called for the thorough cooking of hamburgers to interrupt
transmission of the bacterium.
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To elaborate further, the agent
is the entity necessary to cause disease in a susceptible host.
Classically, we think of an agent as being biological: a bacterium,
parasite, or virus. But an agent can also be a physical force, as in the
example of motor vehicle crashes; a chemical, as in an environmental
problem; or a nutritional imbalance, as with rickets. When we talk about
agents, several characteristics are important to consider:
- Infectivity—the capacity to cause
infection in a susceptible host.
- Pathogenicity—the capacity to cause
disease in a host.
- Virulence—the severity of disease
that the agent causes in the host.
The host is the person,
or in a more generic definition, the organism, that is susceptible to the
effect of the agent. The status of the host is quite important and is
generally classifiable as susceptible, immune, or infected. Finally, and
also quite important, is that the host's response to exposure can vary
widely, from showing no effect to manifesting subclinical disease,
atypical symptoms, straightforward illness, or severe illness.
The environment is the
conditions or influences that are not part of either the agent or the
host, but that influence their interaction. A wide variety of factors,
including physical, climatologic, biologic, social, and economic
conditions, can come into play. For instance, in a study of motor vehicle
injuries, the agent (mechanical energy) and the host (driver) could be
affected by the topography, the weather, and the actions of other drivers.
In many infectious disease outbreaks, social and economic conditions cause
overcrowding and lead to high levels of exposure.
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Agent, host, and environment alone are
not sufficient to cause an epidemic; an adequate chain of transmission
must be present. This process requires a source for the agent, a portal of
exit, a mode of transmission, and a portal of entry. The first element,
the source for the agent, is often the place where the
agent originates, where it lives, grows, and multiplies, but this is not
always the case. The agent that causes botulism (Clostridium botulinum),
for example, originates in soil, but the source of most botulism
infections is improperly canned food containing C. botulinum
spores.
The second element, a portal of
exit, is a pathway by which the agent can leave the source. This
pathway is usually related to the place where the agent is localized. For
instance, the agents causing tuberculosis and the flu are released through
the respiratory tract, whereas agents for many stomach ailments are
released through the digestive tract. Agents found in the blood, such as
hepatitis B and HIV, can be released through cuts or needles.
Once the agent leaves the source, a mode
of transmission, or means of carrying it to the host, is needed.
This can happen in a number of ways, some of which are direct and some
indirect. Direct transmission includes contact with soil
or plants as well as contact between people. In indirect
transmission, the agent can be airborne, vector
borne, or vehicle borne. In airborne
transmission, the agent is carried from the source to the host suspended
in air particles. Vector-borne diseases are transmitted indirectly by a
live carrier, usually an arthropod, such as mosquitos, fleas, or ticks.
Vehicle-borne diseases are carried by inanimate objects, such as food or
water, blood, or items like handkerchiefs, bedding, and surgical
instruments.
Finally, there must be a pathway into the
host, a portal of entry, that gives the agent access to
tissue where it can multiply or act. Often the agent enters the host in
the same way that it left the source. This is the case with the flu virus,
which leaves the source through the respiratory tract and enters a new
host through the respiratory tract.
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A Few More Words about Field
Epidemiology
Field epidemiology is, in the most
general terms, the practice or application of epidemiology to control and
prevent health problems. Epidemiologists are mobilized under a variety of
circumstances, prime ones being when a problem is acute and unexpected and
when quick action is required. The Legionnaires' disease outbreak in
Philadelphia, mentioned at the beginning of this discussion, is an
excellent example. These criteria are also met when a commercial product
presents an imminent threat to public health and safety, as was the case
with L-tryptophan and EMS. High levels of community concern often mandate
a quick response. Involvement of the press is occasionally the driving
force behind an investigation, and political pressure is also often part
of the equation.
Field investigations are action oriented,
with the main goal being to solve a pressing public health problem.
Uppermost in investigators minds is the need to institute the controls
necessary to safeguard health as soon as possible, and often, as in the
example of L-tryptophan and EMS, this step is taken before the entire
investigation is complete. Limited control over the situation, little time
for planning a study, and limited data sources and laboratory samples
challenge investigators. However, the obligation remains to do the best
science possible under the circumstances.
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