applications of epidemiology

THE APPLICATIONS OF
EPIDEMIOLOGY.
Epidemiologic methods can be used
for a number of distinct purposes .

DISEASE SURVEILLANCE.
• Perhaps the most basic question that can be
asked about a disease is “What is the
frequency with which the disease occurs ?”.
To answer this question , it is necessary to
know the number of persons who acquire the
disease (cases) over a specified period of
time , and the size of the unaffected
population.

• Measures of frequency of occurrence of a disease , are used
to characterize the patterns of the occurrence of the disease ,
,and the medical surveillance of the disease . Typically , the
criteria used to defined the occurrence of a disease depend
on current knowledge of a disease ; such criteria may become
more defined as the causes of a disease are delineated and
new diagnostic tests are introduced .For example in 1982 the
CDC created an initial , relatively simple surveillance
definitions for AIDS : “A disease , at list moderately indicative
of a defect in cell-mediated immunity , occurring in a person
with no known cause for diminished resistance to that
disease. “

• A more specific definitions became possible once the
causative agent , HIV , was identified and tests for the
detection of antibodies to the virus were developed . In 1987
the CDC surveillance definition was expanded to incorporate
clinical conditions that are indicative of AIDS .A 1993 revision
include three additional indicator conditions (pulmonary
tuberculosis , recurrent pneumonia , or invasive cervical
cancer ) , or the presence of severely depressed CD4 T
lymphocyte count .In 2000 , the CDC integrated monitoring of
both HIV infection and AIDS .

• The identification of patients with a disease
can occur through various mechanisms , most
commonly by physician and laboratory
reporting . In the United States , a number of
diseases , including AIDS , must be reported to
public health authorities . Monitoring the
patterns of occurrence of a disease within a
population is referred to as surveillance.

SURVEILLANCE DATA.
• There are many potential benefits from the collection of
surveillance data.
• This type of information (1) can help to identify the new
outbreak of an illness , such as AIDS, (2) can provide clues , by
considering the population groups that are most affected by
the illness, to possible causes of the condition ,(3) can be used
to suggest strategies to control or prevent the spread of
disease ,(4) can be used to measure the impact of disease
prevention and control efforts , and finally , (5) can provide
information on the burden of illness , data that are necessary
for determining health and medical service needs.

The course of the AIDS pandemic in the
United States.
• From 1985 through 1992 there was an unrelenting
rise in the number of newly reported cases .
• From 1993 through 1998 there was a progressive fall
in the number of newly cases report .
• Between 1999 and 2001 the level of AIDS cases was
unchanged , with a slight increase in 2002 .
• It should be noted that the information relates to the
number of newly diagnosed cases per year.

• Changes in the counts of new cases can be affected
by a number of factors , including among others ,
changes in the following :
1. Frequency with which the disease occurs.
2. Definition of the disease .
3. Size of the population from which the cases
develop .
4. Completeness of the reporting of the cases.

• With respect to point 2, the same surveillance definition was
used consistently for all years , minimizing any confounding
influence of a change in definition of the disease over time.
• With regard to point 3 , growth in the size of the population
of the United States could not explain more than a trivial
amount of the rise in the cases seen between 1985 and
1992 .the national population grew at only about 1% per
year , whereas the average annual increase in reported
persons with AIDS exceeded 30 %.Moreover , the declines in
AIDS cases observed between 1993 and 1998 occurred while
the population of the United States continued to grow .
• Concerning point 4 the overall completeness of reporting of
AIDS cases is estimated to be about 85 %

INCIDENCE RATE.
• For surveillance purposes , the size of the source population from which
cases usually is estimated from census data .
• The frequency of a disease occurrence is then expressed as the number of
new cases developing within a specified time among a standard number
of unaffected individuals. For example , during 2002 over 42,000 cases of
AIDS were reported in the United States , the US populations in 2002 was
about 288,000,000 .Dividing the number of reported cases by the size of
the population yields 0,00015 cases per person during that year.
Epidemiologists typically express such frequencies of disease occurrence
for for a population of a specified size , say 100,000 persons .By
multiplying 0.00015 by 100.000 , the number 15 is obtained . That is to
say , within a standard population of 100.000 persons 15 would have been
reported as developing AIDS during 2002. This measure of the rapidity of
incidence rate
disease occurrence is referred to as an .

SEARCHING FOR CAUSES.
• The study personal and environmental characteristics ,
epidemiologists often rely an interviews, review of records,
and laboratory examinations. Associations between these
characteristics and the occurrence of disease can arise by
coincidence, by noncausal linkages to other features , or by
cause-and effect relationships.
• The epidemiologist is primarily interested in the last
category ,ie, determinants of disease development , also
known as risk factors. Identification of risk factors can
provide a better understanding of the pathways leading to
disease acquisition ,and consequently , better strategies for
prevention.

• Returning to the AIDS example, early epidemiologic studies
played an important role in determining the cause of the
disease. Within the first 5 months after recognition of this
syndrome , the CDC has received reports on 70 patients with
AIDS in 4 urban centers . Of these individuals , 50 homosexual
male patients with AIDS were interviewed; also interviewed
were 120 unaffected homosexual male comparison subjects.
Persons who are affected with a disease are referred to by
epidemiologists as cases, and unaffected comparison
persons are called controls. Comparison of the responses
from cases and controls revealed that the AIDS patients had a
higher number of sexual partners. This type of investigation is
referred to as a case-control study.

COHORT STUDY DESIGN.
• Comparison historical exposures reported by cases
and control can provide suggestive evidence of a
cause and effect relationship. This type of
information , however, may be distorted or
biased by the fact that ability of cases and
controls to recall earlier exposures differs. Such bias
could be avoided by using a cohort study
design, in which exposure is assessed among
unaffected persons, and subjects are then observed
for subsequent development of illness.

DIAGNOSTIC TESTING.
The purpose is to obtain objective evidence of the
presence or absence of particular condition. This
evidence can be obtained to detect disease at its
earliest stages among asymptomatic persons in
general population, process referred to as
screening .In other circumstances, diagnostic tests
are used to confirm a diagnosis among persons with
existing signs or symptoms of illness .

• Occasionally , a positive test result will incorrectly
suggest that infection is present in an unaffected
person . This type of outcome is referred to as a
false positive , because the positive test result was
in error. A test with a very low percentage of false-
positive results is said to have a high specificity.
Another type of error occurs when a test incorrectly
suggests that infection is not present (negative test
result) in an affected person. This type of outcome is
referred to as a false negative, because the
negative test result was in error .A test with a very
low percentage of false-negative results is described
as having high sensitivity .

NATURAL HISTORY.
• After being informed of a new diagnosis , patients most frequently ask “
What will happen to me ? “This question can not be answered with
absolute certainty because of variations in outcome across individual
patients .Consider for example a patient newly diagnosed as being HIV
positive . In this instance, with the advent new treatment it is reasonable
to question whether the full syndrome of AIDS will develop , and if it does
how long it will take to occur.Natural history indicate the typical
course of an illness , regardless of whether it can be treated effectively
.there are several ways to characterize the natural history of an illness
.One straightforward measure is the case fatality, which represents the
percentage of patients with a disease who die within a specified
observation period. For example , among the 11,740 reported adolescent
and adult patients diagnosed with AIDS in 1985 in the United States ,
10,946 are known to have died before 1998 . In other words , the case
fatality was 10,946
• 11,740 x 100%= 93.2%.

• Another method of characterizing the natural
history of a disease is to estimate the typical
duration from diagnosis to death (survival
time).
• The length of survival that is met or exceeded
by 50% of the study population is referred to
as the median survival time .

SERCHING FOR PROGNOSTIC
FACTORS.
• Analysis of survival can be employed to
identify groups of patients with unusually
favorable (or unfavorable) clinical outcomes.
Characteristics that relate to the likelihood of
survival are referred to as prognostic
factors.

TESTING NEW TREATMENT.
• In the United States , all new medications must be tested and
proved effective before they can be introduced into routine
clinical care . The standard approach used to evaluate
treatment effectiveness is the randomized controlled
clinical trial. The term controlled means that patients
(experimental subjects) who receive the new medication are
compared with patients (control subjects) who receive either
an active substance (placebo) or a standard treatment if one
exists. Randomized refers to a method of assignment of
subjects to either the experimental or control group that is
determined by chance rather than patient preference or
physician selection . The principles of randomized controlled
clinical trials can be demonstrated by study that has
contributed to a revolution in the treatment of HIV infected
persons.

• The results of this trial and other similar
studies clearly demonstrated the short –term
therapeutic benefit of combined treatment
with reverse transcriptase inhibitors and
protease inhibitors . As the search for even
more effective treatments continues ,
randomized controlled clinical trials will serve
as the definitive approach to establishing
therapeutic superiority .

STUDY QUESTION.
• Incidence rate ?
• Risk factors?
• Case control study ?
• Cohort study?
• False positive?
• False negative?
• Natural history?
• Median survival?
• Prognostic factors?
• Randomized controlled clicinal trial?

What are the most common practical
applications of epidemiology

• The common practical applications of
epidemiology include investigation of
infectious diseases. These are done on a
routine basis and are conducted by health
departments to investigate common
infectious diseases, particularly food borne
illness, and food borne disease.

• . Epidemiology comes into practice in the
health care system, and in hospitals where
nurse epidemiologists and other
epidemiologists explore the causes and
outbreaks of hospital borne infections.

• Then a third kind of trend is to do with the use
of epidemiology to evaluate policies that are
adopted and that our government set up;
policies that relate to health, curtailment of
smoking, to changes in our diet, to elimination
of trans-fats and fatty foods from our diet,
and the elimination of sugary snacks from
schools, and so forth.

Community diagnosis
• Epidemiology provides the facts about
community health; it describes the nature and
relative size of the problems to be dealt with,
and ‘maps’ are produced of such scales as are
required or possible. Results are sometimes
surprising—at any rate in contrast with the
type of problem of which there is general
awareness and concern in the public health
movement

• Over 10% of sickness absence in male
industrial workers in 1951 was ascribed to
‘bronchitis’ (16 million days). ‘Psychological’
disorders accounted for >13 million days;
gastric and duodenal ailments for over 11
million, ‘rheumatism and arthritis’ for over 11
million.

The individual's chances
• The risks to the individual—or at any rate
their order of magnitude—of suffering an
accident as a schoolboy cyclist or an elderly
pedestrian, of developing leukaemia for a
radiologist, of producing malformation from
rubella or breast cancer from chronic mastitis,
can be estimated only if the experience of
whole populations of individuals is known and
the relevant averages can thus be calculated.

Identification of syndromes
• This use again relates directly to clinical
medicine. Broad descriptive clinical and
pathological categories often include very
different elements. Their different statistical
distribution, and their different behaviour
among the population, may make it possible
to distinguish such elements from each other,
and thus help to identify characteristic
syndromes.

Clues to causes
• The main function of epidemiology is to
discover groups in the population with high
rates of disease, and with low, so that causes
of disease and of freedom from disease can be
postulated.

• The most obvious and direct examples are the
original observations on the nutritional
deficiencies (scurvy, beriberi, pellagra and
goitre); the geographical study of cancer
(especially of the skin and liver); the industrial
cancers (bladder, for instance); and industrial
accidents (of coal-miners or railway workers)

• The biggest promise of this method lies in
relating diseases to the ways of living of
different groups, and by doing so to unravel
‘causes’ of disease about which it is possible to
do something.

Conclusion
• Epidemiology is today the Cinderella of the
medical sciences. The proposition might,
however, be advanced that public health
needs more epidemiology, and so does
medicine as a whole, and, it may be said,
society at large.

• Epidemiology, moreover, as a tried instrument
of research—with its modern developments in
sampling and surveys, small-number statistics,
the follow-up of cohorts, international
comparisons, field experiment and family
study; and with its extensions to problems of
genetics as well as environment, to
physiological norms as well as disease, the
psychological as well as the physical, morbidity
as well as mortality

• Epidemiology now offers the possibility of a
new era of collaboration between public
health workers and clinical medicine. Such a
collaboration could be on equal terms, each
making their particular contribution to the
joint solving of problems. There is abundant
evidence today that clinicians would very
much welcome such a development.

• Collaboration between clinician, laboratory
scientist and epidemiologist might be more
successful. The possibilities are at present
unlimited, if often neglected.

applications of epidemiology

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applications of epidemiology