Chapter 1. Introduction in epidemiology

1.1. Epidemiology as a science and practice1.2.History1.3.Definition 1.4.Modern branches of Epidemiology 1.5.Basic principles and epidemiologic approach1.6.Main tasks1.7.Epidemiological method and types of epidemiologic studies

1. 1. Epidemiology as a science and practice

EPIDEMIOLOGY is the basic science and most fundamental practice of public

health and preventive medicine.

As a science,

Epidemiology is the study of the distribution and determinants of health-related

states and events in specified populations and the application of this study to the con-

trol of health problems. (J.M. Last, 1995 )

The practice of epidemiology is the scientific process that detects, investigates,

and analyzes health problems, followed by applying this information to the control

and prevention of these problems, and also the evaluation of control and prevention.

This practice requires health problems to be the subject of public health surveillance,

epidemiological investigation, and analysis.

The term “epidemiology” derived from three Greek roots (epi meaning upon, demos

meaning people, and logia meaning study). It is unknown exactly who the first used this

term.

Thus, the term epidemiology was originally applied to the study of outbreaks of acute

infectious diseases and was defined as the science of epidemics. Historically, the earliest

concern of the epidemiologist was to investigate, control and prevent epidemics.

Human society is suffering from epidemics from the beginning of his history till our

days. There are some description of severe epidemics in the Indian’s Vedas, Egyptian pa-

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pyruses, ancient Greek and Chinese writing , the Bibles. We know about fucinid’s plague,

which occurred in 430-425 B.C. and was described by Greek historic Funicid. The fist

pandemic of plague in 531-580, called the Ustinian’s plague, helped to destroy the Visan-

tic Empire. In 14th century the second pandemic of plague killed one quarter of all popula-

tion of Europe (25 million people). Our ancestors also knew the epidemics of such infec-

tious diseases as smallpox, louse-born typhus, cholera, influenza and others.

Definition of epidemiology

Epidemiology is the study of the distribution and determinants of health-re-

lated states and events in specified populations and the application of this study to

the control of health problems. (Last JM, 1995 )

This definition requires some comments. Epidemiology studies the frequency of dis-

ease and other events: usually incidence and prevalence. Distribution relates to time,

place, and person. Determinants include both causes and factors that influence the risk of

disease. Some diseases have a single necessary cause, some diseases have many causal

factors and conditions, influencing on the risk of diseases to occur.

There are many others definitions of Epidemiology, but this definition is common

used and the most acceptable.

Modern branches of epidemiology

Epidemiology is divided into epidemiology of infectious diseases (communicable dis-

eases) and the epidemiology of noninfectious diseases (chronic diseases).

Epidemiology of infectious diseases – is independence branch of the medical sci-

ence, which investigates the causes of occurrence and spread of communicable dis-

eases in the human society and applies this knowledge for the fight, prevention and,

finally, eradication some of these diseases.

The object of investigation is the epidemical process (prosess of infectious diseases).

Epidemiology of noninfectious diseases (chronic diseases) – is the using epidemi-

ological methods for investigation the causes and conditions (determents) of occur-

rence and spread noncommunicable diseases and other health-related states and

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events in human population and applying of this study to the control of health prob-

lems.

The object of investigation is frequency (incidence and prevalence) of diseases and

other conditions in population.

Those two branches of epidemiology have different basic principles, concepts, laws.

They are connected by the epidemiological method.

There are some other branches of epidemiology. In some textbooks you can see envi-

ronmental epidemiology, nutrition epidemiology and others. It requires some comments.

In our Russian preventive medicine there are some distinguishes between Epidemiology

and Hygiene. In our schools there are Hygiene of nutrition, of children and adolescents,

occupational hygiene, environmental hygiene and ecology. These disciplines are studied at

the Department of General Hygiene and Ecology.

In foreign schools, for example, in American’s school of epidemiology, all those dis-

ciplines are called Epidemiology.

An increasingly broad interface between clinical medicine and epidemiology is called

now clinical epidemiology – the basis science of evidence-based medicine, which is used

epidemiological method.

Molecular epidemiology means using the genetic biological methods for epidemiolo-

gical investigations.

1.5. Main tasks of epidemiology

The main goals of the epidemiology are the following:

Characterising the frequency and distribution diseases and other conditions in popula-

tion

Identifying of factors that cause the occurrence and spread of diseases

Providing the surveillance of diseases (communicable and noncommunicable) and other

conditions

Identifying of factors or conditions that can be used for prophylactic of disease and

other conditions.

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Evaluating prophylactic means and measures

Reducing the morbidity and mortality from diseases, preventing the occurrence and

spread of communicable and noncommunicable diseases and conditions

Identifying the health problems of the community and define the main directions of pro-

phylactic.

Defining the needs of health service.

Predicting the future health problems and health needs of a population

Epidemiology is closely connected with such sciences as

- Microbiology (We need the information about etiology, features of a causal agent, labo-

ratory methods of investigation of patient and environmental

- Immunology (Definition of level of specific immunity, laboratory methods for defini-

tion antibody titer, immunocompromised person, immunologic monitoring for estimation

of herd immunity, spread of infection, effectiveness of vaccination and other)

- Clinic of infectious diseases (understanding of pathogenesis, clinical features of infec-

tious diseases, period of communicability)

- Biostatistics (math methods for epidemiological investigations)

- Public health and public health service (information about public health, problems of

prophylaxis, organization and management of public health service)

- Hygiene and Ecology (knowledge about pathway and factor of transmission (water,

food), environmental, preventive measures)

- Biology (knowledge about vectors, helminthes, protozoa)

- Others.

1.6. Basic principles and epidemiologic approach for investigation of human’s dis-

eases

Epidemiology now refers to the study of the distribution and determinants of diseases

or conditions in a defined population.

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Epidemiology is based on two fundamental assumptions

Diseases do not occur by chance

Diseases are not distributed randomly in the population; thus, their distribu-

tion indicates something about how and why that disease process occurred.

Epidemiologic approach to investigate health problems means:

Investigation on the level of population

An epidemiologist is trained to identify and prevent diseases in a given population,

while a clinician is trained to identify and treat disease in an individual. So, clinician stud-

ies a case of disease, epidemiologist studies fist of all morbidity - frequency of diseases

(incidence, prevalence).

Complex investigation

An epidemiologist studies diseases in a population with many variables over which

one has no control, while a basic scientist studies diseases in a laboratory, modifying one

variable at a time.

Using the epidemiologic method, integrating many methods from different

disciplines (multidisciplinary method).

There is the aphorism that “microbiology, immunology, hygiene… - they are bring-

ing grains on the mill of epidemiology”.

Since the epidemiologic method is a multidisciplinary method, epidemiologists have

diverse backgrounds, including human and animal medicine, entomology, microbiology,

statistics, computer programming, administration and others.

Statistical data design (application of statistical methods).

Design of epidemiological investigations: observation-description-analyses-

evidence-conclusion.

An epidemiologist not only promotes hypothesis about determinant (cause), but he

has to test it in the analytic investigation and prove it in the experimental study (trial).

Quantity evaluation of prophylactic means and measures.

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Only in epidemiologic studies you can estimate efficacy and effectiveness means and

measures (such as vaccines, medicine, manipulation, operation and others).

1.7. Epidemiological method

Epidemiologic method – is the complex of methods from different disciplines, us-

ing in epidemiologic studies.

Epidemiological method includes epidemiological investigation of case, statistical

methods, bacteriologic methods, virologic methods, genetic molecular methods, immuno-

logic methods, formal logistic methods and others.

All these methods are used in epidemiologic studies for different goals and may be

classified in connection with type of study:

Descriptive and evaluating methods (observation, description, variable statistics, labor-

atory methods) – in descriptive studies.

Analytic methods (statistical methods, formal logistic methods) – in analytic studies.

Experimental methods (laboratory experiment, epidemiologic experiment, statistical

methods) – in experimental studies.

Methods of mathematical modelling – in experimental studies

.1.8. Types of epidemiologic studies

Types of epidemiologic studies include: Descriptive studies

- Populations (correlation studies)- Individuals

case report case series cross-section surveys

Analytic studies-Case-control studies-Cohort studies

Experimental studies- Controlled epidemiologic experiment (clinical trials, field trials)- Uncontrolled epidemiologic experiment ()

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- Natural epidemiologic experiment- Experimental epizootic studies

Descriptive studies describe the distribution of cases in relation to person, place, and

time.

Characteristics of persons include basic demographic factors, such as age, sex, race,

marital status, occupation and life-style variables, such as consumption of various food, to-

bacco smoking, drug use and other.

Characteristics of place refer to the geographic distribution of a disease, including

variations among counties or within countries, such as between urban and rural areas.

With regard to time, descriptive studies may examine seasonal patterns. Descriptive

studies may follow secular (multiyear) trend of disease occurrence over time.

So, descriptive studies allow

study acute outbreaks of disease

obtain information about health problem

formulate hypotheses about disease determinants and transmission, that can be

tested in analytic studies.

Populations (correlation) studies uses data from entire populations to compare disease

frequency between different groups during the same period of time or in the same popula-

tion at different points of time.

Case reports is careful detailed investigation of a one case of a disease. It’s very im-

portant, basic type of descriptive studies.

The individual case reports may be expended to a case series. For example, In 1981

4 cases of Pneumocystis carinii pneumonia were described in young previously healthy

homosexual men. It was the fist step towards the investigation of AIDS.

Cross-sectional surveys. Extensive information is the periodically collected by ques-

tionnaire from representative sample of individuals throughout some population. Partici-

pants are asked about personal and demographic characteristics, illnesses, health habits,

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utilization health care resources. Such data are necessary for public health administrates

for evaluation of health status and health care needs.

Analytic studies, such as the retrospective (case-control) studies and the prospective

(cohort) studies, identify causal relationships or factors associated with disease. Analytic

studies are observational studies. Investigator doesn’t influence on the result by any means

and measure.

Analytic studies do not prove cause and effect, but they are used to generate hypo-

theses that can be tested

Case-control study:

The scheme of Case-control study is the following:

to assemble a group of individuals with a particular disease as well as a compa-

rable group of persons without it

to obtain information about their previous medical history and heath habits.

to compare the frequency of the factor’s exposure in case- and control-groups

If there is a statistically significant difference (relevant difference) and in case-group

the exposure of factor higher in comparison with the control-group, the factor may be re-

ferred to high-risk factors.

This approach allowed investigators to look backward in time to assess quickly the

effect of a relevant exposure without having to wait many years for the disease to develop.

This is retrospective study.

The fist investigation of this kind was the classic study of cigarette smoking and lung

cancer published by Doll and Hill in 1950. They obtained information on smoking history

from over 700 men and women with lung cancer and a similar number of patients hospital-

ized for nonmalignant conditions.

We often use case-control study for investigation of outbreaks of infectious diseases.

Cohort study:

The scheme of Cohort study is the following:

to assemble a group of individuals without the disease of interest,

to classify them with respect to their exposure status at the start of study,

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and to monitor the subsequent development of the disease in exposed and non-

exposed individuals over time.

to compare incidence rates.

This is prospective study.

An early and important study of this kind was the Framingham Heart Study, in which

a group of almost 5200 residents of Framingham, Massachusetts was assemled and has

since been followed for over 35 years to explore the relationships of a wide variety of risk

factors with coronary heart disease.

Experimental studies are carefully designed to prove an association between a factor

and disease outcome. In experimental studies investigator influence on the individuals by

using the means or conducting measures. Such investigations as vaccine field trials and

clinical studies that evaluate therapy are the kinds of experimental epidemiological studies

Epidemiology – the science and practice - can help find, investigate, analyse,

control, and prevent a wide range of health problems.

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