Epidemiology

Chapter 1

Introduction

Major Contents Definition of Epidemiology Main Methods of Epidemiology Contributions of Epidemiology to the

Medical Science

Section 1 Definition of Epidemiology

Definition Distinguish classical Epidemiology

and clinical Epidemiology Branch of Epidemiology

Definition :

Epidemiology is a discipline to study distribution (or occurrence and development) of diseases and health conditions in human populations and the determines influencing the distribution as well as to study the measures for the prevention and control of disease and promotion of health.

Three points: 1. Objects: human population,not individual

Classifying of medicine Study level

Elementary Medicine molecular or sub molecular Immunology Biochemistry;

Cell biologyClinical Medicine individual patients Internal Medicine;surgeryPreventive Medicine populations Epidemiology; Environmental Hygiene

2.Range and Contents Range: infectious and noninfectious diseases; health conditions ; injury Contents: distribution of diseases and health conditions; influencing factors; take steps to control

3.Aim Aim of Epidemiology is to prevent and

control diseases, and to promote health.

Distinguish between classical Epidemiology and clinical Epidemiology

Classical Epidemiology: Study the community origin of health

population, particularly those related to nutrition, the environment,human behavior,social, and spiritual state of a population.

Clinical Epidemiology is to use similar designs and statistical tools.

However, their objects are patients,

aim is to improve the diagnosis and treatment of diseases and the prognosis for patients.

For example: esophagus cancerClassical Epi: Epidemiologists usually study a

population of some cities or counties, survey the distribution of this cancer.In which time ,which population,which place, incidence rate is highest? Then, to study what reasons cause this result.

And then, according to those reasons,carrying out measures to decrease incidence rate or prevent the appearances of esophagus cancer.

Clinical Epi: Aims : how to improve the diagnosis method ; find patients as soon as early; study which method is better or best to treat this disease; to estimate the patients’ prognosis, such as may he be dead quickly? May he be mended? How much time maybe he lives?

Epidemiology is used in clinical medicine to:

describe the natural history of diseases discuss disease causality

- proximate: biological mechanisms of disease- distal: social and environmental causes of disease

provide disease surveillance - essential for evaluating community health problems

- and setting disease control priorities

Branches of Epidemiology With the developing of Epi and other disciplines,

more and more methods and theories of Epi are applied and produce some branches of Epi.

Such as, Environmental Hygienic Epi; Occupational Hygienic Epi; Injury Epi; Molecular Epi;

If Epidemiology is applied to study some diseases,there are Tumour Epi; Vascular Disease Epi ,diabetes Epi.

Section 2 Methods of Epidemiology

Descriptive Study Observational Methods

Analysis Study

Clinical Trail

Experimental Methods Field Trail

Community Trail

1.Observational Methods

Descriptive Study(Descriptive Epidemiology)

Main job: to descript the distribution of diseases or health condition in different place, population and time.

How can we get these datum?We can use methods as below:cross-sectional

study or ecology study.

Main aim:to supply clues of cause of diseases.

If we know the distribution of disease,we can know the incidence rate is more in which place,which population and which time. Then we survey or study the factor which exists more frequency in this population or place. Maybe, this factor is the risk factor of this disease.

For example: lung cancerAt the begin,we don’t know the cause

of lung cancer.Through descriptive study, we can

know the characters of distribution of this disease.

Population: incidence rate in male>female in older>younger

Place: consumption of tobacco is more,incidence rate is higher.

Time: current year> before time

Raise clue or hypotheses of cause:Smoking?

True or not? Analytic Study

Case-Control StudyAim: to test the hypotheses of cause of disease.

Steps of case-control study:① Selecting two groups of subjects:

patients group (who have the interested

disease) controls group ( without the disease)

② Surveying the exposures history which maybe have relationship with the disease using questionnaire.③ Comparing the frequencies of exposur

e in two groups.

④ If group1> group 2 : draw a conclusion---there are relationship between the exposure and the disease.

Pay attention to : not a cause-effect association.

  Cohort StudiesAim: also to test the hypotheses ,moreover tes

t cause-effect association, because the direction is prospective.

Steps of cohort study: ① Selecting two groups: exposure group (presence of exposure to

a particular factor) Non-exposure group (absence of factor)

② Following-up for a specified period time:

In general,the follow-up period must be at least several years in order to allow for an adequate number to develop the outcome,so that meaningful comparisons can be made.

③ Comparing the incidence rates of researching disease between exposure group and non-exposure group.④ If group 1 > group 2 , draw a conclusion:

there is a cause-effect association between exposure and disease.

2.Experimental Study

Experimental studies may be viewed as a type of prospective cohort study because participants are identified on the basis of their exposure status and followed to determine whether they develop the disease.

The distinguishing feature of this study is that the exposure status of each participant is assigned by the investigator.

Steps of experimental studies: ① Selecting a number of objects and

then divided them into two groups at random.

Experimental group ( must be treated by one or more

intervention factors) Control group ( untreated or control

factors)

②Following-up a certain period of time , and determined the incidence rates of disease of two groups.

③ Comparing the incidence rates of two groups.

④Judging the effect of interventions.

Three types: Study unite AimClinical trail individuals of patients to estimate a kind of

new drug or treatment

methodField trail health individual who to estimate the effect of a has no interesting disease kind of vaccineCommunity trail population to estimate or judge a kind of prevention method

Section 3 Contributions of Epidemiology to the Medical Science

Investigating the Models of

Transmission of A New Disease Determining Preventable

Causes(or risk factor) of Disease Preventing and Controlling Diseases

Usually refer to infectious diseases.When a new disease emerges in some

population ,the first step we want to do is to control and eliminate it. But we must know the cause or transmission of this disease. Then ,according to this to take steps.

1.Investigating the Models of Transmission of A New Disease

However, to determine the cause of a disease need a very long time,it’s so complicate. Maybe, when we know the cause ,there are so many people suffered from this disease,even died from it. It’s too late.

On the contrary, studying the transmission mode of a new disease maybe simply a little. From the transmission mode, we can know the agent spread from one person to another by means of which route.

If we have known the transmission of agent, we can take steps to cut the transmission route to control this disease.

For example: AIDS(Acquired Immuno-deficiency Syndrome)The first patient appeared in 1981. Agent---HIV was found and separated in 1

983.Till now, so many people have suffered fr

om HIV,however we have no methods for this virus.

Transmission route: sexual activity;sharing of needles ;exchange of blood and blood products.

Prevention and controlling methods: use condoms;avoid sharing

needles;and institute programs to exchange needles and screen blood.

2.Determining Preventable

Causes(or risk factor) of Disease

Particularly refers to noninfectious diseases,such as hypertension, coronary heart disease(CHD),and cancers.

For these diseases, maybe we don’t know the true or direct cause, but if we know the risk factors of this diseases we can prevent them.

Risk factor refers to the factor which can increase the incidence rate of disease , if it is not exist ,the incidence rate will decrease.

So risk factor is the preventable cause of disease.

For example: CHDTill now, we don’t know the true cause

of CHD.But we can using epidemiology methods to investigate the risk factors.

Researchers found that hypertension, smoking can increase the incidence rate of CHD. Then we can take steps according to these factors. Such as getting rid of smoking, decreasing blood pressure by use of drugs or other methods.

We can see the incidence rate decrease also. So, we call hypertension and smoking are the preventable causes of CHD.

3.Preventing and Controlling Diseases

For chronic noninfectious diseasesThe development of disease is often

an irregularly progress.There is no obvious limit from healthy to ill.It’s a continuous variable progress.

Natural history of disease: refers to the course of disease over

time,unaffected treatment.Natural history can be divided into 4

stages.

(1)Stage of Susceptibility: In this stage,disease has not

developed,but risk factors have been present.

For example,CHDRisk factors: smoking, hypertension,high

serum cholesterol levelIf someone is healthy now, but he is a

smoker,or his blood pressure is high. He is in the susceptibility of CHD.

(2)Stage of pre-symptomatic disease(sub-clinical)

At this stage,there is no manifest disease, no obvious symptom or sign, but pathogenetic changes have started to occur.

For the patients in this stage, we usually rely on some tests or examinations to find them.

For example, lung cancer.In sub-clinical stage,patients has no

symptom or sign, they have been found usually through health examination.Through x-ray test, we can see there is small cancer tissue in lung.

(3)Stage of Clinical DiseaseBy this stage sufficient end-organ

changes have occurred,so that there are recognizable signs or symptoms to disease.

(4) Stage of disabilitySome diseases run their course and

then resolve completely with or without treatment.

However,there are a number of conditions which cause disability.

Disability is defined as “any temporary or long-term reduction of a person’s activity as a result of an acute or chronic condition.” Such as blind,deaf,lameness.

Note that the emphasis is on loss of function rather than on structural defect.

Three-step prevention:

primary prevention

secondary prevention

tertiary prevention

(1)Primary preventionPrimary prevention is aimed at

susceptible stage of disease. It is prevention of disease by altering susceptibility or reducing exposure for susceptible individuals.

Because in this stage, people have no disease,but may be they have already exposed some risk factors(or cause of disease),we can according to these risk factors to prevent disease.

In broad sense, cause of disease does not refer to a certain cause of disease, it is the cause of many diseases.

For example,smoking, drinking, fat have relation with so much diseases, such as lung cancer, hepatitis cancer, CHD,hypertension and so on.

So,we can take steps according to these factors ,such as get rid of smoking,don’t drinking,control weight ,or loss weight.

Through these methods we can prevent many diseases.This is one category of primary prevention.It is suitable for the diseases which the cause is undetermined.

There also has another ---specific protective measures.

If we know the risk factor of a disease, we can take this measure.

For infectious diseases, we know the pathogen ,so we can immunize people using antibody to this pathogen.

(2)secondary prevention aim to pre-clinical and clinical stageMain contents: three early---find early diagnose early treat early

Find early (basis)When someone is in the pre-clinical stage of

disease, his tissue may be occur change but he has no obvious symptom or specific sign.

So we can not find him by seeing.But, we can find him early through census, or sampling survey depending on more sensitive methods.

Suspicious patients

diagnose early (core)For suspicious patient, we can

diagnose them exactly by using of more specificity examination or test.

Treat early (aim)For patients diagnosed definitely, we

should treat them according to their special conditions.

In secondary prevention,even if we can not stop the occurrence of disease,but we can slow its progression ,limit disability and improve their life quality.

(3) Tertiary prevention

This consists of limitation of disability and rehabilitation where disease has already occurred and left residual damage.

If someone has suffered from a kind of disease already and has obvious symptom,he should be treated according to his symptom.

Doctor should also prevent him occurring syndrome and sequela,and prevent disability.

However,if someone has occurred disability,the major work we should do is rehabilitation,which includes two aspects: social and vocational rehabilitation.

Social rehabilitation means we should make the people who are disability feel their roles in society and self sufficiently. We can use various methods to maintain disable people’s social status.

Vocational rehabilitation:We should supply suitable work for

them according to their different characterization of every disable people.

Important points:1.definition of epidemiology: object ,range and aim of Epi.2.main methods of epidemiology:typing3.content of three-step prevention