Cause of Disease
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Cause of Disease Yang Haiyan [email protected] 2008.11.

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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.


Population

Distribution

Cause

Prevention


  • Definition and Models of Cause of

    Disease

  • Classification of Cause of Diseases

  • Process of Study on Cause of Disease

  • Evaluation of Presence of A Valid

    Statistical Association

  • Criteria in the Judgment of Cause of

    Disease


Section 1 definition and models of cause of disease
Section 1 Definition and Models of Cause of Disease

  • Definition of Cause of Disease

    Cause of disease is the factors which can increase the incidence rate of disease. And if one or more of these factors don’t exist, the incidence rate of population will decrease.


In epidemiology, we can also describe these factors as risk factors. That means these factors can be indirect causes of disease .

For example,

smoking--?--?--?--?---lung cancer


  • Models of Cause of Disease

    (1)Organism Human Disease

    only suitable for infectious diseases.

    For example, tubercle germ---tuberculosis(TB)

    However, the organism alone is an essential factor, but not a sufficient factor for infectious diseases.


Host

(2)Triangle Model

Environment

Pathogen

For example, influenza virus influenza.


In fact, this model is also suitable for infectious diseases. But, in this model, environments are attended.

For example:

tubercle + low level of immunity

tuberculosis


Social Environment diseases. But, in this model, environments are attended.

(3)The Wheel Model

Biologic

Environment

Host

G

Physical Environment

Genetic Core


The relative size of the different components of the wheel depend upon the specific disease.

For hereditary diseases, the genetic core would be relatively large.

For a disease likemeasles, the genetic core would be smaller, the state of immunity of the host and biological sector of environment would be larger.


Section 2 classification of cause of diseases
Section 2 Classification of Cause of Diseases depend upon the specific disease.

Host Factor (Intrinsic)

Genetic Factor

Immunity

Personality


(1) depend upon the specific disease. Genetic Factor

Most diseases have relationship with genetic factor , but the degree is different.

Some diseases only due to genetic factors, such as color blinding.


Some diseases depend upon the specific disease. partly due to genetic factors,partly due to environment factors.

For example, ABO blood type

Type A gastric cancer

Type O duodenal ulcer


(2) depend upon the specific disease. Immunity

low immunity---tuberculosis

(3) Personality

Type A: CHD (coronary heart disease)

Type B: Cancer


Environmental Factors (Extrinsic depend upon the specific disease. )

(1)Biological Environment

such as bacteria, virus

(2)Social Environment

In Japan stomach cancer


(3) depend upon the specific disease. Physical Environment

Including heat, light, air, water, radiation, atmospheric pressure, and so on.

Air pollution chronic respiratory disease, such as chronic bronchitis and lung cancer

(4)Chemical Environment

Such as heavy metal, trace element.


Section 3 Process of Study on depend upon the specific disease.

Cause of Disease


1 depend upon the specific disease. Descriptive study to understand the distribution of disease formulate etiologic hypotheses

For example: smoking and lung cancer


For example: depend upon the specific disease. lung cancer

At 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: depend upon the specific disease. 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


2 depend upon the specific disease. Analysis study and experimental Epidemiology

test the hypotheses

case-control study:

Frequency of smoking in group 1 > group 2

Relationship exists

Patients with lung cancer

survey

Two groups

inpatients


But depend upon the specific disease. , We can not estimate cause-effect relationship.

Why? The direction of this kind study is retrospective, cause effect.

If we want to know whether there is cause—effect relationship, we must do cohort study.


Follow up depend upon the specific disease.

smoking

Two groups

Non-smoking

  • Cohort study

    Incidence rate of lung cancer in smoking group >

    Non-smoking group

    Because the direction of cohort study is prospective,

    cause effect

Cause-effect relationship exits


Experimental Epidemiology depend upon the specific disease.

Clinical trails

Intervention test

Filed trail


Experimental group depend upon the specific disease.

Smoking people

Example of intervention test:

Lung cancer ----- smoking

Control group

Follow up

Give up smoking

Comparing incidence rate

Continue smoking

If group 1< group 2

Cause-effect relationship


3 depend upon the specific disease. Judgment of A Cause-effect Relationship

In this chapter, we mainly talk about judgment of cause-effect relationship.

But ,before this we must do a work firstly,that is:


Section 4 Evaluation of Presence of depend upon the specific disease.

A Valid Statistical Association


In epidemiology study ,we can find relationship between cause and disease, but it is just a statistical association. That means it’s may be the true effect of an exposure on the development of disease, and it also possible has other explanation:

① it’s maybe spurious association

② it’s maybe indirect association


1 Spurious Association cause and disease, but it is just a statistical association. That means it

In the course of epidemiology such as design, selecting subjects, dividing groups,collecting data and analysis data, if there is difference between this two groups, the result we obtained is error, we can also call the relationship as spurious association.


For example cause and disease, but it is just a statistical association. That means it: smoking --- lung cancer

In case-control study

malefemale

Patient group: 80% 20%

Control group: 50% 50%

Result : smoking frequency

patient group > control group


A cause and disease, but it is just a statistical association. That means it

2 Indirect association

C

B

Indirect association


smoking cause and disease, but it is just a statistical association. That means it

Example:

Stomach ulcer

Lung cancer


Section 5 Criteria in the Judgment of Cause of Disease cause and disease, but it is just a statistical association. That means it

There are about 6 criteria in the judgment of cause of disease. After epidemiological study has been done we shall use these criteria to deduce the cause of disease.


1 cause and disease, but it is just a statistical association. That means itTime Sequence

The exposure occurs in former and the disease occurs in later, this is an essential condition to judge a cause-effect relationship.


It is easy to judge time sequence in prospective cohort study , or randomized trail.

But in case-control study or cross-sectional study, it is difficult.


2 Strength of the Association study , or randomized trail.

We usually express strength of the association by use of risk ratio (RR) .

RR is more, the possibility of association between exposure and disease is larger.

incidence rate of exposure group

RR=

incidence rate of non exposure group


RR of smoking and lung cancer is 9 study , or randomized trail.

RR of smoking and CHD is only 2

It is showed that smoking is more likely the cause of lung cancer than CHD.

In general, if the design and analysis are all correct and RR is more than 3 or 4, we can believe that the exposure is the cause of this disease.


3 Biologic Credibility study , or randomized trail.

If there is a known or postulated mechanism which supports a cause and effect relationship, it can enhance the relationship.


For example, study , or randomized trail.smoking ---lung cancer

There were so many chemical carcinogens in tar, such as arsenic, carbon monoxide, and so on. Mean while , let dog inhale the cigarette smoking ,the dog occurred lung cancer.


4 study , or randomized trail.Reeligible of Association

To a cause-effect relationship, if different researchers use different methods in different time or different place among different populations, they all show similar or same results, this can support association much more.


For example study , or randomized trail.,

Doll and Hill smoking and lung cancer

more than 7 times cohort study

more than 35 times case-control study


5 study , or randomized trail.Dose-Response Relationship

When the degree of exposure has changed, if the morbidity of disease changed corresponding, the possibility of a cause-effect relationship is more large.

For example, the more amount population smoke every day, the higher mortality rate they dead from lung cancer.


6 If eliminating the exposure,the morbidity rate of disease will decrease.

For example, after having got rid of smoking ,we found incidence rate and mortality rate of lung cancer decrease.This enhance the persuasiveness to cause-effect association.

In fact, this list also corresponding the second sentence in definition of cause of disease.


These 6 are criteria when we judge a cause-effect relationship . But that is not to say, judging a relationship must accord with all of this 6 criteria.

Time sequence is essential. For the other 5criteria, the more criteria it accords with, the higher possibility the hypotheses establishes.


Summary
Summary relationship . But that is not to say, judging a relationship must accord with all of this 6 criteria.

  • Epidemiologists are concerned with discovering the causes of disease in the environment, nutrition, life-style, and genes of individuals and populations-------that is, the causes or factors that when removed or modified will be followed by a reduction in the disease burde



Important contents
Important contents: relationship between a factor and a disease

  • Definition of cause of disease

  • Process of Study on Cause of Disease

  • Criteria in the Judgment of Cause of

    Disease


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