Epidemiology 代丽萍 Dai Liping
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 medicineStudy level Elementary Medicinemolecular or sub molecular Immunology Biochemistry; Cell biology Clinical Medicineindividual patients Internal Medicine;surgery Preventive Medicinepopulations 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 cancer Classical 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 StudyObservational 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 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: 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 Study Aim:to test the hypotheses of cause of disease. Steps of case-control study: ①Selecting two groups of subjects: patients group (who have the interested disease) controls group ( without the disease)
②Surveying the exposures history which maybe have relationship with the disease using questionnaire. ③Comparing the frequencies of exposure in two groups.
④If group1> group 2 : draw a conclusion---there are relationship between the exposure and the disease. Pay attention to : not a cause-effect association.
Cohort Studies Aim: also to test the hypotheses ,moreover test cause-effect association, because the direction is prospective.
Steps of cohort study: ①Selecting two groups: exposure group (presence of exposure to a particular factor) Non-exposure group (absence of factor)
②Following-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.
③Comparing the incidence rates of researching disease between exposure group and non-exposure group. ④If 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. ③Comparing the incidence rates of two groups. ④Judging the effect of interventions.
Three types: Study unite Aim Clinical trailindividuals of patientsto estimate a kind of new drug or treatment method Field trailhealth individual whoto estimate the effect of a has no interesting diseasekind of vaccine Community trailpopulation 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
1.Investigating the Models of Transmission of A New Disease 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.
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 1983. Till now, so many people have suffered from 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: CHD Till 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 diseases The 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,CHD Risk factors: smoking, hypertension,high serum cholesterol level If 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 Disease By this stage sufficient end-organ changes have occurred,so that there are recognizable signs or symptoms to disease.