Introduction to Epidemiology by

Dr. M.Logaraj, M.D,Dept. of community medicine

How these inferences are arrived

Normal pulse rate 60-90/minute Measles & chicken pox mostly occurs during

spring Cancer of stomach more common in Japanese Lung cancer is common among smokers Paracetomol is 90% effective in headache Metronidazole is effective against Amoebiasis

Epidemiology What is epidemiology ? We study health and disease

1. By observing individuals

2. By laboratory of experimental animals

3. By measuring the distribution of health problem in population

Third one is epidemiology – putting people into groups

John snow- epidemic of cholera

Located the home of each person of cholera in London 1854

Found out association between source of water supply and cholera

Cholera – spread by contaminated water before discovery of the organism of cholera

John snow

British Doctors Study

By Doll & Hill The relationship between cigarette smoking

& lung cancer in 1950 a follow up study on British doctors

Epidemiology

Epidemiology derived from Greek word Epi-- on or upon demos-- people logos-- the study The study upon the people or population

Definition

The study of the distribution and determinants of health related states or events in specified population, and the application of this study to control of health problems

Disease frequency Disease distribution Disease determinants

Disease frequency

Epidemiology is concerned with the frequency and pattern of health events in a population

Measurements of frequency of disease, disability or death in the form of rates and ratios

Rates are essential for the comparison of frequency in different population

Comparison may yield important clue for the etiology or formulation of etiological hypothesis

Distribution of disease

Time characteristics include annual occurrence, seasonal occurrence and daily or even hourly occurrence during an epidemic

Place characteristics include geographic variation, urban- rural differences.

Personal characteristics include age, sex, race, marital status, socioeconomic status, Behaviour and environmental exposure

This aspect of study is called as descriptive study

Descriptive epidemiology provides what, when, and where of health related events

What is the event or disease? What is the magnitude? When did it happen? Where did it happen? Who are affected? The important out come distribution study is

formulation of etiological hypothesis

Determinants of disease

Test the etiological hypothesis and identify the underlying causes or risk factors of disease

This aspect of epidemiology is Analytical epidemiology

Which provides why and how of such events by comparing groups with different rates of disease occurrence.

By searching the differences in the characteristics between the diseased and healthy

Making comparison

The basic approach in epidemiology is to make comparison and draw conclusion

By comparison we try to find out curial differences in the host and environmental factors between those affected and not affected

Basic tools of measurement are necessary for the comparison –rates, ratio and proportion

Rate

There were 500 deaths from motor vehicle accident in city A

In epidemiology compare the rates of accident in city a with city B

Rate – elements denominator, numerator, time specification and multiplier.

Crude rates Specific rates Standardized rates

Ratio

Another measure of disease frequency Shows the relation in size between two

quantities The numerator not a component of the

denominator Sex ratio, doctor population ratio, child

woman ratio etc.

Proportion

Shows the relation in magnitude of the part of the whole

The numerator is always included in the denominator

Proportional mortality rate

Measurements of mortality

Mortality data provides the starting point for many epidemiological studies.

Mortality data is relatively easy to collect and reasonably accurate

The basis of mortality data is the death certificate

International death certificate

For national and international comparison a standardized system of recording and classification death

Part I – deals with immediate cause( pneumonia) and underlying cause of death( strangulated hernia)

Part II – deals with associated disease that contributed to the death( diabetes)

Limitation of mortality data

Incomplete reporting of death Lack of accuracy Lack of uniformity Choosing a single cause of death Changing coding system Diseases with low fatality

Uses mortality data

Can explain the trends and differences in overall mortality

Help in prioritization for health action Allocation of scares resource For assessment and monitoring of public

health programmes Gives important clue for epidemiological

research

Commonly used mortality rates and ratio

Crude death rate – simplest measure, lack comparability

Specific death rate – age, disease, income, religion etc.

Case fatality- killing power of disease for acute and not chronic

Proportional mortality rate –cause, age etc. can be used when population data are not available

Survival rate- usually for five years Standardized rates – direct and indirect

Measurements of morbidity

Any departure, subjective or objective, from a state of physiological well-being

Sickness, illness, disability

Measured by

1. Persons who are ill

2. Illness frequency( spells of illness)

3. The duration

Incidence rate

The number of NEW cases occurring in a defined population during a specific period of time.

Incidence

Number of new cases of specific Disease during a given time period

Population at risk during that period

X 1000

Uses of incidence rate

Taking action to control disease Research into the etiology or causation Research into the pathogenesis Studying distribution of disease Test the efficacy of preventive and

therapeutic measures Used for formulating and testing the

hypothesis

Special incidence rates

Attack rate Used only when the population is exposed to

risk for a limited period of time such as during an epidemic

Usually expressed as a percentage

Secondary attack rate

The number of exposed person developing the disease within the range of the incubation period following exposure to primary case

SAR

Number of exposed person developing the disease within the range of the incubation period

Total number of exposed/ susceptible contacts

X100

Secondary Attack Rate

Limited to application in infectious diseases In disease where there are numerous sub-

clinical cases Useful to determine the disease of unknown

etiology is communicable or not Useful in evaluating the effectiveness of

control measures – immunization

Prevalence

All current cases ( old or new) existing at a given point of time or over a period of time in a given population

Point prevalence Period prevalence

Start of illness

Duration of illness

Incidence - case 3,4,5 & 8

Jan 1 Dec 31

Case 1

Case 2Case 3

Case 4

Case 5

Case 6

Case 7

Case 8

Point prevalence Jan 1- 1, 2 & 7

Point prevalence Dec 31- 1,3,5 & 8 Period prevalence Jan-Dec- 1,2,3,4,5,7,&8

Number of cases Jan- Dec

Relationship between prevalence & incidence

• Depends upon two factors, incidence & duration

P = I X D

Prevalence = incidence X mean duration

Longer the duration of the disease the greater the prevalence A decrease in incidence & duration will decrease prevalence

• For chronic diseases (TB)- high prevalence rate relative to incidence

• For acute diseases ( food poisoning, diarrhoea)- prevalence is relatively low compared to incidence

• For acute disease- no prevalence ( No of episodes)

• Treatment decreasing the duration will decrease the prevalence

• Treatment preventing death but no recovery will increase the prevalence

Uses of prevalence

• To estimate the magnitude of health/disease problems in a community

• Identify potential high risk population

• Useful for administrative & planning purpose ( No of hospital beds, man power need, rehabilitation facilities)

Aims of epidemiology

To describe the distribution and magnitude of health and disease problem in human population

To identify etiological factors in the pathogenesis of disease

To provide data essential to the planning, implementation and evaluation of services for the prevention, control and treatment of disease and setting up priorities

Epidemiology and Clinical medicine Epidemiology Clinical medicine

Population at risk Case or cases

Both sick and healthy Only sick

Relevant data by studying group or population

History taking Sign and symptoms Lab investigation

Patient comes to the doctor Investigator goes to the community

A knowledge of prevalence, etiology and prognosis derived from epidemiological research is important to the clinician for the diagnosis and management of individual patient