Epidemiology- I

Mahmood ur Rahman

Definition of Epidemiology

Epidemiology is the study of the distribution and determinants of diseases or states in population groups and the application of this study to the control of health problems

(Last JM ed. Dictionary of Epidemiology, Oxford University Press, 1995)

'Epidemiology is that field of medical science which is concerned with the relationship of various factors and conditions which determine the frequencies and distributions of an infectious process, a disease, or a physiologic state in a human community'

(Lilienfeld 1978)

Historical Perspective

• Hippocrates - 5th century Association between external

environment and personal characteristics and health

“Whoever wishes to investigate medicine properly should proceed thus: in the first place consider the seasons of the year, and what effects each of them produces. Then the winds, the hot and the cold, especially such are as common to all countries, and then such as are peculiar to each locality. In the same manner, when one comes into a city to which he is a stranger, he should consider its situation, how it lies as to the winds and the rising of the sun; for it influence is not the same whether it lies to the north or the south, to the rising or to the setting sun. One should consider most attentively the waters which the inhabitants use, whether they be marshy and soft, or hard and running form elevated and rocky situations, and then if saltish and unfit for cooking; and the ground, whether it be naked and deficient in water, or wooded and well watered, and whether it lies in a hallow, confined situation, or is elevated and cold; and the mode in which the inhabitants live, and what are their pursuits, whether they are fond of drinking and eating to excess, and given to indolence, or are fond of exercise and labor.” (Hippocrates, “On airs, waters and places” Medical Classics 3:19, 1938).

Historical Perspective• John Graunt – 1662 (Hennekins and Buring

1987)The Nature and Political Observations Made Upon the

Bills of Mortality Systematic statistical approach

• Analyzed births and deaths in London– Excess of males born, higher mortality for males– Infant mortality is very high– Seasonal variation for mortality

Importance of routinely collected information for study of human illness

• William Farr - 1839• Examined mortality and occupation and marital status

Identified important issues in epidemiological investigations • Use of comparison population, influence of multiple factors on

disease

Historical Perspective

• John Snow (1854) – Father of modern epidemiology• Established modern epidemiologic methods

Cholera epidemic in London• Plotted geographical location of all cases – deaths

from cholera

From The Visual Display of Quantitative Data, Edward R. Tufte

John Snow (cont) Went door to door, collecting information on

daily habits• Suspected water supply as source of epidemic

– Broad street pump closed, epidemic stopped

Mode of investigation – “shoe leather” Practical application of epidemiology – use

epidemiological investigation to impact a health problem

How the Epidemiologist Works• Studies origin and distribution of a health

problem• Collection of data

• Constructs a logical chain of inferences to explain the various factors in a society or segment of society that cause a health problem to exist Likened to a detective investigating the

scene of a crime looking for clues• Starts with examination of sick person(s)• Extends investigation to the setting where

illness is occurring• Looks for common denominator that links all

the affected so that the cause of the problem can be eliminated or controlled

Epidemiologic Analyses – Areas of Study

Causal agents related to disease:1. Biological agents – bacteria, viruses,

insects2. Nutritional agents – diet (fats,

carbohydrates, food nutrients)3. Chemical agents – gases, toxic agents4. Physical agents – climate, vegetation,

chemical pollutants (air, water, food)5. Social agents – occupation, stress, social

class, lifestyle, location of residence

Epidemiologist studies:

• Host characteristics: Biological factors

• Age, sex, degree of immunity, other physical attributes that promote resistance or susceptibility

Behavioral factors • Habits, culture, lifestyle

• Social environment Living conditions such as poverty, crowding Norms, values and attitudes

• Socially prescribed standards of living– Use of food and water, food handling practices– Household and personal hygiene

Eras of Epidemiology

1. Sanitary era – early 19th century2. Infectious disease era – between late

19th century and early 20th century3. Chronic disease era – 2nd half of 20th

century4. Eco-epidemiology era – 21st century

Comparing the job of a clinician

and the job of an epidemiologistThe clinician

• Deals with patients• Takes a history • Conducts a physical• Makes a diagnosis • Proposes a

treatment• Follows up the

patient

The epidemiologist • Deals with populations• Frames the question• Investigates• Draws conclusions • Gives

recommendations• Evaluates programmes

Attack rate of acute hepatitis by age and sex, Baripada, Orissa, India, 2004

Cases Population Attack rateper 1000

Age 0-4 1 1012 0.1

5-9 11 21802 2

10-14 37 74004 5

15-44 416 51358 81

45+ 73 56153 13

Sex Male 341 102683 3.3

Female 197 101646 1.9

Person

Uses of epidemiology1. Examine causation2. Study natural history3. Description of the health status of

population4. Determine the relative importance of

causes of illness, disability and death5. Evaluation of interventions6. Identify risk factors

1. Examine causation

Genetic factors

Environmental factors

(Biological, chemical, physical, psychological

factors)

Good health Ill health

Life style related factors

To study historical rise and fall of disease in the

population• The diseases wax and wane

(tuberculosis ) new ones appear (encephalitis, asbestos poisoning, LSD psychosis) old ones are eradicated (smallpox)

Number of countries with smallpox, 1967-1978

A Pertussis by 5-Year Age Groups B. Pertussis by

<1, 4-Year, Then 5-YearAge Groups

   

Lung Cancer Rates in the United States, 1930–1999

 Infant Mortality Rates for 2002, by Race and Ethnicity of Mother

Chip

at

river

Attack rate of acute hepatitis by zone of residence, Orissa, India,

20040 - 0.9 / 1000

1 - 9.9 / 1000

10 -19.9 / 1000

20+ / 1000

Attack rate

Underground water supply

Pump from river bed

Place

Common Source (Vehicle), Single (Point) Exposure

Outbreak of Cholera, London, August-September , 1854

 

Common Source, Continued exposure :

Propogated source

Seasonal fluctuations :

• Malaria and JE are commoner during immediate post monsoon season.

• Asthma shows highest incidence during spring and autumn suggesting specific environmental factors in its causation

Secular trend

Searching for causes and risk factors

• Theories in Primitive and Middle Ages• Supernatural Causes e.g. being possessed by evil

spirits, wrath of gods, punishment for evil deeds during previous births “bad air”

• Contagion Theory• William Farr -Miasma Theory• Germ Theory - Henle-Koch postulates• Epidemiological Wheel Theory• The Theory of “Necessary” and Sufficient”

cause

Role of the host, the agent and the environment in the occurrence of

disease

VECTOR

AGENT

HOSTENVIRONMENT

Biologic, Chemical,

Physical (injury, trauma)Social

Psychological

GenotypeNutritionImmunityBehaviour

SanitationWeatherPollution

Socio-CulturalPolitical

Causes of Tuberculosis

Syndrome identification 

• Epidemiologists are called “lumpers and splitters”• The differentiation of hepatitis A from hepatitis B

and the distinction between several varieties of childhood leukemia.

• “lumping” include the identification of many manifestations of tuberculosis.

• Patterson-kelly syndrome of association between dysphagia and iron deficiency anaemia but when association was tested by epidemiological studies , it was not found.

Individual risk and chances: Life table-The ‘risks’ the average

male in England

Measures of disease frequency

• Prevalence Number of cases of a disease in a

defined population at specified point of time

• Incidence Number of new cases, episodes or

events occurring over a defined period of time

Incidence rate

Number of people who get

the disease or condition

in a specified timeTotal population at risk X FactorI =

Population at risk

Men Women

0-25 years

25-69 years

70+ years

25-69 years

Total population

All women(age groups)

Population at risk

Eg. Population at risk in a study of carcinoma of cervix

Prevalence

Number of people with the disease or condition at a specified time

Total population at risk X FactorP =

How to remember this: think about a bathtub

Community diagnosis :• The definition of indicators is a pre-requisite

Indicators - to estimate the burden of illness and the strategies for control.

• The main health indicators are expressed in terms of crude age-adjusted or age-specific mortality rates (such as infant mortality rates, mortality for children under 5, or maternal mortality rates), disease-specific morbidity rates, and life expectancy at birth.

   

3. Study natural history

Good health

Sub-clinical disease

Clinical disease

Recovery

Death

Natural history of disease timeline

4. Evaluation of interventions

Good Health Ill Health

Treatment, Medical

care

Health promotion

Preventive measures

Public health services

Evaluation

• Evaluation is the process of determining, as systematically and objectively as possible, the relevance, effectiveness, efficiency, and impact of activities with respect to established goals.

• Effectiveness refers to the ability of a program to produce the intended or expected results in the field; effectiveness differs from efficacy, which is the ability to produce results under ideal conditions.

• Efficiency refers to the ability of the program to reproduce the intended results with a minimum expenditure of time and resources.

Descriptive Epidemiology

Collecting all data describing occurrence of disease

• Person• Disease predominates in smokers• Disease predominates immuno-compromised individuals

• Place• Disease is associated with an arid climate• Disease is associated with a tropical climate

• Time• Disease associated with the date of company picnic• A particular year

Disease surveillance

1. Frequency with which the disease occurs

2. Definition of the disease 3. Size of the population

from which the cases develop

4. Completeness of the reporting of the cases.

Types of epidemiologyDistribution − Frequency of health events− By person, time and place

Descriptive epidemiology

Determinant− Search for causes or risk factors− Response to a study hypothesis− Use various epidemiologic methods

Analytic epidemiology

Health, disease or injury Disease-specific epidemiology

Application Applied epidemiology

Epidemiologic triad

ENVIRONMENTAGENT

HOST

DISEASE

Factors Associated with Increased Risk of Human

DiseaseHOST

(Intrinsic)AGENTS ENVIRONMENT

(Extrinsic)

•Age•Gender•Ethnicity•Religion•Customs•Occupation•Heredity•Marital status•Family background•Previous diseases

•Biological (bacteria, etc.)•Chemical (poison, alcohol, smoke)•Physical (auto, radiation, fire)•Nutritional (lack, excess)

•Temperature•Humidity•Altitude•Crowding•Housing•Neighborhood•Water•Milk•Food•Radiation•Air pollution•Noise

EPIDEMIOLOGYScientific basis of public health

Measure of mortality and morbidity

Rates Ratio Proportion

Risk (Cumulative incidence) / Prevalence / Incidence

• Ratio is one number divided by another number (numerator doesn’t have to be included in denominator—and vice versa)

• Proportion is a ratio in which the numerator is included in the denominator It has no dimension because the unit of the

numerator cancels out the unit of the denominator

• Rate is a ratio The numerator is the number of events

• The unit is event (i.e., number of cases) The denominator is the sum of follow-up time

contributed by the people at risk of the event• The unit is time or, more accurately, person-time to

account for duration of time of follow-up