PRINCIPLES OF EPIDEMIOLOGY&DISEASE SURVEILLANCE

PRINCIPLES OF EPIDEMIOLOGY&DISEASE

SURVEILLANCEBY

DR IDOWU AJIBOLA

DEPARTMENT OF COMMUNITY MEDICINE

BOWEN UNIVERSITY TEACHING HOSPITAL

OGBOMOSO

• To define epidemiology and explain its salient features

• Describe the types and forms of epidemiology

• Give brief history of epidemiology

• State the objectives of epidemiology

• Describe epidemiological tools

• Describe disease distribution in terms of person, place and time

• Define types of epidemics

Course objectives

EPIDEMIOLOGY IS THE STUDY OF THE DISTRIBUTION AND DETERMINANTS OF DISEASES AND HEALTH RELATED EVENTS IN SPECIFIED (HUMAN) POPULATIONS, AND THE APPLICATION OF THIS STUDY TO THE CONTROL OF HEALTH PROBLEMS•Last JM, editor. Dictionary of epidemiology. 4th ed. New York: Oxford University Press;2001. p. 61.

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Epidemiology defined

• Study – it means epid is a scientific researchable discipline

• Distribution – describing health problems in terms of frequency and patterns of occurrence: who, where and when

• Determinants – describing factors that are associated with disease occurrence

• Application – applying knowledge gained to prevent&control the health problem

• Health related events - not limited to disease entities only but includes study of other health related issues such lifestyles of people

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KEY WORDS IN THE DEFINITION/CONCEPTS OF EPID

• To identify disease etiology

• To determine the extent/magnitude/burden of a

disease

• To study the natural history of a disease

• To evaluate effectiveness of control programs

• To provide foundation for developing health

policies

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Objectives of epidemiology

• 5TH CENTURY: HIPPOCRATES suggested possible relationship between environmental and personal factors and disease- father of medicine

• 1662: JOHN GRAUNT described patterns of disease in a population; noted seasonal variations and excess male births and deaths- father of vital statistics

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HISTORICAL CONTEXT

• 1839-79: WILLIAM FARR set up a system for routine compilation of the numbers and causes of deaths. Also compared mortality patterns of different groups

• 1849, 1854: JOHN SNOW -described cholera epidemics in London- father of modern epid

• 1879: ROBERT KOCH – identified the tubercle Baccilus and Anthrax under the microscope

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HISTORICAL CONTEXT

• 1930s -1940s – application of epidemiological methods to the study of non-communicable diseases

• post world war ii – application extended to the study of entire range of health related outcomes, behaviour, including knowledge and attitude• doll and hill studies linking smoking and lung cancer

• Framingham studies of cardiovascular diseases

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HISTORICAL CONTEXT

• Generally, there are two main ways of describing epidemiology– Descriptive – assesses variation in the frequency and

distribution of disease in terms of person, place, and time.

– Analytic – searches for the determinants/risk factors/ethiology of a disease or the reasons for relatively high or low frequency in specific population

N.B: Disease frequency often involves counting the number of cases, estimating incidence and prevalence rates

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Types of Epidemiology

MEASURE OF DISEASE FREQUENCY

• Incidence rate= number of new cases occurring per time x1000

Population at risk

• Prevalence rate= Total number of new and old cases per time x1000

population at risk

Prevalence =Incidence rate X time

Prevalence increases when

-incidence rate increases

-when the disease is chronic in nature

Prevalence decreases when;

-incidence is low

-there is effective treatment leading to cure of cases

-the case fatality rate is high

• Field epidemiology – is population-oriented

-It deals with science of human behaviour and tried to explain social causes and consequences of disease

• Clinical epidemiology – focuses on health care and explores factors that affect the diagnosis, treatment and outcome in patient care

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Forms of applied Epidemiology

• Infectious Disease Epidemiology– e.g Ebola& Lassa Fever epidemiology

-Its more heavily dependent on lab support (microbiology, serology).

• Non Communicable Disease Epidemiology – e.g.Cancer epi, Psychiatric epi, etc.

• Molecular Epidemiology

• Environmental Epidemiology

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Forms of Epidemiology cont..

•Diseases are described based on;

-Persons affected

-Place where cases are found

-Time of occurrence of most cases

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Describing a disease-descriptive epidemiology

Person

Place

Time

Cases

0

5

10

15

20

25

1 2 3 4 5 6 7 8 9 10

0

200

400

600

800

1000

1200

0-4 '5-14 '15-

44

'45-

64

'64+

Age Group

Descriptive Epidemiology

Who? Where? When?

• Describing disease based on persons affected-distribution of a disease is described based on characteristics of persons affected such as;

• AGE-some diseases are commoner among adults compared to children e.g type2 DM, hptn

• SEX-diseases such as onchocerciasis, schistosomiasis are commoner among men

• OCCUPATION e.g blood borne infections among doctors

• INCOME e.g STIs are commoner among low incomeearners

• SOCIO-ECONOMIC STATUS e.g STI are commoner among the people in lower socio-economic class

• ETHNICITY/race e.g hptn among blacks,CAD among caucasians

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• PLACE-disease is described based on where most cases occur.

-urban vs rural e.g maternal deaths is higher in rural areas of

Nigeria

-regional distribution – some diseases are commoner in certain

regions as shown below

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Place: International differences

Source: Lucas AO. Gilles HM. Short Textbook of Public Health Medicine for the Tropics, Arnold 2003

Yellow fever endemic regions-Africa

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Example:Regional differences

• Describing diseases based on time of occurrence

• Different types of variation exist with regards to disease occurrence. A disease can be described;

-Based on duration withing which most cases occur:Short term Vs long term variation

-Based on Periodic flunctuation of the disease: Seasonal variation,cyclical variation

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• Occurrence of cases of an illness or other health-related events in specified human population clearly in excess of normal expectation

• Disease Outbreak=Localised epidemics

• Types of Epidemics:

A. Common- source Epidemics

B. Propagated Epidemics

C. Slow Epidemics: Takes long time before epidemics occurs e.g NCDs

Short term variations-Epidemics

• Common source epidemics

a. Common source, point source. Characterized by

People are exposed to the infectious agent at the same place and at the same time. Characteristics include;

-brief and simultaneous exposure

-the epidemic tends to be explosive

-all cases occur within one incubation period of the infectious agent

-the epidemic curve has one peak

-the curve rises and falls rapidly with no secondary waves

e.g epidemics of food poison from food eaten at a party

Common source-point source

b. Common source-continuous exposure.

Exposure to the agent occurs at the same place but at different times.

Characterized by;

-prolonged or repeated exposures

-e.g a prostitutes could be a common source for gonorrhoea infection and other STDs

-a well of contaminated water could be common/repeated source for cholera

• Propagated epidemics

-occurs mostly in diseases that can be transmitted from person to person through direct contacts

-curve shows a gradual rise and tails off over a longer period of time (i.e with secondary waves)

e.g transmission of measles: epidemic occur when the herds immunity drops i.e when the number of susceptible children increases

-Herds immunity is the level of immunity of the whole community against a disease

-Herds immunity can decrease when the number of unimmunized persons in the community rises due to low immunization coverage, emigration of immunized persons or in-migration of unimmunized persons

Ring immunization involves surrounding and infected person with immunized person to limit disease spread.

PROPAGATED EPIDEMICS