EPIDEMIOLOGY

FOR

Health Science Students

Lecture Note Series

Kifle Wolde Michael

Yigzaw Kebede

Kidist Lulu

Jimma University and Gondar University College

 EPIDEMILOGY

FOR

Health Science Students

Lecture Note Series

Kifle Wolde Michael, MD, MPH

Assistant Professor of Public Health

Jimma University

Yigzaw Kebede, MD, MPH

Associate Professor of Public Health

Gondar University College

Kidist Lulu, MD, MPH

Assistant Professor of Public Health

Jimma University

November 2003

Supported by the Carter Center

Jimma University and Gondar University College

Preface

Shortage of teaching materials within higher education institutions is a major

problem in Ethiopia. This fact holds true for epidemiology as well. Very few text
books that are available within the institutes are too vast and detailed. Moreover,
there is no epidemiology lecture note prepared for undergraduate health science
students. This resulted in institutional variations in the coverage of this particular
course. Therefore this lecture note is prepared to alleviate the aforementioned
problem.

This lecture note is mainly prepared for health officer and medical students. We
believe that other health science students will be beneficiaries of this material.
Acknowledgements

We would like to express our gratitude and appreciation to The Ethiopian Public
Health Training Initiative (The Carter Center) for initiating, coordinating and
financing the preparation of this lecture note. We also extend our thanks to
Jimma University and Gondar University College for supporting and permitting us
to work on the teaching material.

We are deeply indebted to Professor Yemane Berhane, an epidemiologist and

Public health specialist at the Addis Ababa University (AAU), Faculty of Medicine
and Dr. Mekonnen Assefa, an epidemiologist and public health specialist at
Jimma University (JU), Faculty of Public health for their meticulous review of the
lecture note and constructive comments.

Last but not least we acknowledge the contributions of our students who directly
or indirectly inspired us to prepare this lecture note.
Contents

Preface
Acknowledgements
Chapter One: Basic Concepts in Public Health
1
Objective
Definitions
Differences between Public health and Clinical medicine
Methods of Community Diagnosis
Exercise
Chapter Two: The Subject Matter of Epidemiology
7
Objective
Definition
History
Scope of Epidemiology
Purpose of Epidemiology
Basic Assumptions in Epidemiology
Types of Epidemiology
Exercise
Chapter Three: Principles of Disease Causation and Models
12
Objective
Definition
Principles of disease causation
Germ Theory
Ecological Approach
Models of disease causation
Epidemiological Triangle Model
Web of Causation Model
 The Wheel Model
Exercise

Chapter Four: Natural History of diseases and Levels of Prevention

20
Objective
Definition
Stages in the Natural History of Diseases
Levels of Prevention
Exercise
Chapter Five: Infectious disease Process
28
Objective
Components of the Infectious Disease Process
Exercise
Chapter six: Sources of Data for Community Health
36
Objective
Census
Vital Statistics
Health Service Records
Morbidity and Mortality Surveys
Exercise
Chapter Seven: Measurements of Morbidity and Mortality
42
Objective
Ratios, Proportions and Rates
Measures of Morbidity
Measures of Mortality
Errors in Measurement and their Sources
Exercise
Chapter eight: Descriptive Epidemiology
61
Objective
Definition
The major characteristics in descriptive epidemiology
Epidemiologic study designs
Descriptive study designs
Exercise
Chapter Nine: Analytic Epidemiology
68
Objective
Definition
Observational analytic studies
Experimental/Intervention studies
Measures of Association
Exercise
Chapter Ten: Evaluation of Evidence
85
Objective
Analysis of cause-effect relationships
Exercise
Chapter Eleven: Epidemic Investigation and Management
93
Objective
Levels of Disease occurrence
Definition of Epidemic
Types of Epidemic
Investigation of Epidemic
Management of Epidemic
Exercise
Chapter Twelve : Epidemiological Surveillance
104
Objective
Definition of Surveillance
Purpose of Surveillance
Types of Surveillance
Steps in Surveillance
Sources of Data
Exercise
Chapter Thirteen: Screening: Program and Evaluation
Objective
Definition
Diseases appropriate for screening
Criteria for establishing screening programs
Screening tests
Evaluation of screening
Exercise

Reference
Chapter One
Basic Concepts in Community Health

Objectives:

At the end of this chapter the student is expected to:

Define health, public health, disease/ illness, and community diagnosis

Distinguish between clinical medicine and public health/community

medicine

Describe the different methods of community diagnosis

Definition
Health is a difficult concept to define. Traditionally, health was equated with
survival, or absence of death. In fact, mortality is still used as a measure of
health. The next stage was to see health as the absence of disease. This
definition is still the most widely used in practice. But nearly everyone agrees that
health is more than the absence of disease, and many attempts have been made
to come up with a broader definition. The World Health Organization (WHO) in
1947 defined health as “a state of complete physical, mental, and social well-
being and not merely the absence of disease or infirmity”. The definition
does emphasize the multidimensionality of health and the existence of positive
health, and it serves as an ideal. Other approaches are less ambitious, referring
to absence of disease, disability, or handicap. The Ottawa Charter for Health
Promotion (World Health Organization, 1986), as described in Epidemiologic
Methods for Health Policy by Spasoff R, states that “to reach a state of complete
physical, mental and social well-being, an individual or group must be able to
identify and to realize aspirations, to satisfy needs, and to change or cope with
the environment. Health is, therefore, seen as a resource for everyday life, not
the objective of living. Health is a positive concept emphasizing social and
personal resources, as well as physical capacities”. This is consistent with the
call in the WHO’s Health for All declaration for all people to attain a level of health
“that will permit them to lead a socially and economically productive life”.

Therefore, health is taken to be a multifaceted concept. It consists of:

 Physical health

 Mental health

 Social health

 Emotional health

 Spiritual health and

 Occupational health

These have a continuous interaction with each other.

Physical health: Efficient bodily functioning, resistance to disease and the
physical capacity to respond to varied events.

Mental health: Capacity to cope with life situations, grow in awareness and
consciousness.

Social health: Good relations with others, a supportive culture and successful
adaptation to the environment.

Emotional health: The ability to control emotions and express them comfortably and
appropriately.
Spiritual health: The ability to discover and articulate a personal purpose in life, learn
how to experience love, joy, peace and fulfillment.
Occupational health: Feelings of comfort and accomplishment related to one's
daily tasks.

Clinical versus community medicine:

Knowledge about human health and disease arises from basic sciences (e.g.,
biochemistry, physiology, pathology), clinical sciences (e.g., medicine, surgery,
obstetrics and gynecology, pediatrics) and population medicine (e.g.,
epidemiology, biostatistics, health service management and planning). In different
settings, population medicine is also referred to as community medicine,
preventive medicine, or social medicine, or, more traditionally, as public health.
Clinical medicine is concerned with diagnosing and treating diseases in
individual patients, while community medicine is concerned with diagnosing the
health problems of a community, and with planning and managing community
health services. In 1920, Winslow defined Public health as a science and an art
of preventing disease, prolonging life, and promoting health and efficiency
through organized community effort for sanitation, control of communicable
disease, health education, etc. It necessitates a systematic way of studying both
the patterns of occurrence of disease in a community and the patterns of delivery
of medical care. Information about the illnesses prevalent in the community also
contributes to diagnosis. Conversely, assessment of the level of occurrence of
disease in a population is dependent on the accuracy of the diagnosis made on
individual patients and on the completeness with which reportable diseases are
made known to public health authorities. This indicates that the two approaches
(clinical and community medicine) are complementary to each other.

Information on the health and disease of a defined community is gathered through

Community Diagnosis. It is defined as the process of identification and detailed
description of the most important health problems of a given community. As patient’s
history, physical findings and laboratory data are the basis for making a clinical diagnosis
there are some methods that allow the making of community diagnosis.
Methods of Community Diagnosis:

1. Discussion with community leaders and health workers

2. Survey of available health records

3. Field survey. Conducting study on a sampled population or total

population.

4. Compilation and analysis of the data.

It is impossible to address all the identified problems at the same time because of
resource scarcity. Therefore the problems should be put in the order of priority
using a set criterion.

Criteria for priority setting

Magnitude (amount or frequency) of the problem

Severity (to what extent is the problem disabling, fatal)

Feasibility (availability of financial and material resource, effective

control method)

Community concern (whether it is a felt problem of the

community)

Government concern (policy support, political commitment)

In summary, in clinical medicine, the procedure consists of history taking,

physical examination and laboratory investigation on individual patient to make
diagnosis that is followed by treatment and follow up. In community medicine the
community diagnosis is first made through field survey, record review and
discussion with the community members. This is followed by intervention on
selected priority problems. The intervention programs are monitored continuously
and evaluated periodically.

Disease, Illness and Sickness

Disease, illness and sickness are loosely interchangeable terms but are better
regarded as wholly synonymous. Disease is literally the opposite of ease. It is
physiological or psychological dysfunction. Illness is the subjective state of a
person who feels aware of not being well and Sickness is a state of social
dysfunction; i.e. a role that an individual assumes when ill.

Many different diseases occur in the community. Some diseases usually last a
short time: days or weeks. Examples are most diarrhoeal diseases, measles, and
pneumonia. These are called acute diseases. Others last much longer, often for
many months or years. These are called chronic diseases. Examples are
tuberculosis, leprosy, diabetes, heart disease and cancer.

Risk Factors

Health workers need to know how healthy people can stay healthy. Many
diseases have known causes. For example Schistosomiasis is caused by
schistosome organism and measles by measles virus. These diseases cannot
occur without these specific causes. But the agent alone may not be responsible
for the onset of the disease. For example in the case of schistosomiasis if
somebody is not working or playing in a cercariae infected water the infection
cannot occur. These factors (the availability of infected water and the behaviour
of the individual) are called risk factors. Risk factor is any factor associated with
an increased or decreased occurrence of disease. A factor associated with an
increased occurrence of a disease is risk factor for the exposed group; and a
factor associated with a decreased occurrence of a disease is a risk factor for the
non exposed group.

Risk factors could be:

1. Factors related to the agent:

  Strain difference

2. Factors related to the human host

 Lack of specific immunity.

3. Factors related to the environment

 Overcrowding, Lack of ventilation

Risk factors may further be classified as:

1. Factors susceptible to change

 smoking habit, alcohol drinking habit

2. Factors not amenable to change

 age, sex, family history

In order to be able to prevent disease, it is vital to identify factors that can be

changed. For some diseases, the specific causes are not known. In such cases
it is very important to identify risk factors, especially those that can be changed
and act on them. Epidemiology is mainly interested in those risk factors that are
amenable to change as its ultimate purpose is to prevent and control disease and
promote the health of the population.

Exercise I

1. Write a list of the most common diseases in your area.

2. What is the difference between clinical and community medicine?

 3. How do you make a community diagnosis?

4. Write all the risk factors for the following diseases:

a) Diarrhoea

b) Tuberculosis

c) HIV/AIDS

5. Which of the risk factors you have listed above are amenable to change?

Chapter Two
The Subject Matter of Epidemiology (Definition, Scope, Purpose)

OBJECTIVES

At the end of this chapter the student is expected to:

 Define epidemiology
  Discuss the history of epidemiology

 be able to discuss the components of the definition and

 describe the scope and purpose of epidemiology

Definition

Epidemiology is the study of the frequency, distribution and determinants of

diseases and other health related states or events in specified populations, and
the application of this study to the promotion of health, and to the prevention and
control of health problems.

Epidemiology offers insight into why disease and injury afflict some people more
than others, and why they occur more frequently in some locations and times
than in others. It is an applied science, with direct and practical applications. This
knowledge is necessary for finding the most effective ways to prevent and treat
health problems. It is considered the basic science of public health.

Components of the definition

“Population” the focus of epidemiology is mainly on the population rather than

individuals.
“Frequency” shows epidemiology to be mainly a quantitative science.
Epidemiology is concerned with the frequency of diseases and other health
related conditions. Frequency of diseases is measured by morbidity rates and
mortality rates.

“Health related conditions” are conditions which directly or indirectly affect or

influence health. These may be injuries, vital events, health related behaviors,
social factors, economic factors etc.

“Distribution” refers to the geographical distribution of diseases, the distribution

in time, or/and distribution by type of persons affected.

The part of epidemiology concerned with the frequency and distribution of

diseases by time, person and place is named Descriptive Epidemiology. It
asks the questions: how many? Where? When? What?

“Determinants” are factors which determine whether or not a person will get
a disease. The part of epidemiology dealing with the causes and determinants of
diseases is Analytical Epidemiology. It asks the questions: how? Why?

History of Epidemiology

Although epidemiological thinking has been traced to the time of Hippocrates,

who lived around 5th century B.C., the discipline did not flourish until the 1940s.

 Hippocrates displayed an extraordinary awareness of the impact of

environment and behavior on personal well–being. Hippocrates therefore
identified forces that epidemiologists today recognize as major
determinants of human health.

 The most important advances in epidemiology is attributed to the English

man John Graunt (1620 – 1674). In his pioneering research, Graunt
noted that biological phenomena, such as births and deaths, varied in
predictable and regular ways. His research laid the groundwork for the
disciplines of both epidemiology and demography. He observed that male
births consistently outnumbered female births. Graunt also noted a
relatively higher urban than rural death rate and seasonal variation in
 mortality rates. His work is summarized in the “Natural and Political
Observations…. Upon the Bills of Mortality”, which was first published in
England in 1662. He analyzed reports of births and deaths, quantified
patterns of disease in a population, noted seasonal variation in mortality,
recognized the value of routinely collected data in providing information
and noted high infant mortality rate (IMR).

 In 1747, Lind used an experimental approach to prove the cause of

scurvy by showing it could be treated effectively with fresh fruit.

 In 1839, William Farr, an English physician, established the tradition of

application of vital statistical data for the evaluation of health problems.

 In 1849, John Snow an English physician formulated and tested a

hypothesis concerning the origin of an epidemic of cholera in London.
Snow postulated that cholera was transmitted by contaminated water.

Epidemiology is a relatively new discipline, and its scope and purposes are
widening from time to time.

Scope of Epidemiology

Originally, epidemiology was concerned with epidemics of communicable

diseases and epidemic investigations. Later it was extended to endemic
communicable diseases and non-communicable diseases.

At present epidemiologic methods are being applied to:

 Infectious and non infectious diseases

 Injuries and accidents
 Nutritional deficiencies
 Mental disorders
 Maternal and child health
 Congenital anomalies
  Cancer
 Occupational health
 Environmental health
 Health behaviors
 Violence etc.

Hence, epidemiology can be applied to all disease conditions and other health
related events.

Purpose/Use of Epidemiology

The ultimate purpose of Epidemiology is prevention and control of disease, in an

effort to improve the health status of populations. This is realized through:

1. Elucidation of the natural history of disease

2. Description of the health status of the population

3. Establishing the determinants/causation of disease

4. Evaluation of intervention

Basic Assumptions in Epidemiology

There are two basic assumptions in epidemiology. These are:

 Non random distribution of diseases i.e. the distribution of disease in

human population is not random or by chance and

 Human diseases have causal and preventive factors that can be identified
through systematic investigations of different populations.

Since distribution of diseases is not random or by chance, we need to identify

what factors lead to the higher level of occurrence of a disease in one area as
compared to others. Epidemiology is also based on the assumption that
diseases have causal and preventive factors and these can be identified by
studying human populations at different places and times.

Exercise
II.1 Why do we need to study the frequency of diseases?

___________________________________________________________
___________________________________________________________
___________________________________________________________
______________________

II.2 Why do we need to study the distribution of diseases?

___________________________________________________________
___________________________________________________________
___________________________________________________________
______________________

II.3 Why do we need to study the determinants of diseases?

___________________________________________________________
___________________________________________________________
___________________________________________________________
______________________

II.4 Which of the following are included under epidemiology? Tick in the
boxes.

□ studying the distribution of malaria in a region

□ studying risk factors for hypertension in the adult population of a region

□ studying why road traffic accidents have increased in a city

□ identifying environmental hazards related with development of cancer

□ studying cultural factors influencing the feeding habit of people in an area

II.5 Why do we need to study epidemiology?

Chapter Three
Principles of Disease Causation and Models

OBJECTIVES
At the end of this chapter the student is expected to:

 Discuss the concept of disease causation

 Describe the principles and models of causation

Disease Causation

Cause of a disease: is an event, condition, or characteristic that preceded the

disease event and without which the disease event either would not have
occurred at all, or would not have occurred until some later time. A common
characteristic of the concept of causation is the assumption of a one-to-one
correspondence between the observed cause and the effect. Each cause is seen
as necessary and sufficient in itself to produce the effect. But the present
understanding is that the cause of any effect must consist of a constellation of
components that act in concert. A “sufficient cause,” which means a complete
causal mechanism, can be defined as a set of minimal conditions and events that
inevitably produce disease; “minimal” implies that all of the conditions or events
are necessary.

Principle of Causation

There are two principles of disease causation. Namely:

1. The single germ theory and

2. The ecological approach

The Germ theory

Luis Pasteur isolated microorganism. This discovery led to Koch's postulate in

1877. It was a set rule for the determination of causation.

Koch's Postulate states that:

 The organism must be present in every case.

 The organism must be isolated and grown in culture.

 The organism must, when inoculated into a susceptible animal,

cause the specific disease.

 The organism must then be recovered from the animal.

The Ecological approach

Ecology is defined as the study of the relationship of organisms to each other as

well as to all other aspects of the environment. Since disease arises within an
ecological system, a basic tenet of epidemiology is that an ecological approach is
necessary to explain the occurrence of disease; disease cannot be attributed to
the operation of any one factor. The requirement that more than one factor be
present for disease to develop is referred to as multiple causation or multifactorial
etiology.

In the ecological view, an agent is considered to be necessary but not sufficient

cause of disease because the conditions of the host and environment must also
be optimal for a disease to develop.

Example: Mycobacterium tubercle bacilli is a necessary but not sufficient

cause for tuberculosis

Etiology of disease: All factors that contribute to the occurrence of a disease.

These factors are related to agent, host and environment.
I. The Agent

A. Nutritive element

Excessive Cholesterol
Deficiency Vitamin, Protein

B. Chemical Agents

Poison Carbon monoxide (CO)

C. Physical Agents Radiation

D. Infectious Agents

Metazoa Hookworm, Schistosomiasis

Protozoa Amoeba
Bacteria M.Tb
Fungus Candidiasis
Virus Measles

II. Host Factors: Influence exposure, susceptibility or response to agents.

 Genetic
o Age
o Sex
 Physiologic state
o Pregnancy
o Puberty
o stress
 Immunologic condition
o Active immunity: Prior infection, immunization
o Passive immunity: Gamma globulin
 Human behavior
o Hygiene
 o Diet handling

* Host factors result from the interaction of genetic endowment with the
environment.

Example:

 Blood group A has been found to be associated with higher

incidence of gastric carcinoma

 Blood group O has been found to be associated with higher

incidence of duodenal ulcer

III. Environmental Factors: Influence the existence of the agent, exposure, or

susceptibility to agent.

A. Biological environment

 Infectious agents
 Reservoirs (man, animal, soil)
 Vectors (flies, mosquitoes)

B. Social environment

Socioeconomic and political organizations affect the level of medical care.

C. Physical environment

 Heat, Light, Water, Air

 Industrial wastes
 Chemical agents of all kinds
 Indoor air pollution

It is the interaction of the above factors (agent, host, and environment) which
determines whether or not a disease develops, and this can be illustrated using
different models.

Disease Models
How do diseases develop? Epidemiology helps researchers visualize disease
and injury etiology through models. There are a number of disease causation
models, however, the epidemiologic triangle, the web of causation, and the wheel
are among the best known of these models.

The epidemiologic triangle

Agent

Host Environment

The most familiar disease model, the epidemiologic triad (triangle), depicts a
relationship among three key factors in the occurrence of disease or injury:
agent, environment, and host.

An agent is a factor whose presence or absence, excess or deficit is necessary

for a particular disease or injury to occur. General classes of disease agents
include chemicals such as benzene, oxygen, and asbestos; microorganisms
such as bacteria, viruses, fungi, and protozoa; and physical energy sources such
as electricity and radiation. Many diseases and injuries have multiple agents.

The environment includes all external factors, other than the agent, that can
influence health. These factors are further categorized according to whether they
belong in the social, physical, or biological environments. The social
environment encompasses a broad range of factors, including laws about seat
belt, and helmet use; availability of medical care and health insurance; cultural
“dos” and “don’ts” regarding diet; and many other factors pertaining to political,
legal, economic, educational, communications, transportation, and health care
systems. Physical environmental factors that influence health include climate,
terrain, and pollution. Biological environmental influences include disease and
injury vectors; soil, humans and plants serving as reservoirs of infection; and
plant and animal sources of drugs and antigens.

The host is the actual or potential recipient or victim of disease or injury.

Although the agent and environment combine to “cause” the illness or injury, host
susceptibility is affected by personal characteristics such as age, occupation,
income, education, personality, behavior, and gender and other genetic traits.
Sometimes genes themselves are disease agents, as in hemophilia and sickle
cell anemia.

From the perspective of epidemiologic triad, the host, agent, and environment
can coexist harmoniously. Disease and injury occur only when there is
interaction or altered equilibrium between them. But if an agent, in
combination with environmental factors, can act on susceptible host to create
disease, then disruption of any link among these three factors can also prevent
disease.

The web of causation

Although the epidemiologic triad has contributed to the understanding of disease

etiology, the process that actually generates disease or leads to injury is much
more complex. This complexity is better portrayed in a second model, the web
of causation

The web of causation was developed especially to enhance understanding of

chronic disease, such as cardiovascular disease. However, it can also be
applied to the study of injury and communicable diseases. The web of causation
de-emphasizes the role of the agent and highlights other factors that encourage
the onset of disease. Using this model, scientists can diagram how factors such
as stress, diet, heredity, and physical activity relate to the onset of the three
major types of cardiovascular disease: coronary heart disease, cerebrovascular
disease (stroke), and hypertensive disease. In addition, the approach reveals
that each of these diseases has a precursor, for example, hypertension, that can
alert a diagnostician to the danger of a more serious underlying condition.
 Stress Diet

Hormones Physical activity

Smoking Obesity Heredity

Blood clotting Hardening of the arteries Hypertension

Heart disease Stroke Hypertensive

disease

The Wheel

A model that uses the wheel is another approach to depict human – environment
relations. The wheel consists of a hub (the host or human), which has genetic
makeup as its core. Surrounding the host is the environment, schematically
divided into biological, social, and physical. The relative sizes of the different
components of the wheel depend upon the specific disease problem under
consideration. For hereditary diseases, the genetic core would be relatively
large. For conditions like measles the genetic core would be of lesser
importance; the state of immunity of the host and the biological sector would
contribute more heavily. In contrast to the web of causation, the wheel model
does encourage separate delineation of host and environmental factors, a
distinction useful for epidemiologic analyses.
 Biologic Environment

Host (man)

Genetic core Social

Environment

Physical Environment

Exercise

III.1 Discuss the difference between a necessary cause and a sufficient

cause with examples.

___________________________________________________________
___________________________________________________________
___________________________________________________________
___________________________________________________________
___________________________________________
Chapter Four: Natural History of Disease and Levels of Prevention

OBJECTIVES

At the end of this chapter the student is expected to:

 Define the natural history of disease and its different stages

 Describe the levels of disease prevention and

 Apply these concepts in relation to diseases/ health problems of public

health importance in the country

Natural history of disease

The “natural history of disease” refers to the progression of disease process in an
individual over time, in the absence of intervention. Each disease has its own life
history, and thus, any general formulation of this process is arbitrary. However, it
is useful to develop a schematic picture of the natural history of diseases as a
frame work within which to understand and plan intervention measures including
prevention and control of diseases.

There are four stages in the natural history of a disease. These are:

Stage of susceptibility
1. Stage of pre-symptomatic (sub-clinical) disease

Stage of clinical disease and

Stage of disability or death
Stage of susceptibility
In this stage, disease has not yet developed, but the groundwork has been laid
by the presence of factors that favor its occurrence.

Examples:

 A person practicing casual and unprotected sex has a high risk of getting
HIV infection.

 An unvaccinated child is susceptible to measles.

 High cholesterol level increases the risk of coronary heart disease.

2. Stage of Pre-symptomatic (sub-clinical) disease

In this stage there is no manifest disease but pathogenic changes have started to
occur. There are no detectable signs or symptoms. The disease can only be
detected through special tests.

Examples:

 Detection of antibodies against HIV in an apparently healthy person.

 Ova of intestinal parasite in the stool of apparently healthy children.

The pre-symptomatic (sub-clinical) stage may lead to the clinical stage, or may
sometimes end in recovery without development of any signs or symptoms.
The Clinical stage
By this stage the person has developed signs and symptoms of the disease. The
clinical stage of different diseases differs in duration, severity and outcome. The
outcomes of this stage may be recovery, disability or death.

Examples:

 Common cold has a short and mild clinical stage and almost everyone
recovers quickly.

 Polio has a severe clinical stage and many patients develop paralysis
becoming disabled for the rest of their lives.

 Rabies has a relatively short but severe clinical stage and almost always
results in death.

 HIV/ AIDS has a relatively longer clinical stage and eventually results in
death.

Stage of disability or death

Some diseases run their course and then resolve completely either
spontaneously or by treatment. In others the disease may result in a residual
defect, leaving the person disabled for a short or longer duration. Still, other
diseases will end in death.

Disability is limitation of a person's activities including his role as a parent, wage

earner, etc…

Examples:

 Trachoma may cause blindness

 Meningitis may result in blindness or deafness. Meningitis may also result

in death.
 Healthy person

Sub clinical disease

Recovery
Clinical disease

Recovery Death
Disability

Figure 1 – A schematic diagram of the natural history of diseases and their

expected outcomes.

Disease Prevention

The major purpose in investigating the epidemiology of diseases is to learn how

to prevent and control them. Disease prevention means to interrupt or slow the
progression of disease. The aim is to push back the level of detection and
intervention to the precursors and risk factors of disease. Epidemiology plays a
central role in disease prevention by identifying those modifiable causes.
 Table 1- Levels of prevention in relation to the stage of disease.

Level of Stage of disease Aim Target

Prevention
Avoiding the emergence and
Primordial Existence of underlying establishment of the social, Total population
condition leading to causation. economic, and cultural patterns and selected
of living that are known to groups
contribute to an elevated risk of
disease.

Example: Smoking,
environmental pollution
The causative agent exists but
Specific causal factors exist. the aim is to prevent the Total population,
Primary development of the disease. selected groups
and healthy
Example: Immunization individuals
The aim is to cure patients and
Secondary Early stage of disease prevent the development of Patients
advanced disease.

Example: Early detection and

treatment of cases of
tuberculosis and STD
The aim is to prevent severe
Tertiary Late stage of disease disability and death Patients
(treatment and rehabilitation)
Example: Leprosy

1) Primary prevention is aimed at preventing healthy people from becoming

sick. The main objectives of primary prevention are promoting health,
preventing exposure and preventing disease. Primary prevention keeps
the disease process from becoming established by eliminating causes of
disease or increasing resistance to disease.

 Health promotion consists of general non-specific interventions

that enhance health and the body's ability to resist disease.
 Improvement of socioeconomic status, provision of adequate
food, housing, clothing, and education are good examples of
health promotion.

Prevention of exposure is the avoidance of factors which may cause disease if an

individual is exposed to them. Examples can be provision of safe and adequate water,
proper excreta disposal, and vector control.
 Prevention of disease is the prevention of disease
development after the individual has become exposed to the
disease causing factors. The timing is between exposure and
biological onset. Immunization can be taken as a good
example.

i. Active immunization- exposing the host to a specific

antigen against which it will manufacture its own
antibodies after three weeks interval.

ii. Passive immunization- providing the host with the

antibodies necessary to fight the disease. It is commonly
given after exposure. Example: Rabies, Tetanus.

Note: Both active and passive immunization act after exposure has
taken place. Immunization does not prevent an infectious organism
from invading the immunized host, but does prevent it from
establishing an infection.

2) Secondary prevention - Detecting people who already have the disease

as early as possible and treat them. It is carried out after the biological
onset of the disease, but before permanent damage sets in. The objective
of secondary prevention is to stop or slow the progression of disease
and to prevent or limit permanent damage.

Examples:

 Prevention of blindness from Trachoma

  Early detection and treatment of breast cancer to prevent its
progression to the invasive stage

3) Tertiary prevention – is targeted towards people with chronic diseases

and disabilities that cannot be cured. Tertiary prevention is needed in
some diseases because primary and secondary prevention have failed,
and in others because primary and secondary prevention are not effective.
It has two objectives:

 Treatment to prevent further disability or death and

 To limit the physical, psychological, social, and financial

impact of disability, thereby improving the quality of life. This can
be done through rehabilitation, which is the retraining of the
remaining functions for maximal effectiveness.

Examples:

 Blindness due to vitamin A deficiency occurs when primary prevention

(adequate nutrition) and secondary prevention (early detection of
corneal ulcers) have failed, and damage to the cornea (keratomalacia)
can not be treated. Tertiary prevention (rehabilitation) can help the
blind or partly blind person learn to do gainful work and be
economically self supporting.

 Diabetes mellitus is a disease that can not really be prevented or cured

i.e. primary and secondary prevention are not effective. Hence, the
goal of tertiary prevention in diabetics is to control the level of their
blood sugar using drugs and/ or diet, and to treat complications
promptly in order to improve the quality of life, prevent permanent
damages such as blindness, and prevent early death.
Exercise:

IV.1 Tick in the boxes if the given statement is true.

□ Not all people with a sub-clinical stage will develop the clinical
disease.
□ A person with polio antibodies in the blood must at some time have
had the clinical disease.
□ A clinical disease will result in recovery only if treated appropriately.

□ A person in the stage of susceptibility may not necessarily develop

the sub-clinical stage of a disease even in the presence of risk
factors for the disease.
□ A person with sub-clinical infection of typhoid does not have signs
or symptoms of typhoid fever.
Exercise:

IV.2 Discuss the natural history and levels of prevention for meningococcal
meningitis, specifying the target population at whom the specific
prevention measure is directed.

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CHAPTER FIVE: THE INFECTIOUS DISEASE PROCESS

OBJECTIVES

At the end of this chapter the student is expected to:

 Discuss the components of infectious process

 Describe different modes of disease transmission


The infectious process of a specific disease can be described by the following
components, which constitute of the chain of disease transmission.

1. The Agent
2. Its reservoirs
3. Its portal of exits
4. Its mode of transmission
5. Its portals of entry
6. The human host

I. The Agents

The agents in the infectious process range from viral particles to complex multi-
cellular organisms. These can be characterized through their:

 Size
 Chemical character
 Antigenic makeup
 Ability to survive outside the host
 Ability to produce toxin etc

Host agent interaction is characterized by infectivity, pathogenicity, virulence

or immunogenicity.

Infectivity: The ability of an agent to invade and multiply in a host, i.e. the
ability to produce infection

Pathogenicity: The ability to produce clinically apparent infection.

Virulence: The proportion of clinical cases resulting in severe clinical

disease.

Immunogenicity: The infection's ability to produce specific immunity.

Factors which can change the above properties for infectious agents are:
  Environmental conditions: may be favorable or unfavorable to the specific
agent
 Dose of the agent: severity of disease may be related to the amount
entering the host body
 Route of infection: the same agent may cause different levels of severity
according to the route of entry into the body
 Host factors (Age, race, nutritional status)

Pathogenic mechanisms

Infectious agents may bring about pathologic effects through different

mechanisms. Some agents may use more than one mechanism at ones, or
sometimes different mechanisms may lead to illnesses with different
characteristics as a result of infection by the same agent. The different
mechanisms employed by infectious pathogens are:

1. Direct tissue invasion

2. Production of a toxin

3. Immunologic enhancement or allergic reaction

4. Persistent or latent infection

5. Enhancement of host susceptibility to drugs.

6. Immune suppression

II. Reservoirs

A reservoir is an organism or habitat, in which an infectious agent normally lives,

transforms, develops and/or multiplies. Reservoirs for infectious agents may be
humans, animals, plants or other inanimate objects.

Some diseases with human reservoirs are:

 Most bacterial and viral respiratory diseases

 Most staphylococcal and streptococcal infections

  STD, mumps, typhoid etc.
All infected humans, whether showing signs and symptoms of the disease or not, are
potential sources of infection to others. A person who does not have apparent clinical
disease, but is a potential source of infection to other people is called a Carrier. Carriers
may be classified as:
 Incubatory carriers: Transmitting the disease during incubation period,
i.e. from first shedding of the agent until the clinical onset.

Example: Measles, mumps

 Convalescent carriers: Transmitting the disease during convalescence

period i.e. from the time of recovery to when shedding stops.

Example: Typhoid fever

 Asymptomatic carriers: Transmitting the disease without ever showing
manifestations of the disease.

Example: Polio, Amoebiasis

 Chronic carriers: Transmitting the disease for a long period / indefinite

transmission.

Example: Viral Hepatitis, Typhoid fever.

Some diseases are transmitted to human beings from animals. These diseases are called
zoonoses.
Examples: Rabies, anthrax, brucellosis etc.

III. Portal of exit

Portal of exit is the way the infectious agent leaves the reservoir. Possible
portals of exit include all body secretions and discharges: Mucus, saliva, tears,
breast milk, vaginal and cervical discharges, excretions (feces and urine), blood,
and tissues.

IV. Mode of Transmission

Modes of transmission include the various mechanisms by which agents are
conveyed to a susceptible host. Transmission may be direct or indirect.

1. Direct transmission

1.1 Direct contact: The contact of skin, mucosa, or conjunctiva with

infectious agents directly from person or vertebrate animal, via
touching, kissing, biting, passage through the birth canal, or during
sexual intercourse.

Example: HIV, rabies, gonorrhea

1.2 Direct projection: projection of saliva droplets by coughing,

sneezing, singing, spitting or talking.

Example: common cold

1.3 Transplacental: Transmission from mother to fetus.

Example: syphilis

2. Indirect transmission

2.1 Vehicle-borne: Transmission occurs through indirect contact with

inanimate objects (fomites): bedding, toys, or surgical instruments;
as well as through contaminated food, water, IV fluids etc.

2.2 Vector-borne: The infectious agent is conveyed by an arthropod to

a host. Vectors may be biological or mechanical.

Biological vector: If the agent multiplies in the vector before transmission.

 Salivarian Example: Malaria by the anophelus mosquito

 Stercorarian Example: Typhus by ticks or lice

Mechanical vector: If the agent is carried by the leg or proboscis.

Example: Trachoma by flies

2.2 Airborne: which may occur by dust or droplet nuclei (dried residue
of aerosols)
 Example: Tuberculosis

2.3 Non vector intermediate host: hosts not playing an active role in
transporting the agent to humans.

Example: Aquatic snails in the transmission of

schistosomiasis.

V. Portal of entry: the site where an infectious agent enters a susceptible

host. These are:

The Mucosa:
Nasal - common cold
 Conjunctival - Trachoma

 Respiratory - Tuberculosis

 Vaginal - Sexually transmitted diseases

 Urethral - Chlamydial infection

 Anal - Sexually transmitted diseases

Injury site: Tetanus

The skin: Hook worm infection (Ancylostomiasis)

VI. Host: The susceptible human host is the final link in the infectious
process. Host susceptibility can be seen at the individual level and at
the community level.

At the individual level: The state of the host at any given time is the
interaction of genetic endowment with the environment over the entire life
span. The relative contributions of genetics and environmental factors in the
susceptibility of the host for diseases are not always clear.

Examples:

 Genetic factors: sex, blood type, ethnicity etc.

 Environmental factors: immunity acquired as a result of past infection

At the community level: Host resistance at the community (population) level is
called herd immunity. Herd immunity can be defined as the resistance of a
community (group) to invasion and spread of an infectious agent, based on the
immunity of a high proportion of individuals in the community. The high
proportion of immunes prevents transmission by highly decreasing the probability
of contact between reservoirs and susceptible hosts.

Conditions under which herd immunity best functions

1. Single reservoir (the human host): If there is other source of infection it
can transmit the infection to susceptible hosts.

2. Direct transmission (direct contact or direct projection): Herd immunity is

less effective for diseases with efficient airborne transmission.

3. Total immunity: Partially immune hosts may continue to shed the agent,
and hence increase the likelihood of bringing the infection to susceptible
hosts.

4. No shedding of agents by immune hosts (no carrier state).

5. Uniform distribution of immunes: Unfortunately, susceptibles usually

happen to live in clusters or pockets because of socioeconomic, religious,
or geographic factors.

6. No overcrowding: Overcrowding also increases the likelihood of contact

between reservoirs and susceptible hosts.

However, these conditions for the operation of herd immunity are seldom fulfilled.

TIME COURSE OF AN INFECTIOUS DISEASE

Pre-patent Period: The time interval between biological onset and the time of
first shedding of the agent.

Incubation Period: Interval between biological onset and clinical onset.

Communicable Period: The time interval during which the agent is shed by the
host.

Latent Period: The interval between recovery and relapse in clinical disease.
Exercise:

Table 2. Identify the components in the chain of transmission for the following diseases.

Chain of transmission Malaria Meningococcal meningitis Tuberculosis

i. Infectious agent

ii. Reservoir

iii. Portal of exit

iv. Mode of transmission

v. Portal of entry

vi. Susceptible host

IV.1. Direct transmission of infectious agents include(s) the following

mode(s) of transmission (tick in the boxes).

 A. Blood transfusion

 B. Accidental injection with contaminated

needle

 C. Organ transplantation

 D. Droplet nuclei

 E. Droplet projection

 F. Drinking from a contaminated water source

 G. Insect bite

Chapter Six
Sources of Data for Community Health

Objectives:

At the end of this chapter the student is expected to:

Identify the sources for health information

Describe the advantages and disadvantages of each source

There are different sources of data on health and health related conditions in the
community. Each source has advantages and limitations. The information
obtained from these sources is used for health planning, programming and
evaluation of health services. The major sources are the following.

Census:
Census is defined as a periodic count or enumeration of a population. Census
data are necessary for accurate description of population’s health status and are
principal source of denominator for rates of disease & death.

It provides information on:

 Size and composition of a population

 The forces that determine these variability

 The trends anticipated in the future.

There are two types of census counts. They are called de facto and de jure. De
facto counts persons according to their location at the time of enumeration, but
excludes those who are temporarily away. De jure counts according to their
usual place of residence and excludes temporary visits.

In Ethiopia census was conducted twice, i.e., in 1984 and 1994. Data was
collected on:

 Age, sex and size of the population

 Mortality, fertility

 Language, ethnicity

 Housing

From these data different health indices could be calculated. Crude birth rate,
crude death rate, age specific mortality rate and sex specific mortality rate are
some of the examples of the indicators that could be calculated.

Limitation
 Conducting nationwide census is very expensive and it generates a
large amount of data which takes a very long time to compile and
analyze. .

 It is carried out every 10 years. Therefore it can’t assess yearly

changes.

Vital statistics:
This is a system by which all births and deaths occurring nationwide are
registered, reported and compiled centrally. Certificate is issued for each birth
and death. It is the source of information for the calculation of birth and death
rates. Cause specific mortality rate can also be calculated since cause of death is
recorded on death certificates. The denominator however comes from census.
The main characteristics of vital statistics are:

 Comprehensive – all births and deaths should be registered.

 Compulsory by law – should be enforced by law.

 Compiled centrally so that it can serve as a source of information.

 Continuous – it should be an ongoing process.

There is no nationwide birth and death registration system in Ethiopia.

Health Service Records: All health institutions report their activities to the
Ministry of Health. The Ministry compiles, analyzes the data and publishes it in
the health service directory. It is therefore the major source of health information
in Ethiopia.

Advantages:

 Easily obtainable

 Available at low cost

 Continuous system of reporting

 Causes of illness and death available.

Limitations:

 Lack of completeness – health service coverage is only 52%.

 Lack of representativeness – a small proportion of diseased

population seeks medical advice.

 Lack of denominator – catchment is not known in majority of

cases.
  Lack of uniformity in quality

 Diagnosis varies across the level of health institutions.

 Lack of compliance with reporting.

 Irregularity and incompleteness of published compilations.

Notification of Infectious Diseases

There are some internationally notifiable diseases. WHO member states report
on Plague, Cholera, and Yellow fever. Moreover, every country has its own list
of notifiable diseases. In Ethiopia, in addition to the above, the following diseases
are notifiable.

 Measles,

 Poliomyelitis,

 Neonatal Tetanus

 Meningococcal Meningitis

 Diarrhea,

 Diarrhea with severe dehydration in under five children

 Bloody diarrhea

 Typhoid Fever

 Tuberculosis,

 Malaria,

 Epidemic Typhus,

 Relapsing Fever

 Viral Hemorrhagic Fever

 HIV/AIDS
  Sexually Transmitted Infection (STI)

 Onchocerciasis

 Dracunculiasis

 Pneumonia in under five children

 Leprosy

The major problems related to this source are low compliance and delays in
reporting.

Health Surveys

These are studies conducted on a representative sample population to obtain

more comprehensive data for monitoring the health status of a population. There
are two types of health surveys:

Surveys of specific diseases: These are studies conducted on each specific

disease. Examples are:

 Expanded Programme for Immunization (EPI)

 Control of Diarrheal Diseases (CDD)

 Prevention and control of HIV/AIDS

 Prevention of Blindness

 Tuberculosis / Leprosy control

Surveys of general health status: These are studies on general health status of
the population. They are based on interview, physical examination and laboratory
tests. They are more reliable as compared to surveys of specific diseases but
also more expensive.

Advantages of surveys based on interview:

 They are more representative of the health condition of the community.

 The denominator is known.

  Data are more uniform in quality.

Limitations:

 Data accuracy is dependent on the memory and cooperation of the

interviewee.

 Surveys are expensive.

Exercise:

1. State the different sources of health information

EG, health survey and census

2. What is the major source of health information in Ethiopia?

Eg, health service record

3. Discuss the problems related to health service records as

source of health data.
Chapter Seven: Measurements of Morbidity and Mortality

OBJECTIVES

At the end of this chapter the student is expected to:

 Describe the principles of measurement in epidemiology

 Calculate different morbidity and mortality measures

Measurement of health

Epidemiology is mainly a quantitative science. Measures of disease frequency

are the basic tools of the epidemiological approach. Health status of a community
is assessed by the collection, compilation, analysis and interpretation of data on
illness (morbidity), on death (mortality), disability and utilization of health
services.

The most basic measure of disease frequency is a simple count of affected

individuals. Such information is useful for public health planners and
administrators for proper allocation of health care resources in a particular
community. However, to investigate distributions and determinants of disease, it
is also necessary to know the size of the source population from which affected
individuals were counted. One of the central concerns of epidemiology is to find
and enumerate appropriate denominators in order to describe and to compare
groups in a meaningful and useful way. Such measures allow direct comparisons
of disease frequencies in two or more groups of individuals.

Ratios, proportions, and rates

The most important epidemiological tool used for measuring diseases is the rate;
however, ratios and proportions are also used.

Ratio

A ratio quantifies the magnitude of one occurrence or condition to another. It

expresses the relationship between two numbers in the
x: y x/y X k
form of or

Example: The ratio of males to females in Ethiopia.

Proportion
A proportion quantifies occurrences in relation to the populations in which these
occurrences take place. It is a specific type of ratio in which the numerator is
included in the denominator and the result is expressed as a percentage.

Example: The proportion of all births that was male

Male births X 100

Male + Female births

Rate

Rate is a special form of proportion that includes the dimension of time. It is the
measure that most clearly expresses probability or risk of disease in a defined
population over a specified period of time, hence, it is considered to be a basic
measure of disease occurrence. Accurate count of all events of interest that
occur in a defined population during a specified period is essential for the
calculation of rate.

Rate = Number of events in a specific period x k

Pop at risk of these events in a specified Period

Example: The number of newly diagnosed breast cancer cases per 100,000
women.

Types of rates

There are three types of rates:

  Crude rates

 Specific rates

 Adjusted rates

Crude rates are summary rates based on the actual number of events (births,
deaths, diseases) in the total population over a given time period. The crude
rates that are widely used in description of populations are the crude birth rate
(CBR) and the crude death rate (CDR). These rates refer to the total population,
and hence, may obscure the possible difference in risk among subgroups of the
total population.

Example: the risk of death differs among different age groups

Crude death rates depend on two factors.

 The probability of dying for individuals

 The age distribution of the population

Advantages:

 Actual summary rates

 Calculable from minimum information

 Widely used despite limitations

Disadvantages:

 Difficult to interpret due to variation in composition (e.g.: age)

 Obscure significant differences in risk between subgroups.

Specific rates

Specific rates apply to specific subgroups in the population, such as a specific

age group, sex, occupation, marital status, etc. When calculating specific rates,
except for cause-specific rates, the denominator should be the population in that
specific group (NOT the total population). As a result, specific rates do not add
up to a crude rate.

** Do not add age specific rates to get crude rate, take the weighted average.

Example: Infant Mortality Rate (IMR), Neonatal Mortality Rate (NMR), Maternal
Mortality Ratio (MMR)

Advantages:

 The rates apply to homogenous subgroups

 The rates are detailed and useful for epidemiological and public health
purposes.

Disadvantages:

It is cumbersome to compare many subgroups of two or more populations

Adjusted rates

Adjusted rates are summary rates that have undergone statistical

transformation, to permit fair comparison between groups differing in some
characteristics that may affect risk of disease. For example: age needs
adjustment due to its marked effect on both diseases and death. When
comparing the crude death rates of two or more places, it is impossible to know
whether the difference is due to age composition, age specific death rate or both.
In Age adjusted rates the difference is exclusively attributed to differences in age
specific mortality rates, since the effect of age composition is artificially removed.

Advantages:

 Summary rates

 Permit unbiased comparison

 Easy to interpret

Disadvantages:

 Fictitious rates
  Absolute magnitude depends on standard population

 Opposing trends in subgroups masked.

Methods of adjustment

1. Direct method

When using the direct method, the adjusted rate is derived by applying the
category specific rates observed in each of the populations to a single standard
population.

Example: Table –2 shows the age-group age-specific mortality rate, and age
distribution of two hypothetical populations, A and B.

Table - 3 Age-specific mortality rate and age distribution of two populations, A and
B.

Age-group ASMR per 1000 ASMR per 1000 Age distribution Age distribution
(years) per year of per year of of population A of population B
population A population B

< 15 2 3 3000 4000

15-44 5 4 4000 5000

>44 20 20 3000 1000

Totals 10,000 10,000

To calculate the crude death rate (CDR):

 Multiply the ASMR in each age group by the number of people in the
same group; this will give the annual number of deaths occurring in the
specific age group.
  Add the number of deaths occurring in each age group to obtain the total
number of deaths

 Then divide the total number of deaths by the total population of each
area.

To calculate the age-adjusted rate:

 Use a standard population for each age group of both areas. Note that
the populations of the groups to be compared have to be equal when
standardizing. For example use 1000 as a standard for each age
category of both populations, or you may use one population either A or
B as a standard.

 Then follow the steps you took when calculating the CDR, but this time
using the standard population.

2. Indirect method

This method implies the process of applying the specific rates of a standard
population to a population of interest to yield a number of "expected" deaths. A
common way of carrying out indirect age adjustment is to relate the total
expected deaths thus obtained to observed deaths through a formula known as
the standardized mortality ratio (SMR).

SMR = Total observed deaths in a population

Total expected deaths in that population

If SMR > 1

More deaths are observed in the smaller population than would be expected on
the basis of rates in the larger (standard) population.
If SMR <1

Fewer deaths are observed than expected.

This method is used to compare two populations, in one of which the ASMR are
not known or are excessively variable because of small numbers.

Measurements of morbidity

Incidence
The incidence of a disease is defined as the number of new cases of a disease
that occur during a specified period of time in a population at risk for developing
the disease.

Incidence rate = Number of new cases of a disease over a period of time

Population at risk during the given period of time

The critical element in the definition of incidence is new cases of disease.

Incidence is a measure of events – the disease develops in a person who did not
have the disease previously. Because incidence is a measure of new events (i.e.
transition from a non-diseased to a diseased state), incidence is a measure of
risk. The appropriate denominator for incidence rate is population at risk. For
incidence to be meaningful, any individual who is included in the denominator
must have the potential to become part of the group that is counted in the
numerator. Thus, if we are calculating incidence for prostate cancer, the
denominator must include only men, because women are not at risk for
developing prostate cancer. Another important issue in regard to the
denominator is the issue of time. For incidence to be a measure of risk we must
specify a period of time and we must know that all of the individuals in the group
represented by the denominator have been followed up for that entire period. The
choice of time period is arbitrary: We could calculate incidence in one week,
incidence in one month, incidence in one year, incidence in 5 years, and so on.

Nevertheless the determination of population at risk is a major problem in the

study of disease incidences. It may require a detailed study based on:

 interviews

 medical records

 or serology for antibodies, which are very expensive and time consuming.

Population fluctuation due to births, deaths, and migration is another problem in

the calculation of the denominator.

Types of incidence

1. Cumulative Incidence (CI): An incidence rate that is calculated from a

population that is more or less stable (little fluctuation over the interval
considered), by taking the population at the beginning of the time period
as denominator. The cumulative incidence assumes that the entire
population at risk at the beginning of the study period has been followed
for the specified time interval for the development of the outcome under
investigation. It provides an estimate of the probability, or risk, that an
individual will develop a disease during a specified period of time.

CI = Number of new cases of a disease during a given period of time

Total population at risk

2. Incidence Density: An incidence rate whose denominator is calculated

using person-time units. Similar to other measure of incidence, the
numerator of the incidence density is the number of new cases in the
population. The denominator, however, is the sum of each individual’s
 time at risk or the sum of the time that each person remained under
observation, i.e., person – time denominator. This is particularly when one
is studying a group whose members are observed for different lengths of
time. In presenting incidence density, it is essential to specify the time
units – that is, whether the rate represents the number of cases per
person – day, person – month or person – year.

Incidence density = Number of new cases during a given period x 10 n

Time each person was observed, totaled for all

* Often used in cohort studies of diseases with long incubation or latency period.

Basic requirements for calculating incidence rates

1. Knowledge of the health status of the study population

To be able to classify people as “diseased" and "not diseased", there should be

adequate basis for assessing the health of the individuals in a population. The
information necessary for this may be obtained from health service records, or
may require screening or making detailed examination of the general population.

2. Time of Onset

Since incidence rates deal with newly developing diseases, identifying the date of
onset is necessary. However, this may be difficult for diseases with indefinite
onsets. For example for cancers the actual date of onset is practically impossible
to identify, therefore the date of onset is usually taken as the date of definite
diagnosis.
 3. Specification of Numerator: Number of persons versus number of conditions

Sometimes one person may have more than one episode of the illness under
study; therefore it is absolutely necessary to indicate whether the numerator
addresses number of conditions or number of persons.

Example: children may have more than one episode of diarrhea in a one-
year period. Hence, it is possible to construct two types of incidence rates
from this.

i. Number of children who developed diarrhea in one-year period

Number of children at risk

ii. Number of episodes of diarrhea in children in one-year period

Number of children at risk

4. Specification of Denominator:

The denominator for incidence studies should consist of a defined population that
is at risk of developing the disease under consideration. It should not include
those who have the disease or those who are not susceptible to the disease.

5. Period of Observation:

Incidence rates must be stated in terms of a definite period of time. It can be any
length of time. The time has to be long enough to ensure stability of the
numerator. Person-time denominator must be used for unequal periods of
observation. This helps to weigh the contribution of each study subjects when
there is attrition because; individuals die, move away or get lost to follow up.

Prevalence rate
Prevalence rate measures the number of people in a population who have a
disease at a given time. It includes both new and old cases. There are two types
of prevalence rates.

1. Period Prevalence rate

2. Point Prevalence rate

Period Prevalence rate measures the proportion of a population that is affected

with a certain condition during a specified period of time.

Period Prevalence rate = No. of people with the condition during a specific period of
time

Total population

Point Prevalence rate: measures the proportion of a population with a certain

condition at a given point in time. This is not a true rate; rather it is a simple
proportion.

Point Prevalence rate = All persons with a specific Condition at one point in time

Total population
**The basic requirements for prevalence study are similar to that of incidence
study except for “time of onset”.

Relationship between incidence and point prevalence

Since point prevalence rate includes both new and pre-existing cases, it is
directly related to the incidence rate. Point prevalence rate is directly proportional
both to the incidence rate and to the average duration of the disease.

Point Prevalence rate ~ IR x D

Uses

Prevalence rates are important particularly for:

 Chronic disease studies

 Planning health facilities and manpower

 Monitoring disease control programs

 Tracking changes in disease patterns over time

Incidence rate is important as:

 A fundamental tool for etiologic studies of acute and chronic diseases

 A direct measure of risk

High prevalence may reflect an increase in survival due to change in virulence

or in host factors or improvement in medical care.

Low prevalence may reflect:

 A rapidly fatal process

 Rapid cure of disease

 Low incidence

Limitations of prevalence studies

 Prevalence studies favor inclusion of chronic over acute cases.

  Disease status and attribute are measured at the same time; hence,
temporal relations cannot be established.

Measurements of Mortality: mortality rates and ratios

Mortality rates and ratios measure the occurrence of deaths in a population using
different ways. Rates whose denominators are the total population are commonly
calculated using either the mid - interval population or the average population.
This is done because population size fluctuates over time due to births, deaths
and migration.

Below are given some formulas for the commonly used mortality rates and ratios.

Crude Death rate (CDR) = Total no. of deaths reported during a given time interval X
1000
Estimated mid interval population

Age- specific mortality rate = No. of deaths in a specific age group during a given
time X1000
Estimated mid interval population of sp. age group

Sex- specific mortality rate = No. of deaths in a specific sex during a given time X
1000
Estimated mid interval population of same sex

Cause- specific mortality rate = No. of deaths from a specific cause during a given
time X 100,000
Estimated mid interval population

Proportionate mortality ratio = No. of deaths from a sp. cause during a given time x
100
Total no. of deaths from all causes in the same time

Case Fatality Rate (CFR) = No. of deaths from a sp. disease during a given time x
100
 No. of cases of that disease during the same time

Fetal Death Rate = No. of fetal deaths of 28 wks or more gestation reported
during a given time
No. of fetal deaths of 28 wks or more gestation and live births
in the same time

Perinatal Mortality Rate = No. of fetal deaths of 28 wks or more gestation Plus no. of infant
deaths under 7 days
No. of fetal deaths of 28 wks or more gestation plus the no. of live births
during the same

Neonatal Mortality Rate = No. of deaths under 28 days of age reported during a
given time x 1000
No. of live births reported during the same time

Infant mortality rate (IMR) = No. of deaths under 1 yr of age during a given time X
1000
No. of live births reported during the same time interval

Child mortality rate (CMR) = No. of deaths of 1-4 yrs of age during a given time X 1000
Average (mid-interval) population of same age at same time

Under- five mortality rate = No. of deaths of 0-4 yrs of age during a given time X
1000
Average (mid-interval) population of the same age at same
time

Maternal Mortality Ratio = No. of pregnancy associated deaths of mothers in a given

time x 100000
No. of live births in the same time

When calculating (using) mortality rates it is important to understand their interpretations

and how they differ from each other. For example case fatality rate; proportionate
mortality ratio, and cause specific death rates are often confused. They all have the same
numerator, i.e. number of deaths from a specified cause, occurring in a specified
population, over a specified period of time.
The case fatality rate asks the question: “what proportion of the people with the
disease die of the disease?”
The proportionate mortality ratio asks the question:”out of all the deaths
occurring in that area, what proportion are due to the cause under study?”

The cause specific death rate asks the question: “out of the total population,
what proportion dies from a certain disease within a specified period of
time?”

**Unlike all specific rates, the cause specific death rate has the total population
as denominator.

Other commonly used indices of health

Crude Birth Rate (CBR) = No. of live births reported during a time interval X 1000
Estimated mid-interval population

General Fertility Rate = No. of live births reported during a given time interval X 1000

Estimated no. of women 15-44 years of age at mid interval

LBW ratio = No. of live births of weight less than 2500 gms during a given time x 100
No. of live births reported during the same time interval

Attack rate = No. of new cases of a sp. disease reported during an epidemic x k
Total population at risk during the same time
Exercise:

i. Please answer the following questions by choosing from the

measures of mortality and morbidity given below and writing the
correct letter in the space provided.

A.CFR B. PMR C. CSMR D. Incidence rate

E. Point prevalence rate F. Period prevalence rate G. Attack
rate

_____1. The proportion of adults who were treated for malaria at an out
patient clinic in the year 1995 (Mesk.1- Pagume5).
_____2. Proportion of adults who had malaria in Jimma town during an
epidemic in the month of Meskerem, 1995.
_____3. The proportion of deaths from malaria as compared to deaths to
other causes in the year 1995.
_____4. The proportion of deaths from malaria out of those adults who
developed malaria in Jimma town in the year 1995.
The proportion of people in Jimma town who were found to have malaria during a
survey carried out on Meskerem 1, 1995. In 1995, there were 100 cases of
malaria in Village A and 300 cases in Village B. In which of the two villages was
malaria of greater public health importance.