Natural histroy of disease anand ync

Learning Objectives
• By the end of this lecture students will be
able to:
• Describe natural history of diseases and their
implications for prevention of diseases.
• Describe spectrum of diseases and their
implications for on prevention of diseases.
ANANDA.S ASST.PROFESSOR YNC

Introduction
• Natural history of disease refers to the
progress of a disease process in an individual
over time, in the absence of treatment or
prevention.
• Key concept in Epidemiology
• Each disease has its own, unique natural
history.

• The process begins with exposure to or
accumulation of factors capable of causing
disease.
• Without medical intervention, the process
ends with
• – recovery ,
• – disability,
• – or death.

1.Prepathogenesis phase
• This refers to the period preliminary to the
onset of disease in man.
• The disease agent has not yet entered man,
but the factors which favour its interaction
with the human host are already existing in
the environment.
• This situation is frequently referred to as “man
in the midst of disease” or “man exposed to
the risk of disease”.

2. Pathogenesis phase
• This phase begins with entry of the disease
“agent” in the susceptible human host.
• After the entry, agent multiplies and induces
tissue and physiological changes, the disease
progresses through the period of incubation
and later through the period of early and late
pathogenesis.
• The final outcome of the disease may be
recovery, disability or death.

• Germ theory
• Theory of epidemiological triad
• Multifactorial causation theory
• Web of causation
• Spectrum of disease
• Iceberg of disease
THEORIES AND MODELS OF
DISEASE CAUSATION

1. GERM THEORY
• Proposed by Robert Koch and Louis Pasteur.
• Every human disease is caused by a microbe
or germ, which is specific for that disease and
one must be able to isolate the microbe from
the diseased human being.
•This theory attributes micro-organisms
as the only cause of disease.
Causitive
agent
Man Disease
Cause EffectANANDA.S ASST.PROFESSOR YNC

2. THEORY OF EPIDEMIOLOGICAL
TRIAD
• This model is also called as ecological model
and evolved through the study of infectious
disease.
• Acc. To this model there are three elements
or major factors which are responsible for a
particular disease causation. i.e. agent, host
& environment.

• The agent refers to primary factor, without
which a particular disease can not occur.
• The host refers to human beings who come
in contact with the agent.
• The environment includes all, that is
external to the host and agent but that may
influence interaction between them.
agent
host environmentANANDA.S ASST.PROFESSOR YNC

EPIDEMIOLOGICAL TRIAD
• 1.Agent factors
A substance, living or non-living, or a force,
tangible or intangible, the excessive presence
or relative lack of which may initiate or
perpetuate a disease process

• Biological agents:
These are living agents of disease, viruses,
rickettsiae, fungi, bacteria, protozoa and
metazoa
• Nutrient agents:- These are proteins, fats,
carbohydrate, minerals and water. Any excess
or deficiency of the intake of nutritive
elements may result in nutritional disorders.
e.g. Anaemia, goitre, obesity and vitamin
deficiencies are some of the current
nutritional problems in many countries.

• Physical agents:-Exposure to excessive heat,
cold, humidity, pressure, radiation, electricity,
sound.
• Chemical agents :-
o Endogenous: e. g. Ureamia, jaundice, ketosis.
o Exogenous : e. g. allergens, metals, fumes.
• Mechanical agents:- Exposure of chronic
friction and other mechanical forces may
result in crushing, tearing, sprains,
dislocations and even death.

• Social agents:- It is also necessary to consider
agents of disease. These are poverty, smoking,
abuse of drugs and alcohol, unhealthy
lifestyles, social isolation, maternal
deprivation

2 Host factors
• The human host is referred to as “soil” and the
disease agent as “seed” . In some situations,
host factors play a major role in determining
the outcome of an individuals exposure to
infection.eg. Tuberculosis.
The host factors may be classified as;
• Demographic
• Biological

• Social and economic characteristics such as
socioeconomic status, education, occupation,
stress , marital status , housing, etc.
• Lifestyle factors such as personality traits ,
living habits, nutrition, physical exercise, use
of alcohol, drugs and smoking, behavioral
patterns.

3 Environmental factors
• For human beings the environment is not
limited, as it normally is for plants and
animals, to a set of climatic factors.
• For Example, for man, social and economic
conditions are more important than the mean
annual temperature.

• Physical environment
• Biological environment
• Psychosocial environment
• Physical environment:
The term “physical environment” is applied to
non-living things and physical factors (e.g.. Air,
water, soil, housing, climate, geography, heat,
light, noise, debris & radiation)

• Biological environment:- The biological
environment is the universe of living things
which surrounds man, including man himself.
The living things are the viruses and other
microbial agents, insects, rodents animals and
plants
• Psychosocial environment:- “Those factors
affecting personal health, health care and
community well-being that stem from the
psychosocial make-up of individuals and the
structure and functions of social groups

3. MULTIFACTORIAL CAUSATION
THEORY
• Disease caused by multiple factors .
example:- coronary heart disease.

4. WEB OF CAUSATION
Given by macmohan and pugh.
According to him disease never depends upon
single isolated cause rather it develops from a
chain of causation in which each link itself is a
result of complex interaction of preceding
events these chain of causation which may be
the fraction of the whole complex is known as
web of causation.

5. Spectrum of disease
• The term “spectrum of disease” is a graphic
representation of variations in the manifestations
of disease.
• At the one end of disease spectrum are
subclinical
• infections which are not ordinarily identified,
and at the other end are fatal illnesses. In the
middle of spectrum lie illnesses ranging in
severity from mild to severe.
• These different manifestations are the result of
individuals’ different states of immunity and
receptivity.

6. Iceberg of disease
• The floating tip of the iceberg represents what
the physician sees in the community, i.e., clinical
cases. The cast submerged portion of the iceberg
represents the hidden mass of disease, i.e.,
latent, inapparent, pre-symptomatic and
undiagnosed cases and carriers in the
community.
• The “waterline” represents the demarcation
between
apparent and inapparent disease.

e.g., Hypertension, Diabetes,
Anaemia, mental illness, Polio etc.ANANDA.S ASST.PROFESSOR YNC

Concepts of prevention
• Prevention is the process of intercepting or
opposing the “cause” of a disease and thereby
the disease process.
• LEVELS OF PREVENTION :-
• - Primordial prevention
• - Primary prevention
• - Secondary prevention
• - Tertiary prevention

Primordial prevention
• It is the prevention of the emergence or
development of risk factors in population groups
in which they have not yet appeared.
• For example, many adult health problems (e.g.,
obesity and hypertension) have their early origin
in childhood, so efforts are directed towards
encouraging children to adopt healthy lifestyles(
e.g, physical exercise, healthy dietary habits etc.)
• The main intervention in primordial prevention is
through individual and mass education.

Primary prevention
• It can be defined as “ action taken prior to the
onset of disease, which removes the
possibility that a disease will ever occur.
• It signifies intervention in the pre-
pathogenesis phase of a disease.
Two types of strategies :-
• - Population( mass ) strategy
• - High risk strategy

• Population strategy :-
- directed at whole population irrespective of the
individual risk levels.
- directed towards socio-economic, behavioural and
lifestyle changes.
• High risk strategy :-
- Includes identification of “High risk groups” in the
population and bring preventive care to these risk
group.
- e.g., People having the family history of
Hypertension, Diabetes .

Two types of modes of intervention :
- Health promotion
- Specific protection
• Health promotion – It is the process of enabling
people to control over, and to improve health.
- Health education
- Environmental modifications
- Nutritional interventions
- Lifestyle and behavioural changes

Specific protection
Examples
- Immunization
- Chemoprophylaxis
- food fortification e.g., iodized salt
- Protection against occupational hazards
- Protection against accidents e.g. , use of
helmets
- Avoidance of allergens etc.

Secondary prevention
• Secondary prevention can be defined as “
action which halts the progress of a disease
at its incipient stage and prevents
complications.
• It is applied in the early pathogenesis stage of
disease.

• The specific interventions used is :- Early
diagnosis and treatment.
• e.g. , screening for disease for breast cancer
(using mammography) and cervical cancer
(using pap smear).
Medical examinations of school children, of
industrial workers and various disease
screening camps

Tertiary prevention
• These include all measures undertaken when the
disease has become clinically manifest or
advanced, with a view to prevent or delay death,
reduce or limit the impairments and disabilities,
minimize suffering and to promote the subject’s
adjustment to irremediable conditions.
• Tertiary prevention has two types of approaches :
- disability limitation
- rehabilitation.

• Disability Limitation : These include all
measures to prevent the occurrence of
further complications, impairments,
disabilities and handicaps or even death.

The sequence with which a disease turns
into handicap is as follows :
– Disease : This is a pathological process and it’s
manifestations which indicate a departure from the
state of perfect health.
– Impairment : This is the actual loss or damage of a part
of body anatomy or an aberration of the physiological
functions that occurs consequent to a disease.
– Disability : This is defined as the inability to carry out
certain functions or activities which are otherwise
expected for that age / sex, as a result of the
impairment.

– Handicap : This is the final disadvantage in life
which occurs consequent to an impairment or
disability, which limits the fulfillment of the
role a person is required to play in life.

• Rehabilitation : This is the second component
of tertiary prevention.
• • Rehabilitation stands for the combined and
coordinated usage of all the available medical,
social, educational and vocational measures,
for training the person to the highest level of
functional ability.

• Medical rehabilitation :
– This is done through medical / surgical procedures to
restore the anatomy, anatomical functions and
physiological functions to as near normal as possible.
• Vocational rehabilitation :
– It includes steps involving training and education so as
to enable the person to earn a livelihood.
• Social rehabilitation :
– This involves steps for restoration of the family and
social relationships.
• Emotional and Psychological rehabilitation :
– This involves steps to restore the confidence, personal
dignity and confidence.

DYNAMICS OF DISEASE
TRANSMISSION

Communicable diseases are transmitted
from the reservoir/ source of infection to
susceptible host.
There are three links in the chain of
transmission
1. Reservoir
2. Mode of transmission
3. Susceptible host

SOURCE OR
RESERVOIR
MODE OF
TRANSMISSION
SUSCEPTIBLE
HOST

1. SOURCE & RESERVOIR
• SOURCE
• The source defined as the “person,
animal, object or substance from which
an infectious agent passes or
disseminated to the host”.

• RESERVOIR
• A reservoir is defined as “any person,
animal, arthropod, plant, soil or
substance in which an infectious agent
lives and multiplies , on which it
where it reproduces itself in
depends primarily for survival, and
such
manner that it can be transmitted to a
susceptible host”.

• Eg:-
In hookworm infection, the reservoir is
man and the source of infection is soil
contaminated with infective larvae.
In typhoid fever the reservoir is a case
or carrier and the source of infection is
faeces or urine of patient or
contaminated food and water.

Types of reservoir
1. Human reservoir
2. Animal reservoir
3. Reservoir in non living things

1. HUMAN
RESERVOIR
The most important source or reservoir
of infection for human is man himself.
Human may be
Case
Carrier

a. CASES
• A case is defined as “ a person in the
population having the particular
disease, health disorder or condition
under investigation”.
• The presence of infection in host may
be :-
Clinical
Sub clinical
Latent

• 1. clinical illness:-
• Clinical illness may be mild or
moderate, typical or atypical, severe or
fatal.
• Mild cases may be more important
source of infection than severe cases.

• ii. Sub clinical cases
• Sub clinical cases are also known as in
apparent, missed or abortive cases.
• The disease agent may multiply in the
host but does not manifest itself by
signs and symptoms.
• Sub clinical infection may be detected
only by laboratory tests.

• iii. Latent infection
• Infectious agent lies dormant within the
host without symptoms.

Primary case::- first case of a
communicable disease introduced into
the population.
Secondary case:- develops from
contact with primary cases.
Index case:-first case which comes to
the attention of investigator

b. CARRIERS
• In some due todiseases,
treatment
either
or immune
the disease
inadequate
response,
completely eliminated,
agent
leading
is not
to a
carrier state.
• A carrier is defined as an infected person
or animal
infectious
that harbours a specific
agent and serves as a
potential source of infection for others.

• The elements in a carrier state are:-
The presence of disease agent in the body.
The absence of recognizable signs and
symptoms
Spread of disease agent in the discharges
or excretions.

Classification of
carriers
A. TYPE
 Incubatory
 Convalescent
 Healthy
B. DURATION
 Temporary
 Chronic
C. PORTAL OF EXIT
 Urinary
 Intestinal
 Respiratory

A. TYPE
• (a) Incubatory carriers:
• Carriers which spread the infectious
agent during the incubation period of
disease.
• This usually occurs during the last few
days of incubation period.
• Eg:- measles, mumps, polio, influenza,
hepatitis B

• (b) Convalescent carriers:-
• Carriers which continue to spread
disease during the period of
convalescence.
• Eg ;-Typhoid, Dysentery, Cholera,
Diptheria

(c) Healthy carriers:-
• They are victims of sub clinical infection
who have developed carrier state
without suffering from overt disease.
• A person whose infection remains
subclinical may or may not be a carrier.
E.g-Poliomyelitis, cholera, meningococcal
meningitis, salmonellosis, diphtheria.

B. DURATION
• (a) Temporary carriers:-
• Carriers which spread infectious agent
for short period of time
• (b) Chronic carriers:-
• Carriers which spread infectious agent
for indefinite period
e.g., Typhoid fever, hepatitis B, dysentery,
cerebra-spinal meningitis, malaria, gonorrhoea,
etc

C. PORTAL OF EXIT
 Urinary e.g. Typhoid
 Intestinal e.g. Typhoid, cholera
 Respiratory e.g. influenza
 Others –(skin eruptions, open wounds,
blood) e.g. hepatitis B and HIV

2. ANIMAL RESERVOIR
• The source of infection may sometimes
be animals and birds.
• The diseases and infections which are
transmissible to man from vertebrate
are called zoonoses.
• Eg :- Rabies, Yellow Fever, Influenza

3. RESERVOIR IN NON LIVING
THINGS
• Soil and inanimate matter can also act
as reservoir of infection
e.g- Soil may harbour agents that cause
Tetanus

2. MODE OF
TRANSMISSION
• Communicable diseases are transmitted
from reservoir to host in many different
ways
SOURCE
OR
RESERVOIR
MODE OF
TRANSMISSION HOST

• 1. Droplet contact
• 2. Droplet infection
• 3. Contact with soil
• 4. Inoculation into skin or
mucosa
• 5. Trans placental
A. DIRECT
TRANSMISSION
• 1. Vehicle borne
• 2. Vector borne
• 3. Air borne
• 4. Fomite borne
• 5. Unclean hands and fingers
B. INDIRECT
TRANSMISSION

A. DIRECT TRANSMISSION
• 1. Direct contact
• Infection may be transmitted by direct
contact from skin to skin, mucosa to
mucosa, or mucosa to skin.
• Eg :- STD, AIDS, leprosy, leptospirosis,
skin and eye infections

• 2. Droplet infection
• This is direct projection of spray of
droplets of saliva and nasopharyngeal
secretions during coughing, sneezing,
talking or spitting.
• The droplet spread is usually limited to a
distance of 30-60 cm between source and
host
• Eg :-Respiratory Infections, Common
Cold, Tuberculosis, Diphtheria

• 3. Contact with soil:-
• The disease agent may be acquired by
direct exposure to the disease agent in
the soil
• Eg :- hook worm, tetanus, mycosis
• 4. Inoculation into skin:-
• Disease agent may be inoculated
directly into the skin or mucosa
• Eg:-rabies virus by dog bite, Hepatitis B
by contaminated needles

• 5. Transplacental transmissin:-
• Disease agents can be transmitted
transplacentally.
– S-Syphilis
– T-Toxoplasma
– O-Other infections(AIDS, varicella, Hepatitis B)
– R-Rubella virus
– C-Cytomegalo virus
– H-Herpes virus

B. INDIRECT TRANSMISSION

1. VEHICLE BORNE
• Vehicle borne transmission implies
transmission of the infectious agent
through the agency of water, food, raw
vegetables, fruits, milk, blood etc.

2. VECTOR BORNE
In infectious disease epidemiology, vector
is defined as an arthropod or any living
carrier (e.g., snail) that transports an
infectious agent to a susceptible individual.
Transmission by a vector may be
mechanical or biological.

Mechanical transmission
The infectious agent is mechanically transported
by a crawling or flying arthropod through soiling
of its feet by passage of organisms through its
gastrointestinal tract and passively excreted.
There is no development or multiplication of the
infectious agent on or within the vector.

Biological transmission
The infectious agent undergoing replication or
development or both in vector and requires
an incubation period before vector can
transmit.

3. AIR BORNE
1. Droplet nuclei --"Droplet nuclei" are a type
of particles implicated in the spread of airborne
infection. They are tiny particles (1-10 microns
range)
Diseases spread by droplet nuclei include
tuberculosis, influenza, chickenpox, measles etc
2. Dust- Some of the larger droplets which are
expelled during talking, coughing or sneezing.
e.g- staphylococcal infection, pneumonia,
tuberculosis

4. FOMITE BORNE
• Fomites are inanimate articles or
substances other than water or food
contaminated by infectious agents.
• Eg- soiled clothes, syringes,
instruments etc.
Diseases transmitted by fomites include
diphtheria, typhoid fever, bacillary dysentery,
hepatitis A, eye and skin infections

5. UNCLEAN HANDS
• Lack of personal hygiene favour person-
person transmission of infection.
Examples include staphylococcal and
streptococcal infections, typhoid fever, dysentery,
hepatitis A and intestinal parasites.

Indirect Transmission 5 ‘F’
• Food
• Flies
• Fomite
• Finger
• Fluid

3. SUSCEPTIBLE HOST
SOURCE
OR
RESERVOIR
MODE OF
TRANSMISSION
SUSCEPTIBLE
HOST

SUCCESSFUL PARASITISM
• 4 stages are there in successful
parasitism
PORTALOF
ENTRY
SITE OF
ELECTION
PORTALOF
EXIT
SURVIVAL IN
THE
ENVORNMENT

1. PORTAL OF ENTRY
• Infectious agent enter the host
• Eg :-
Respiratory tract
Alimentary tract
Genito urinary tract
Skin

2. SITE OF ELECTION
• In the body the disease agent finds
appropriate tissue for multiplication and
survival

3. PORTAL OF EXIT
The disease agent finds a way out of
the body
4. SURVIVAL OUTSIDE
After leaving the human body the
organism must survive in the external
environment for sufficient period till a
new host is found

Diseases prevention and control
• Prevention of source or reservoir
• Early diagnosis
• Notification
• Epidemiological Investigation
• Isolation
• Treatment
• Quarantine
• Interruption of the disease transmission
• Prevention of susceptible host
• Immunization
• Health educationANANDA.S ASST.PROFESSOR YNC

THANK YOU

Natural histroy of disease anand ync

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