Transcript Concept of disease causation Manish Chaudhary Introduction Up to the time of Louis pasture( 1822-1895), various concepts of disease causation were in practice e.g.

Concept of disease causation
Manish Chaudhary
Introduction
Up to the time of Louis pasture( 1822-1895),
various concepts of disease causation were in
practice e.g. the supernatural theory of
disease, the theory of humours, the concept
of contagion, miasmatic theory of disease,
theory of spontaneous generation etc.
Discoveries in microbiology marked a turning
point in aetiological concept.
Germ theory of disease
For long time, there was darkness about the
causation of the disease.
In 1873, Louis Pasteur advanced the germ
theory of disease.
He demonstrated the presence of bacteria in
air. The emphasis shifted from the empirical
causes (e.g. bad air) to microbes as a sole
cause of the disease.
The germ theory of the disease is generally
referred as a one to one relationship between
the causal agent and disease. The disease
model according to this theory is
Disease agent
Man
Disease
Koch’s Postulates in germ theory
• the organism must be present in every case of
the disease;
• the organism must be able to be isolated and
grown in pure culture;
• the organism must, when inoculated into a
susceptible animal, cause the
specific disease;
• the organism must then be recovered from the
animal and identified.
The criticism has also appeared to oppose this
theory. It is now found that a disease is rarely
caused by a single agent alone, but rather
depends upon a number of factors which
contributes to its occurrence.
Other theories
Epidemiological triad
Multifactorial causation
Web of causation
Epidemiologic triad
• Epidemiologic triad concept of disease
causation involves the role of 3 factors- agent,
host and environment for the appearance of
disease.
Agent
Environment
Disease
Host
Epidemiological Triad cont……
• Thus, the 3 factors which refer as the causative
factors of disease are called epidemiological triad.
This model helps epidemiologists to focus on
different classes of factors especially with regards
to infectious diseases.
• This model clears that “the mere presence of
agent, host and favorable environmental factors
in the pre- pathogenesis period is not sufficient to
start the disease in man. What is required is an
interaction among these factors to initiate the
disease process in man.”
1. Agent Factors
The disease agent may be defined as a substance,
living or nonliving or a force tangible or
intangible, excessive presence or relative lack of
which may initiate or perpetuate a disease
process.
A disease may have single agent, a number of
independent alternative agents or a complex of
two or more factors whose combined presence is
essential for the development of the disease.
The disease agent may be broadly classified broadly
into following groups:
• Biological agents: these are living agents of the
disease i.e. viruses, fungi, bacteria, protozoa etc.
• Nutritive agents: These can be proteins, fats,
carbohydrate, vitamins, minerals and water. Any
excess or deficiency of the intake of nutritient
elements may result in nutritional disorders.
Anaemia, Protein energy malnutrition, goiter are
some of the examples of the nutritional problems
in many countries.
• Physical agents: Exposure to excessive heat, cold,
humidity, pressure, radiation, electricity, sound
etc may result in illness.
Agent factors cont…..
• Chemical agents:
– Endogenous:some of the chemical may be
produced in the body as a result of derangement
of function e.g. urea, bilirubin, ketones, uric acid
etc.
– Exogenous: agents arising outside of human host
e.g. allergens, metals, fumes,dusts, gases etc.
these may be acquired by inhalatioin, ingestion or
inoculation.
Agent Factors cont….
• Mechanical agents: Exposure to chronic friction
and other mechanical force may result in
crushing, tearing, sprains, dislocations and even
death.
• Social agents: It is also necessary to consider
social agents of the disease. There are poverty,
smoking, abuse of drugs and alcohol, unhealthy
life styles. Loss of job, no one to help, loss of
relatives etc. all causes stress and mental
disorder.
Host Factors
• Demographic characteristics: age ,sex, ethnicity
• Biological characteristics: genetic factors, blood
groups, enzymes, immunological fators etc.
• Social and economic characteristics : Economic
status, education, occupation, marital status,
stress, housing condition
• Life style factors: living habits, nutrition, physical
exercise, use of alcohol, drug and smoking,
behavioural pattern etc.
Environmental Factors:
• The external environment is defned as “ all
that which is external to the individual human
host, living and non living and with which he is
in constant interaction.”
• For descriptive purpose, the environment of
man has been devided into three component:
• Physical environment
• Biological environment
• Psycho-social environment
Environmental factor cont….
• Physical environment : non living things and physical
factors (e.g. air, water, soil, housing, climate, geography,
heat, light, noise, waste, radiation)
• Biological environment: the universe of living things which
surrounds man, including man himself ( viruses and other
microbiological agents , insects, animals and plants. )
• Psycho-social environment: They include cultural values,
customs, habits, beliefs, attitude, morals , religions,
education, lifestyles, health services, social and political
organizations.
Multifactorial Causation
This theory was propounded by Pottenkofer of Munich.
This theory describe that the disease is not caused by
single agent like in germ theory but many other factors.
The use of chemotherapy, antibiotics and other drugs
only communicable diseases began to decline and
replaced by modern disease like lung cancer, coronary
heart disease, mental illness etc,
These disease could not be explained on the basis of
germ theory of disease nor could they be prevented by
traditional method of isolation, immunization or
improvement of sanitation.
Multifactorial Causation
The idea of the single cause of the disease was
oversimplification and there are other factors
in aetiology of diseases-social, economic,
cultural genetic and psychological which are
equally important .
For example, tuberculosis is not merely due to
mycobacterium tuberculosis, other factors
such as poverty, overcrowding , smoking,
malnutrition, contribute its occurrence.
ii) Multifactorial causation
• Multiple factor leads to the diseases
• Common in non-communicable diseases
e.g.
Smoking
Air pollution
Exposure to asbestos
Reaction at cellular level
Lung cancer
b. Interaction of multiple individual causes
Smoking
+
Air pollution
Reaction at cellular level
+
Exposure to asbestos
Lung cancer
Table 1: Age-standardized lung cancer death rates (per 100 000 population) in
relation to tobacco use and occupational exposure to asbestos dust
Web of Causation
This model of disease causation was
suggested by MacMahon and Paugh.
This model is ideally suited in the study of
chronic disease where disease agent is often
not known but is the out come of interaction
of multiple factors.
Web of causation considers all the
predisposing factors of any type and their
complex interrelationship with each other.
Web of causation
Change in life style
Abundance of food
D
Obesity
Stress
Smoking
Lack of physical activity
Hyperlidemia
Emotional
Disturbance
Aging &
other factor
Hypertension
Increase catacholamine
Changes in walls of
arteries
thrombotic activity
Coronory atherosclerosis
Coronary occlusion
Myocardial Infarction
Myocardial Infarction
Risk Factors:
• The causative agent of the non-communicable
diseases is still unknown. Where the disease
agent is not firmly established, the aetiology is
generally discussed in terms of risk factor.
Generally risk factor have following two
characteristics• Exposure to the factor is associated with the
development of disease
• If the factors are reduced or modified by
interventions , the disease occurrence is also
reduced.
• The term "risk factor" is commonly used to
describe factors that are positively associated
with the risk of development of a disease but that
are not sufficient to cause the disease. The
concept has been found useful in a number of
practical prevention programs.
• Some risk factors (e.g. tobacco smoking) are
associated with several diseases, and some
diseases are associated with several risk factors.
• Some risk factors may be modified (i.e.
smoking, physical activity, obesity etc) and
others cannot be modified (e.g. age, sex, race,
genetic factor etc).
• Epidemiological methods (e.g. case control
and cohort studies) are needed to identify risk
factors and estimate the degree of risk
Examples of risk factors
Disease
Risk factors
Heart disease
Smoking, high blood pressure, elevated serum cholesterol,
diabetes, obesity, lack of exercise
Cancer
Smoking, alcohol, solar radiation, ionizing radiation, work
site hazards, environmental pollution, medications,
infectious agents, dietary factors.
Road
traffic Alcohol, non use of seat belts, helmets, speed, road way
accidents
traffic
Diabetes
Obesity, diet
Cirrhosis of liver
Alcohol
Risk group
• The part of population which is susceptible to a disease
is called risk group.
• This approach is developed and promoted by WHO to
identify precisely the "risk group" in the population by
population by certain defined criteria and direct
appropriate action to them first.
• It has been summed up as "something for all, but more
for those in need, in proportion to the need."
• It is managerial device for increasing the efficiency of
the health service within the limits of existing
resources.
Population at risk
Guidelines for defining at risk group for Health services
1. Biological situation
• Age group: infant (low birth weight),
adolescent, elderly
• Sex: female in the reproductive age group
• Physiological state: pregnancy, high blood
pressure
• Genetic factor: family history of genetic
disorders
2. Physical situation
• Rural, urban slums
• Poor housing, over crowding
• Unsafe water supply
• Proximity to industries
3. Socio cultural situation
• Lower social class
• Family disruption
• Poor access to health care
Thank You