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 theoriesEpidemiological triadMultifactorial causationWeb 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

Host

Disease

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 CausationThis 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 Reaction at cellular level Lung cancer  Exposure to asbestos

b. Interaction of multiple individual causes

Smoking +Air pollution Reaction at cellular level Lung cancer + Exposure to asbestos

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 Stress

Abundance of food Smoking Emotional Aging & D Disturbance other factor

Obesity Lack of physical activity Hypertension

Hyperlidemia 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 factorsDisease Risk factorsHeart disease Smoking, high blood pressure, elevated serum cholesterol,

diabetes, obesity, lack of exerciseCancer Smoking, alcohol, solar radiation, ionizing radiation, work

site hazards, environmental pollution, medications, infectious agents, dietary factors.

Road traffic accidents

Alcohol, non use of seat belts, helmets, speed, road way traffic

Diabetes Obesity, dietCirrhosis 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

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