Concepts of epidemiology and models of disease causation

Overview

Epidemiology is "the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems" (the widely used definition by John Last).

• Distribution: who gets the disease, where and when (person, place, time).
• Determinants: why they get it (causes and risk factors).
• Application: using the findings to prevent and control disease.

Aims and uses of epidemiology

• Describe the size and distribution of health problems in a community.
• Identify causes and risk factors.
• Provide data to plan and evaluate health services.
• Study the natural history of disease.
• Evaluate preventive and treatment measures.
• Investigate outbreaks.

Types of epidemiology

See observational study designs.

Health and disease

WHO definition of health (1948): "a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity". It is criticised as an ideal that is hard to measure. Spiritual health is often added as a fourth dimension in teaching.

Disease is a departure from normal function; illness is how the person experiences it; sickness is the social role of being unwell.

Natural history of disease

The course of a disease from start to finish without treatment:

1. Pre-pathogenesis phase: before the person is affected; the agent, host and environment interact. Primary prevention acts here.
2. Pathogenesis phase: the disease begins in the host: an early, often subclinical stage, then clinical disease, then recovery, disability or death. Secondary and tertiary prevention act here.

See levels of prevention.

The iceberg phenomenon

Clinically diagnosed cases are only the tip of the iceberg. Below the surface lie undiagnosed, unreported and subclinical cases and carriers. Diseases with a large hidden part include hypertension, diabetes, anaemia, tuberculosis and mental illness.

Models of disease causation

Germ theory (single cause)

One agent → one disease. Too simple: many people infected with a germ never become ill.

The epidemiological triad

Disease results from the interaction of three factors:

• Agent: biological (bacteria, viruses), chemical, physical, nutritional (excess or lack).
• Host: age, sex, genetics, immunity, nutrition, behaviour.
• Environment: physical (climate, water, housing), biological (vectors, animals) and social (poverty, crowding, customs).

Best suited to infectious diseases.

Web of causation

Proposed for chronic, non-communicable diseases, which have many interacting causes and no single agent. Example: coronary heart disease linked to smoking, diet, hypertension, diabetes, lack of exercise, stress and genes.

It shows that disease can be prevented by cutting a few strands of the web, even without removing all causes.

Wheel model

The host is the hub, with a genetic core, surrounded by the biological, social and physical environment. The size of each part varies with the disease: a large genetic core for inherited disorders; a large environmental part for measles or diarrhoea.

Multifactorial causation

Most diseases have several causes. A risk factor is an attribute or exposure linked to a higher chance of disease (for example smoking for lung cancer).

Association and causation: Bradford Hill's criteria

An observed association may be due to chance, bias or confounding. If these are excluded, Hill's criteria help judge whether it is causal:

1. Strength of association (a large relative risk).
2. Consistency across different studies and places.
3. Specificity (one cause, one effect; weakest criterion).
4. Temporality: the cause comes before the effect.
5. Biological gradient (dose-response): more exposure, more disease.
6. Biological plausibility.
7. Coherence with known facts.
8. Experimental evidence (removing the cause reduces disease).
9. Analogy.

Disease control terms

Pakistan and Afghanistan are the two countries where wild poliovirus transmission has continued, which is why polio eradication campaigns remain a national priority.

Patterns of occurrence: sporadic (scattered cases), endemic (constant presence in an area), epidemic (clearly more cases than expected) and pandemic (an epidemic across many countries or continents).

Quick revision

• Epidemiology: distribution, determinants, application to control.
• Descriptive: time, place, person; generates hypotheses. Analytical: tests them.
• Iceberg: hidden subclinical and undiagnosed disease.
• Triad (agent, host, environment): infections. Web of causation: chronic diseases.
• Hill's criteria: temporality is essential.
• Eradication: worldwide (smallpox). Elimination: regional.

Written by The House Job from Excel Community Medicine and Park's Textbook of Preventive and Social Medicine (named in the UHS PGET syllabus).

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