NATURAL HISTORY & SPECTRUM OF DISEASESDr A Adeniran MBBS, MPH, FMCPH

Consultant Public Health PhysicianLagos State University (College of Medicine)Nigeria

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.

Natural History of Disease

Natural History of Disease: Natural course that a disease would take when it has not been

affected by any treatment or any other intervention.

The natural history of a disease describes the course of the disease in an individual starting from the moment of exposure to the causal agents till one of the possible

outcomes occurs.

• Induction : time to disease initiation

• Incubation:– time to symptoms (infectious disease)

• Latency: time to detection (NCDs) or to infectiousness in CD

Natural history Phenomena

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Natural history of disease

Susceptiblehost

TIME

Incubation period

Death

Recovery

Exposure Onset

Latent

Infection

No infection

Clinical disease

Infectious

- Pre-pathogenesis phase- Pathogenesis phase

The natural history of disease can be divided into two stages

Agent

Host Environment

Pre-pathogenesis period

Pathogenesis period

Advanced disease

Convales-cence

Early pathogenesis

Early disease

DisabilityDeathRecoveryChronic state

Natural history of disease

Pathogenesis phase:The pathogenesis phase is divided into  Stage of subclinical disease Stage of clinical diseaseStage of disability

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Natural history of disease

Define the Following and State their roles in the natural history of diseases

• Infectivity• Pathogenicity• Virulence

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Natural history & Spectrum of disease

Susceptible Host

Subclinical Disease

Clinical Disease

Outcome:Stage of Recovery, Complications, Disability, or Death

Point of Exposure

Screening

Onset of symptoms

Diagnosis sought

Natural History of DiseaseDetectable subclinical disease

Importance of studying natural history of disease

• Natural history is as important as causal understanding for the prevention and control of disease.

• The earlier you can become aware of the disease the more likely you will be able to intervene and save lives.

• Decides appropriate intervention at appropriate stage of disease

Spectrum of Disease

The full range of manifestations of a diseasee.g from precursor state, to subclinical and mild cases, to florid and very severe disease

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Spectrum of disease

The concept that an exposure can lead to varying outcome: signs, symptoms and severity of the same

disease in the population is the spectrum of disease.

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Natural History& Spectrum of Diseases

The outcome will depend on the interactions of host, agent and environmental factors.

Why do we have

varying degrees of

severity or outcome?

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Classification of diseases according to clinical severity (spectrum of disease)

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The relation of severity of illness to disease statistics.

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The pyramid and iceberg of disease

1 Diseased, diagnosed & controlled

2 Diagnosed, uncontrolled

3 Undiagnosed or wronglydiagnosed disease

4 Risk factors for disease

5 Free of risk factors

Diagnosed disease

Undiagnosed orwrongly diagnosed disease

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• Cases of illness correctly diagnosed by clinicians in the community often represent only the “tip of the iceberg.”

• Many additional cases may be too early to diagnose or may remain asymptomatic.

• Examples: Tuberculosis, meningitis, polio, hepatitis A, AIDS.

Iceberg Phenomenon

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The risk is that persons with in-apparent or undiagnosed infections may be able to transmit

infection to others

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Implications of the concepts of natural history and spectrum of disease

• Persons with in-apparent or undiagnosed infections can transmit infections to others.

• Control measures must be directed toward all infections capable of being transmitted to others;• both clinically apparent cases and • those with in-apparent or undiagnosed infections.

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Summary

Questions