Natural History of Disease

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Natural History of Disease
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Background

• Infectious disease epidemiology
• the occurrence of infectious disease in a given
  host is dependent on the presence of disease in
  other members of the population and the length of
  time that infected hosts are able to transmit
  disease to others
• understanding these characteristics of a
• disease allow us to develop rational measures
  to control disease

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Definition Stages

• Definition The course of a disease from onset
  (inception) to resolution.
• Stages

Stage of pathologic onset
Pre-symptomatic stage
Clinically manifest disease
Progress to a fatal termination
Remission and relapses
Regress spontaneously, leading to recovery
Risk Factors
Precursors
Effect of Treatment
Prognostic factor
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Risk factor

• Risk factor An aspect of personal behavior or
  life style, an environmental exposure, or an
  inborn or inherited characteristic, that, in the
  basis of epidemiologic evidence, is known to be
  associated with health-related condition (s)
  considered important to prevent.
• Risk marker increased probability of a specified
  outcome not necessarily a causal factor
• Determinant can be modified by intervention,
  thereby reducing the probability of occurrence of
  disease or other specified outcomes

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The Natural history of disease in a patient
Preclinical Phase
Clinical Phase
(A)
(P)
(S)
(M)
(D)
(T)

• A Biologic onset of disease
• P Pathologic evidence of disease if Sought
• S Signs and symptoms of disease
• M Medical care sought
• D Diagnosis
• T Treatment

Gordis L. Epidemiology. WB Saunders Company. 1996
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THE NATURAL HISTORY OF A DISEASE THE NATURAL HISTORY OF A DISEASE THE NATURAL HISTORY OF A DISEASE THE NATURAL HISTORY OF A DISEASE
STIMULUS to the HOST HOST REACTION HOST REACTION RECOVERY
interrelation of Agent, Host and Environmental factors Latent Period (Pre-symptomatic) Symptoms, Signs(Clinical) with or without Defects, Disability
PREPATHOGENESIS PERIOD OF PATHOGENESIS PERIOD OF PATHOGENESIS PERIOD OF PATHOGENESIS
Health Promotion Specific Protection Early Diagnosis and Prompt Treatment, Early Diagnosis and Prompt Treatment, Disability Limitation Rehabilitation
PRIMARY PREVENTION SECONDARY PREVENTION TREATMENT TERTIARY PREVENTION
(Leavell's Level of Application of Preventive
Medicine)
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Susceptible host
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• Latent period
• the time interval from infection to development
  of infectiousness
• Infectious period
• the time during which time the host can infect
  another susceptible host
• Non-infectious period
• the period when the hosts ability to transmit
  disease to other hosts ceases
• Incubation period
• the time interval between infection to
  development of clinical disease

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• e.g Chicken pox
• an infectious disease caused by the
  varicella-zoster virus
• the latent period for chicken pox is shorter than
  the incubation period, so a child with chicken
  pox becomes infectious to others before
  developing symptoms

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Susceptible host
Infectious
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• Other examples?
• HIV (AIDS)
• latent period relatively short
• infectious period occurs (many years) before the
  onset of symptoms

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Susceptible host
Infectious
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• e.g Malaria
• caused by protozoan parasites of the genus
  Plasmodium
• the stages of the parasite that are infective to
  mosquitoes occur about 10 days after the
  development of symptoms
• latent period is around 10 days longer than the
  incubation period, so early treatment of symptoms
  could have an important effect on transmission

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Natural history of disease
Susceptible host
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Latent Period of Chronic Disease

• Definition "Interval between exposure to a
  disease-causing agent and the appearance of
  manifestations of the disease"
• cf. incubation period in infectious disease

1) brief exposure
Two conditions
2) prolonged or continuous exposure
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Primary Prevention

• 'Preventing the occurrence of disease or injury
  by modifying risk factors.'
• 'Various aspects are considered to produce
  effective primary prevention program. Especially,
  advancing knowledge of disease causation must be
  required.

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Primary Prevention

• Guidelines for effective prevention
  programs(RB Wallace, GD Everett,1986)
• Programs must be based on scientific evidence.
• Prevention programs should be supported by
  effective data system.
• Programs should be flexible.
• Programs must be sensitive to ethical issues.
• Programs should be targeted to the recipients
  most in need.
• Programs should muster a variety of community
  resources.
• Effective prevention requires legislative action
  and social policy decisions.
• Programs should be continuous.

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Primary Prevention

• General health promotion
• 'Proper nutrition, mental hygiene, adequate
  housing, and appropriate balance between work and
  play, est and exercise, and useful and productive
  place in society, are among the best recognized
  factors ontributing to maintenance of optimum
  health.(Commission on Chronic illness, USA,
  1957)
• Specific protection
• Health Promotion
• 'Health promotion is any combination of
  educational, organizational, economic, and
  environmental supports for behavior and
  conditions of living conducive to health (LW
  Green, 1992).'

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Criteria for the Development of Health Promotion
and Education Programs

• A health promotion program should address one or
  more risk factors which are carefully defined,
  measurable, modifiable, and prevalent among the
  members of a chosen group, factors which
  constitute a threat to the health status and the
  quality of life of target group members.
• A health promotion program should reflect a
  consideration of the special characteristics,
  needs, and preferences of its target groups(s)

From APHA Technical Report
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Criteria for the Development of Health Promotion
and Education Programs

• health promotion programs should include
  interventions which will clearly and effectively
  reduce a targeted risk factor and are appropriate
  for a particular setting
• A health promotion program should identify and
  implement interventions which make optimum use of
  available resources.
• From the outset, a health promotion program
  should be organized, planned, and implemented in
  such a way that its operation and effects can be
  evaluated.

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Secondary Prevention

• 'Early detection and intervention, preferably
  before the condition is clinically apparent, and
  has the aim of reversing, halting, or at least
  retarding the progress of a condition.
• 'It sometimes happens that a patient first
  becomes aware of a disease when it is already too
  late for it to be successfully treated.

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Tertiary Prevention

• 'Minimizing the effects of disease and disability
  by surveillance and maintenance aimed at
  preventing complications and premature
  deterioration'
• Medical rehabilitation
• Social rehabilitation

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