Transcript The Natural History of Disease

Natural History of
Disease
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

Definition & Stages
• Definition ; The course of a disease from onset
(inception) to resolution.
• Stages
Progress to a fatal termination
Stage of
pathologic
onset
Pre-symptomatic
stage
Risk Factors
Precursors
Clinically
manifest disease
Remission and relapses
Regress spontaneously,
leading to recovery
Effect of Treatment
Prognostic factor
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
The Natural history of disease in a
patient
Preclinical Phase
(A)
(P)
Clinical Phase
(S)
(M)
(D)
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
(T)
THE NATURAL HISTORY OF A DISEASE
STIMULUS to
the HOST
interrelation of
Agent, Host and
Environmental
factors
PREPATHOGE
NESIS
Health
Promotion
Specific
Protection
HOST REACTION
Latent Period (Presymptomatic)
Symptoms,
Signs(Clinical)
RECOVERY
with or without Defects,
Disability
PERIOD OF PATHOGENESIS
Early Diagnosis and Prompt
Treatment,
Disability Limitation
Rehabilitation
PRIMARY
SECONDARY
TREATMENT
TERTIARY PREVENTION
PREVENTION
PREVENTION
(Leavell's Level of Application of Preventive Medicine)
TIME
Death
Clinical disease
Infection
Susceptible
host
Recovery
No infection
Incubation period
Latent
Exposure
Non-infectious
Infectious
Onset




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 host’s ability to transmit
disease to other hosts ceases
Incubation period
the time interval between infection to
development of clinical disease

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

TIME
Death
Infection
Clinical disease
Susceptible
host
Recovery
No infection
Incubation period
Latent
Exposure
Infectious
Non-infectious
Onset

Other examples?

HIV (AIDS)
latent period relatively short
 infectious period occurs (many years) before the
onset of symptoms

TIME
Death
Infection
Clinical disease
Susceptible
host
Recovery
No infection
Incubation period
Latent
Exposure
Infectious
Onset
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

Natural history of disease
TIME
Death
Infection
Clinical disease
Susceptible
host
Recovery
No infection
Incubation period
Latent
Exposure
Infectious
Onset
Latent Period of Chronic
Disease
Definition; "Interval between exposure to a diseasecausing 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
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.‘
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.
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).'
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
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.
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.‘
Tertiary Prevention

'Minimizing the effects of disease and disability
by surveillance and maintenance aimed at
preventing complications and premature
deterioration'

Medical rehabilitation

Social rehabilitation