Natural History of Disease Atok
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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 & StagesDefinition & Stages
• Definition ; The course of a disease from onset Definition ; The course of a disease from onset (inception) to resolution. (inception) to resolution.
• StagesStages
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
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
(A) (P) (S) (M) (D) (T)
The Natural history of disease in a patient
A ; Biologic onset of diseaseP ; Pathologic evidence of disease if SoughtS ; Signs and symptoms of diseaseM ; Medical care soughtD ; DiagnosisT ; Treatment
Preclinical Phase Clinical Phase
Gordis L. Epidemiology. WB Saunders Company. 1996
THE NATURAL HISTORY OF A DISEASE
STIMULUS to the HOST 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
Health PromotionSpecific
Protection
Early Diagnosis and Prompt Treatment,
Disability Limitation
Rehabilitation
PRIMARY PREVENTION
SECONDARY PREVENTION TREATMENT TERTIARY PREVENTION
(Leavell's Level of Application of Preventive Medicine)
Susceptiblehost
TIME
Incubation period
Death
Recovery
Exposure Onset
Latent Infectious Non-infectious
Infection
No infection
Clinical disease
Latent periodLatent periodthe time interval from infection to development the time interval from infection to development of infectiousnessof infectiousness
Infectious periodInfectious periodthe time during which time the host can infect the time during which time the host can infect another susceptible hostanother susceptible host
Non-infectious periodNon-infectious periodthe period when the host’s ability to transmit the period when the host’s ability to transmit disease to other hosts ceasesdisease to other hosts ceases
Incubation periodIncubation periodthe time interval between infection to the time interval between infection to development of clinical diseasedevelopment of clinical disease
e.g : Chicken poxan 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
Susceptiblehost
TIME
Incubation period
Death
Recovery
Exposure Onset
Latent Infectious Non-infectious
Infection
No infection
Clinical disease
Other examples?HIV (AIDS)
latent period relatively short infectious period occurs (many years) before the
onset of symptoms
Susceptiblehost
TIME
Incubation period
Death
Recovery
Exposure Onset
Latent Infectious
Infection
No infection
Clinical disease
e.g : Malariacaused 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
Susceptiblehost
TIME
Incubation period
Death
Recovery
Exposure Onset
Latent
Infection
No infection
Clinical disease
Infectious
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
Two conditions
1) brief exposure
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 resou
rces.– Effective prevention requires legislative action and soci
al policy decisions.– Programs should be continuous.
Primary Prevention
General health promotion– 'Proper nutrition, mental hygiene, adequate housing, and appropriate ba
lance 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