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