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 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

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

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 Promotion Specific Protection Early Diagnosis and Prompt Treatment, Disability Limitation Rehabilitation PRIMARY PREVENTION SECONDARY PREVENTION TREATMENT TERTIARY PREVENTION (Leavell's Level of Application of Preventive Medicine)

TIME Onset Exposure Incubation period Latent Infectious Non-infectious Death Recovery Clinical disease Infection No infection Susceptible host Onset Exposure Incubation period Latent Infectious Non-infectious

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 Onset Exposure Incubation period Latent Infectious Non-infectious Death Recovery Clinical disease Infection No infection Susceptible host Onset Exposure Incubation period Latent Infectious Non-infectious

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

TIME Onset Exposure Incubation period Latent Infectious Death Recovery Clinical disease Infection No infection Susceptible host Onset Exposure Incubation period Latent Infectious

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 Recovery Clinical disease Infection No infection Susceptible host Onset Exposure Incubation period Latent 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 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 Specific protection '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

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