بسم الله الرحمن الرحيم “Epidemiology" Introduction

Instructional Objectives At the end of the lecture, the students would be able to do the following: 1- Define epidemiology. 2- Itemize the historical evolution of epidemiology. 3- State the elements of a case definition and state the effect of changing the value of any of the elements. 4- List and describe primary applications of epidemiology in public health practice. 5- List the three components of the epidemiologic triad. 6- List and describe the different stages of natural history of disease.

“Epidemiology" is simply the study of health and disease in populations. It’s a basic science of public health. The discipline of epidemiology is the principles of studying the occurrence of illness and related states and events among populations .” " Epidemiology " comes from the Greek: epi… (among, upon), demos…. (people), and logy ….(study)

“The study of the distribution and determinants of health related Definition: “The study of the distribution and determinants of health related states and events in populations, and the application of this study to control health problems” [John M. Last, Dictionary of Epidemiology]).

The functions of epidemiology may be summarized as: 1-Discover the agent, host, and environmental factors that affect health, in order to provide the scientific basis for the prevention of disease and injury and the promotion of health. 2-Determine the relative importance of causes of illness, disability, and death, in order to establish priorities for research and action. 3-Identify those sections of the population which have the greatest risk from specific causes of ill health, in order that the indicated action may be directed appropriately. 4-Evaluate the effectiveness of preventive and therapeutic health programs and services in improving the health of the population.

5-Study the natural history of disease from its precursor states through its manifestations and clinical course. 6-Conduct surveillance of disease and injury occurrence in populations and of the levels of risk factors – passive (receive reports), active (poll practitioners, conduct surveys). 7-Investigate outbreaks (e.g., hospital-acquired infections, disease clusters, food-borne and water-borne infections) to identify their source and controlling epidemics (e.g., measles, rubella, coronary heart disease, overweight).

Classic and recent examples of epidemiologic investigation: Cholera (John Snow) - waterborne transmission, natural experiment Puerperal fever (Ignatius Semmelweis) - hygienic prevention Pellagra (Joseph Goldberger) - “epidemic” disease was not communicable Rubella and congenital birth defects (Gregg) - prenatal exposure Retrolental fibroplasia - iatrogenic disease

Fluoride and dental caries - community epidemiology; environmental Classic and recent examples of epidemiologic investigation: Lung cancer and smoking - coming of age of chronic disease epidemiology Fluoride and dental caries - community epidemiology; environmental prevention Poliomyelitis immunization trial - a massive experiment that demonstrated the effectiveness of the vaccine against this greatly feared virus. Cardiovascular disease - longitudinal community studies; community intervention trials

Classic and recent examples of epidemiologic investigation: Breast cancer screening – a large-scale randomized trial of effectiveness of cancer early detection through screening. Psychiatric disorder - challenges in disease classification and assessment HIV - a new or newly-recognized virus that has transformed the public health and epidemiology landscape with respect to infectious diseases in general and sexually-transmitted infections specifically. Tuberculosis - reminding epidemiology of its roots; control of a pathogen is very different from its eradication.

Characteristics of epidemiology: “Epidemiology” focuses on diseases among human populations (studying sick and well) in terms of: Person: age, sex, marital status, health conditions, exposure history, or other characteristics. Place: geography, worksite, occupation, migration status…etc Time : seasonal variation, epidemics….etc Studying group of general population rather than a dealing with group of highly-selected individuals studied in a clinic or laboratory as in clinical sciences.

Sources of data for epidemiologic studies include: 1- Population data: .Vital statistics (birth rates, death rates, pregnancy rates, abortion rates, low birth weight). .Demographic, economic, housing, geographical, and other data from the Census and other government data-gathering activities. .Summaries of disease and injury reporting systems and registries. .Workplace monitoring systems. .Environmental monitoring systems (e.g., air pollution measurements). .Production and sales data.

Sources of data for epidemiologic studies include: 2-Individual-level data .Vital events registration (births, deaths, marriages). .Disease and injury reporting systems and registries. .National surveys. .Computer data files (e.g., health insurers). .Medical records. .Questionnaires - in person, by telephone, mailed. .Biological specimens (routinely or specially collected).

Natural history of disease: Defining observing, and measuring health and disease requires an appreciation of the concept of natural history – the evolution of a pathophysiologic process. "Natural" refers to the process in the absence of intervention. Natural history encompasses the entire sequence of events and developments from the occurrence of the first pathologic change (or even earlier) to the resolution of the disease or death. The natural history of a disease may be described through a staging classification. Staging can aid in defining uniform study groups for research studies, determining treatment regimens, predicting prognosis, and in providing intermediate end-points for clinical trials.

Disease results from complex interaction between man, an Host Agent Environment Disease results from complex interaction between man, an agent and the environment.

Scheme diagram represents the natural history of disease Usual time of diagnosis by symptoms & signs Prepathogenesis phase Pathogenesis phase Clinical phase End result of disease in man Recovery, death or disability Scheme diagram represents the natural history of disease

*Prepathogenesis phase: period before disease. The disease agent has not yet entered into man. The pathological process did not start yet. Here the person is exposed to disease by its risk factors. *Pathogenesis , presymptomatic phase : the pathological process of disease starts here. If the disease is infectious it begins with the entry of the disease agent, i.e. m o into the susceptible human body . * Clinical phase: begins with the development of symptoms and signs of disease. These periods may vary in time according to type of the disease e.g. days months even years .

development of symptomatic disease". Incubation period may be long or For infectious diseases, it is important to notice Incubation period which refers to the "time from infection (entry of the disease agent) to development of symptomatic disease". Incubation period may be long or short according to type of disease. For non-infectious disease Induction "period of time from causal action until disease initiation".

Ice berg phenomenon of AIDS Ice berg phenomenon : this phenomenon is explained as when the tip of iceberg is seen above the sea while the base of the iceberg which is the biggest portion is below the sea level. e.g. AIDS (Tip of iceberg) 500000 AIDS cases detected and diagnosed 1.2million AIDS cases undetected 8-10 million persons with HIV infections Scheme diagram represents Ice berg phenomenon of AIDS The more evident picture of disease is usually observed and reported (i.e. cases with symptoms and signs). While sub-clinical or latened cases are not registered unless during screening. e.g. moderate to severe diarrhea cases among infants are admitted to hospitals. While mild and self limited cases treated at home are not admitted. So counting on hospital records will not give us the accurate picture of disease severity.