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EPI Notes

An overview of epidemiology, its definition, key concepts like disease distribution and determinants, practical applications, and historical roots tracing back to Hippocrates.

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    Chapter 1: What is epidemiology? The study of the distribution and determinants of disease frequency in human populations. (1970) The study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems. Disease Distribution and Determinants Disease distribution: frequency and pattern of health events in a population Determinants: Any factor (event, characteristic, or other definable entity) that brings about a change in a health condition. What is Epidemiology? Application Applying the knowledge gained by the studies to control the health problems. • Cancer screening • Health education • Health promotion History and Evolution of Epidemiology Epi= on or upon demi=people ology= the study of; body of knowledge Literally meaning "the study of what is upon the people" History and Evolution of Epidemiology- Hippocrates: Hippocrates: • The father of medicine and the first epidemiologist. • Describes the distinction between " epidemic " and "endemic"; endemic —> epidemic—> epidemiology History and Evolution of Epidemiology • In the early time, some described epidemiology as the study of epidemics - An epidemic occurs when there are significantly more cases of the same disease than past experience would have predicted. Epidemiologic Triad for infectious disease Host Agent Environment Public Health

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    Public health is defined as the science of protecting the safety and improving the health of communities through education, policy making and research for disease and injury prevention. Why we study on epidemiology? Public Health • Epidemiology is basic science of PUBLIC HEALTH; • Epidemiology is a PUBLIC HEALTHapproach; EPIDEMIOLOGY has the methodology to: • Determine the extent of disease in the community;* Study the natural history and prognosis of disease; • Identify associations and potential etiology(causes) of a disease and risk factors for disease; • Evaluate new preventive and therapeutic measures and new modes of health care delivery In summary, it may provide a basis for public health initiatives and policies. Epidemiology and Prevention Risk factors • Modifiable risk factors • Non-modifiable risk factors High risk population • Three types of prevention Primary prevention Secondary prevention Tertiary prevention Primary Prevention: Prevention of disease by controlling risk factors (e.g., non-smoking promotion; vaccination; lifestyle changes) Etiologic Study How can we know those risk factors? • It is the most fundamental application of epidemiology; • It aims to identify etiologic (causal) associations between exposure(s) and outcome(s) Secondary prevention: Reduction in consequences of disease by early diagnosis and treatment (e.g., pap smear for cervical cancer screening Mammography for breast cancer etc.) • Screening high risk population • Diagnostic tests

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    Tertiary prevention: Reduction in complications of disease; limits the progression of disability; treatment of symptoms and rehabilitation. • Prognosis • Survival analysis Examples of Clinical Practice uses population data • Criteria of diagnoses are usually defined and determined from large groups of patients. Prognosis is based on experience of large groups of patients with the same disease, stage of disease, and treatments. Selection of therapy is based on the results of large treatment studies, such as clinical trials. Epidemiology and Clinical Practice Epidemiology is critical not only to public health but also to clinical practice. —> clinical epidemiology Epidemiology and basic medical/biological science • Epidemiology may provide clues from population for further medical or biological research; • Results from medical or biological research also needs to be verified in population. • Examples: Epidemiologic studies found use of low dose aspirin is associated with reduced risk of colon cancer. Then the researchers in basic science further demonstrated that the biological pathway from aspirin to colon cancer through in vitro and in vivo studies. Study Types of Epidemiologic research Experimental- descriptive and analytical observational- descriptive and analytical Descriptive and Analytic Epidemiology Descriptive epidemiology: -When was the population affected? -Where was the population affected? -Who was affected? Analytic epidemiology: -How was the population affected? -Why was the population affected? Descriptive epidemiology • Descriptive epidemiology is antecedent to analytical epidemiology. Time (when) Place (where) Person (who)

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    Descriptive epidemiology- Time Time • Seasonal patterns • Calendar time • Temporal trends • Time since an event • Physiologic cycles • Age (time since birth) Descriptive epidemiology- Place Place • Climate • Geology • Presence of agents or vectors • Population density • Economic development • Health care service Descriptive epidemiology- Person Person -Age, gender, ethnicity -Education, occupation, SES -Life styles (smoking, alcohol consumption,diet, physical activity) -Comorbidity -Genetic predisposition Analytic epidemiology Causal inference The Epidemiological Triad of a Disease (Review) Who gets disease and why - Epidemiologists study sick and well people to determine the crucial difference between those who get disease and those who are spared Host:A person or an animal that affords subsistence or lodgementto an infectious agent under natural conditions. Types include: an obligate host, definitive (primary) host, intermediate host and a transport host. The organism that is harboring the disease Vector:An insect or any living carrier that transports an infectious agent from an infected individual or its wastes to a susceptible individual or its food or immediate surroundings. Both biological and mechanical transmissions are encountered Agent:A microbe that causes disease, Bacteria, Viruses, Fungi, Protozoan parasites, etc. Environment: External factors that either allow or cause disease transmission to occur

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    Vector: A living organism that transmits an infectious agent to humans Infectious diseases An infectious disease (also called communicable disease, contagious disease) •A specific infectious factor (usually biological agent or its toxic products) •Directly or indirectly transmittedfrom man to man, from animal to man, from animal to animal, or from the environment (through air, water, food, etc.) to man. Chain of Infection An infectious agent leaves a host (or reservoir) —> Through a portal of exit —> By a mode of transmission —> Enters through a portal of entry —> A susceptible host Modes of Transmission Direct Transmission: Direct contact- Skin, blood, droplet indirect, Indirect Transmission: Common vehicle- Water, air soil... Vector- mosquito, ticks.. 1.Direct Person-to-person by means of direct contact. 2.Indirect (1) Single exposure (2) Multiple exposures (3) Continuous exposure Clinical and Sub-clinical Disease •Clinical disease is characterized by signs and symptoms. •Nonclinical (Inapparent) Disease -Preclinical Disease -Subclinical Disease -Persistent (Chronic) Disease -Latent Disease Incubation Period •The incubation period is defined as the interval from receipt of infectionto the time of onset of clinical illness. •Each infectious disease has a characteristic incubation period. It depends upon: -Rate of growth of organism in the host -Dosage of the infectious agent -Portal of entry -Immune response of the host

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    If you become infected today, the disease with which you are infected may not develop for a number of days or weeks. During this time, the incubation period, you feel completely well and show no signs of the disease. Quarantine a state, period, or place of isolation in which people or animals that have arrived from elsewhere or been exposed to infectious or contagious disease are placed. People come from a different country placed in a certain area Need a certain incubation period to show the symptom If we isolate person for enough time, symptoms can show up Primaiy case and Secondary case •Index case •Secondary case For each case, if they have been infected, we have one person who has been infected (index case) bring in the virus. Index case infects everyone then they are they secondary case Carrier and Carrier Status •An individual (a person or animal ) who harbors the organism; •No clinical symptoms or serologic response(e.g., no antibody) •Can still infect others, although the infectivity is often lower than with other infections. •Carrier status may be of limited duration or may be chronic, lasting for months or years. ex- Typhoid mary- looks healthy Typhoid Mary •Carried Salmonella typhi. •Over a period of many years, she worked as a cook in the New York City area, moving from household to household under different names. •She was considered to have caused at least 10 typhoid fever outbreaks that included 51 cases and 3 deaths. Sporadic •A disease that occurs infrequently and irregularly. •The cases are few and separated widely in time and place that they show no or little connection with each other; •May be the starting point of an epidemic when the conditions are favorable for its spread Endemic •Endemicis defined as the constant presence of a disease or infectious agent within a given geographic area or population group Epidemic •Epidemicis defined as an increase, often sudden, in the number of cases of a disease abovewhat is normally expected in that population in that area

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    Outbreak •Outbreak carries the same definition of epidemic, but is often used for a more limited geographic area ex- a malaria outbreak in a refugee camp - food borne outbreak on a cruise ship Pandemic •An epidemic that has spread over several countries or continents, usually affecting a large number of people. •e.g. H5N1 Influenza pandemics. •*avian influenza, or bird flu Immunity and Susceptibility Ex- Measles occurred in the Faroe Islands in epidemic form when infected individuals entered the isolated and susceptible population. Herd Immunity The resistance of a group to an attack by a disease to which a large proportion of the members of the group are immune Attack Rate Number of people at risk in whom a certain illness develops/ Total number of people at risk Attack Rate Ratio Attack rate (exposed)/Attack rate (non-exposed) Steps in an Outbreak Investigation 1.Confirm existence of the outbreak 2.Appraise existing data -Case identification -Tabulation and spot map -Identification of responsible agent 3.Formulate and test the hypothesis 4.Draw conclusion and implement control measures Outbreak Investigation Step #2: Appraise existing data •Characterize cases in terms of time, place, person (1)Person: compare groups (2)Time: draw epidemic curves (3)Place: construct spot maps Review- Descriptive epidemiology •Descriptive epidemiology is antecedent to analytical epidemiology. Person: Who? Compare groups-Cross tabulation

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    Time When? -draw epidemic curve -The curves may show different patterns of epidemic outbreak Place: Where? - construct spot maps Common, Point Source Outbreak (single exposure, common vehicle outbreak •The exposure period is brief with all cases occurring within one incubation period •Single peak characterized by a sharp upward slope, followed by gradual downward slope •Example: foodbome outbreak at a single event (e.g., wedding party) Common, Continuing Source Outbreak •The exposure period is lengthy; thus, disease duration exceeds one incubation period •Cases rise gradually and likely plateau (rather than peak) •Example: Contaminated drinking water from a faucet at a park Propagated Outbreak -Person-to-person transmission of disease; multiple waves of infection if secondary/tertiary cases occur -Typically a series of progressively taller peaks, separated by a distance approximately equal to the average incubation period Outbreak Investigation- Step #3: Formulate and Test the Hypothesis -What are the possible sources of the outbreak? -What is the likely agent? -What is the most likely method of spread? -What methods can be used for control the disease? ...... Test the hypothesis: case control study; retrospective cohort study Importance of Studying Infectious Diseases Epidemiology •Changes of the pattern of infectious diseases •Discovery of new infections •The possibility that some chronic diseases have an infective origin. Chapter 3: Disease Surveillance (CDC definition) "Ongoing systematic collection, analysis, and interpretation of health data essential to the planning, implementation, and evaluation of public health practice closely integrated with the timely dissemination of these data to those who need to know. " -Infectious diseases -Malformation -Drug adverse effects -Cancer registry

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