Chapter 2 Principles of Epidemiology in Public Health Practice PDF

BIO 52 Epidemiology\ CHAPTER 2 ***Principles of Epidemiology in Public Health Practice*** Epidemiology: Definition and Key Concepts *Epi --* on, upon *Demos --* people *Logos* -- study MacMahon : the study of the distribution and determinants of disease frequency in man. Lilienfeld : the stud...

BIO 52 Epidemiology\ CHAPTER 2 ***Principles of Epidemiology in Public Health Practice*** Epidemiology: Definition and Key Concepts *Epi --* on, upon *Demos --* people *Logos* -- study MacMahon : the study of the distribution and determinants of disease frequency in man. Lilienfeld : the study of the distribution of a disease or a physiologic condition in human populations and of the factors that influence this distribution Present definition : the study of factors affecting the health and illness of populations, and serves as the foundation and logic of interventions made in the interest of **public health** and **preventive medicine**. It is considered a cornerstone methodology of **public health research**, and is highly regarded in **evidence-based medicine** for identifying risk factors for disease and determining optimal treatment approaches to clinical practice. In the study of **communicable and non-communicable diseases**, the work of epidemiologists ranges from **outbreak investigation** to **study design**, **data collection**, and **analysis** including the development of **statistical models** to test hypotheses and the **documentation of results** for submission to peer-reviewed journals. Components of the Definition a. Population b. Distribution c. Factors General Objectives of Epidemiology a. Prevention of disease b. Maintenance of health Specific Objectives of Epidemiology a. For planning and report a. Epidemic/Endemic Occurrence b. Communicable/Non-communicable diseases c. Study of Health Services d. Research in health and other related fields **5 Ws of Descriptive Epidemiology\ **Descriptive epidemiology covers **time, place**, and **person** What = health issue of concern Who = person Where = place When = time Why/How = causes, risks factors, modes of transmission **Analytic Epidemiology** - Concerned with the search for *causes and effects*, or the why and the how - Epidemiologists use analytic epidemiology to quantify the association between exposures and outcomes and to test hypotheses about causal relationships. - Epidemiologic studies fall into two categories: **experimental** and **observational** **Historical Evolution of Epidemiology** A. Circa 400 BC - **Hippocrates** suggested that environmental and host factors such as behaviors might influence the development of disease B. 1662 - **John Graunt**, a London haberdasher (men's clothing dealer) - Published landmark analysis of mortality data in 1662 C. 1800 - **William Farr**, considered FATHER OF MODERN VITAL STATISTICS AND SURVEILLANCE, developed many of the basic practices used today in vital statistics and disease classification D. 1854 - **John Snow**, anesthesiologist, FATHER OF FIELD EPIDEMIOLOGY, for conducting a series investigations in London that warrant the title. E. 19^th^ and 20^th^ centuries - **Mid and late-1800s**, most focused on [acute infectious diseases] - **1930s and 1940s**, [noninfectious diseases] - **1960s and early 1970s**, health workers applied epidemiologic methods to [eradicate naturally occurring smallpox worldwide] - **1990s**, biological warfare and bioterrorism after 9/11/01 - New infectious diseases continued to challenge epidemiologists as new infectious agents emerged (Ebola, HIV/AIDS), were identified (Legionella, SARS), or changed (drug resistant Mycobacterium tuberculosis, Avian influenza) ***The Dynamics of Disease Transmission*** The Ecological Concept of Disease Health -- is a state of complete physical and mental, and social well-being; it is the successful defense of the host against forces tending to disturb the body's equilibrium Disease -- is the failure of the body's defense mechanism to cope with the forces tending to disturb the body's equilibrium Premises of the Ecological Concept of Disease Disease = IMBALANCE of forces BETWEEN HOST and AGENT Imbalance -- depends on the nature of the agent and the host The nature of the agent and the host and their interaction between the environment. Interaction can be shown in the ecological models: a. the lever b. the epidemiologic triangle  c. the wheel d. the web of causation - effects never depend on single isolated cause, but rather develop as the result of chains of causation at which each link itself is the result of "a complex genealogy of antecedents" - Implication of multiplicity of causes -- interrupt the causation of disease by cutting the chains  When does DISEASE occur? 1. **Presence** of infectious organism 2. If it is taken in massive doses 3. If the agent increases in virulence 4. If the host if overcome **The FACTORS OF DISEASE** *AGENT* -- any element, substance, or force whether living or non-living; the presence or absence of which can initiate or perpetuate a disease process Types: A. Non-living agents a. Chemical agents i. Exogenous -- chemicals w/c arise from outside the host ii. Endogenous -- chemicals w/c arise from inside the host b. Physical and Mechanical Agents iii. extreme temperature, lightning, electricity, physical trauma c. Nutrient Agents iv. Deficiency agents v. Excess in nutrients vi. Hypersecretion of hormones B. Living Agents d. Bacteria e. Fungi f. Protozoa g. Cestodes h. Nematodes i. Trematodes j. Viruses Characteristics of LIVING AGENTS - Inherent characteristics - Physical - Biological requirements - Chemical requirements - Viability and resistance Characteristics directly **RELATED TO MAN !!!!** a. Infectivity -- the ability to gain access and adapt to the human host to the extent of finding lodgment and multiplication b. Pathogenecity -- the ability of an agent to set up a specific reaction, local or general, clinical or sub-clinical c. Virulence -- a measure of severity rather than constancy of the reaction produced and is usually measured in terms of fatality d. Antigenicity -- the ability to stimulate the host to produce defense mechanisms Characteristics of Agents in Relation to the Environment a. Reservoir of sources -- the source of infections (man, animals, soil, inanimate organic matter) b. Modes of transmissions -- the mechanisms by which an infectious agent is transported from the reservoir to a susceptible host a. Contact b. Vehicle c. Vector d. Airborne Host factors -- the individual exposed to the agent; the patient; carrier Environmental factors -- sum total of an organism's external surrounding, conditions, and influences that affect its life and development Conditions that create opportunities for pathogens a. Immune suppression b. Changes in the normal microbiota c. Introduction of a member of the normal flora into an unusual site in the body Modes of Disease Transmission a. Contact Transmission a. Direct contact (handshakes) b. Indirect contact (drinking glasses, sneezing and coughing, toothbrushes) b. Vehicle Transmission c. Airborne d. Waterborne e. Foodborne c. Vector Transmission f. Mechanical (insect bodies) g. Biological (lice, mites, mosquitoes, ticks) For MQs to be successful in their transmission, they must: - Survive passage from one host to another or from the reservoir to the host - Attach to or penetrate the host's tissues - Withstand (for a period of time) the host's defense mechanism - Induce damage to or malfunction of the host's tissues ***The Concepts of Cause*** Importance of studying causes of disease - Prevention - Diagnosis - Application of correct treatments Definition of Cause of a Disease - An event, condition, characteristic or a combination of these factors that plays an essential role in producing an occurrence of a disease Statistical Association -- the proportion of persons exhibiting both events is either significantly higher or significantly lower than the proportion predicted on the basis of simultaneous consideration of the separate frequencies of the 2 categories Causal association -- change in one party to the association alters the other A cause is termed **necessary** if a disease *cannot* develop in its absence. A necessary cause may not be sufficient cause because other risk factors are needed to cause the disease. A cause is termed **sufficient** when it inevitably *produces or initiates a disease*. Not usually a single factor but often comprises several components. More than one factor should be present for disease to develop. Factors affecting the development of disease: - Host Factors (intrinsic) - The state the host at any given time is a result of the interactions of genetic endowment with environment; affect susceptibility to disease - Environmental Factors (extrinsic) - Influence exposure and sometimes indirectly affect susceptibility as well The Natural History of Disease The course of disease over time (unaffected by treatment) The course of disease from onset (inception) to resolution Stages of the Natural History of the Disease I. **Stage of Susceptibility** The disease has not developed but the groundwork has been laid by the presence of risk factors which favor its occurrence. Risk Factor is an attribute or exposure that increases the probability of occurrence of disease or other specified outcome. **Level of Prevention:** Primary **Mode of Intervention:** Health Promotion Program II. **Stage of Pre-symptomatic disease** No manifest disease but pathogenic changes have started to occur Inapparent or sub-clinical disease process **Level of Prevention**: Secondary **Mode of Intervention:** Early Detection and initial treatment III. **Stage of Clinical Disease** IV. **Stage of Disability** Residual defect of short or long duration, leaving the person disabled to a lesser or greater extent. **Mode of Intervention:** Rehabilitation to improve the quality of life. ***Using Epidemiology to Identify the Cause of Disease*** Demography - In determining the nature and extent of public health problems, we usually need to know something about the population at risk of experiencing the health problem - Science of population - Two Greek words (?): **snyos** -- people; **ypagly** -- to draw or write - by Achille Gillard 1855 - *"The natural and social history of human species or the mathematical knowledge of populations, of their general changes, and of their physical, civil, intellectual, moral conditions."* Information we need to know about the population: - How large the population is - What comprises the population - Where the population is found 3 Human events 1. Population size a. Place b. Person c. Time - Natality - Mortality - Migration 2. Population composition or structure d. Age e. Sex f. Marital status g. Occupation h. Education 3. Distribution of population in space i. How people are distributed j. May be small or a very large area Uses of Demography in Public Health Work - To identify and characterize health problems besetting a community. - To plan, prioritize and implement health programs rationally. - To control and prevent health problems. - To study determinants or reasons for the occurrence of certain diseases. [Integral to Public Health] Health workers should also know something about the growth and dispersal of population groups in the past to be able to predict future developments and their possible consequences Sources of Demographic Data - Census - Sample Surveys - Voter's Registry Census - The total process of collecting, compiling, and publishing demographic, economic and social data pertaining, at a specified time or times, to all persons in a country or delimited territory. - National population census is conducted once every ten years Sources of Health Data a. Primary Data - Data gathered through actual interview or survey from chosen respondents. - First hand information - Issue: Reliability of given information b. Secondary Data - Data gathered previously recorded documents or registry - Data gathered from archives or database - No actual interview or survey Methods of Data Collection a. Review of documented sources b. Making observations a. Physical examination b. Laboratory tests c. Asking questions c. Standardized interview d. Scheduled interview e. Self-administered questionnaires Data Useful for Epidemiologic Studies a. Vital events b. Disease statistics c. Data on physiologic and pathologic conditions d. Statistics on health resources and services e. Statistics pertaining to the environment f. Demographic data g. Socio-cultural characteristics of the population Desired Data Qualities a. Accuracy b. Timeliness c. Completeness Count -- basic measure of disease frequency Example of counts - Incident cases - Prevalent cases Prevalent cases - Refer to the number of existing cases of one point in time - Include both **old** and **new** cases of the disease - No follow-up time involved Incident cases - **New** cases during an interval of time - Implies a change in status of an incident case from a non-diseased state to a diseased state - Change from non-diseases to diseased state should occur within this period Crowding Index -- total number of persons in the household / number of rooms in the house **The higher the index, the easier it will be for the disease transmission to occur.** 5Ws of Descriptive Epidemiology WHAT = health issue of concern WHO = person WHERE = place WHEN = time WHY/HOW = causes, risk factors, modes of transmission Analytic Epidemiology -- search for causes and effects, the why and the how Epidemiologists use analytic epidemiology to quantify the association between exposures and outcomes and to test hypotheses about causal relationships. Experimental studies In an experimental study, the investigator determines through a *controlled process* of the exposure for each individual (clinical trial) or community (community trial), and then tracks the individuals or communities over time to detect the effects of the exposure. Observational studies In an observational study, the epidemiologist simply *observes the exposure and disease status* of each study participant. John Snow's studies of cholera in London were observational studies. The two most common types of observational studies are cohort studies and case-control studies; a third type is cross-sectional studies Cohort study - the epidemiologist records whether each study participant is exposed or not, and then tracks the participants to see if they develop the disease of interest (the investigator observes rather than determines the participants' exposure status). - After a period of time, the investigator compares the disease rate in the exposed group with the disease rate in the unexposed group. The unexposed group serves as the comparison group, providing an estimate of the baseline or expected amount of disease occurrence in the community. - If the disease rate is substantively different in the exposed group compared to the unexposed group, the exposure is said to be associated with illness Retrospective Cohort Study - An alternative type of cohort study. In this type of study both the exposure and the outcomes have already occurred. Just as in a prospective cohort study, the investigator calculates and compares rates of disease in the exposed and unexposed groups. - Retrospective cohort studies are commonly used in investigations of disease in groups of easily identified people such as workers at a particular factory or attendees at a wedding. Case-Control study - In a case-control study, investigators start by enrolling a group of people with disease (at CDC such persons are called case-patients rather than cases, because case refers to occurrence of disease, not a person). - As a comparison group, investigator then enrolls a group of people without disease (controls). Investigators then compare previous exposures between the two groups. - The control group provides an estimate of the baseline or expected amount of exposure in that population. If the amount of exposure among the case group is substantially higher than the amount you would expect based on the control group, then illness is said to be associated with that exposure Cross-sectional study - In this third type of observational study, a sample of persons from a population is enrolled and their exposures and health outcomes are measured simultaneously. - The cross-sectional study tends to assess the presence (prevalence) of the health outcome at that point of time without regarding the duration. For example, in a cross-sectional study of diabetes, some of the enrollees with diabetes may have lived with their diabetes for many years, while others may have been recently diagnosed. - From an analytic viewpoint the cross-sectional study is weaker than either a cohort or a case-control study because a cross-sectional study usually cannot disentangle risk factors for occurrence of disease (incidence) from risk factors for survival with the disease. - On the other hand, a cross-sectional study is a perfectly fine tool for descriptive epidemiology purposes. Cross-sectional studies are used routinely to document the prevalence in a community of health behaviors (prevalence of smoking), health states (prevalence of vaccination against measles), and health outcomes, particularly chronic conditions (hypertension, diabetes). ***Outbreak Investigation by CDC: 10 steps*** Outbreak/Epidemic -- occurrence of health-related event clearly excess of the normal expectancy An epidemic may include any kind of disease, including noninfectious conditions. There is no general rule about the number of cases that must exist for an outbreak to be considered an epidemic. Rather, an epidemic exists when the number of cases exceeds that of what is expected on the basis of past experience for a given population. For example, one case of smallpox would constitute an outbreak. There is no rule on geographic extent An outbreak could be in only one area or in several countries. When an epidemic spreads in several countries, usually affecting many people, it is called a **pandemic** Most flu epidemics that occur during the winter are pandemics. AIDS is considered a pandemic disease. An outbreak may encompass any time period. It may last a few hours (bacterial food poisoning), a few weeks (hepatitis) or several years (acquired immunodeficiency syndrome, or AIDS) **Endemicity** refers to the usual permanence of a disease or infection in a defined geographic area or population group. Therefore an endemic disease is a disease that happens constantly in an area. For example, hepatitis A is endemic in most states of the United States, especially in the southern part of the country **Definition:** Spreading rapidly and extensively by infection and affecting many individuals in an area or a population at the same time. *Example: an epidemic outbreak of influenza.* - An outbreak of a contagious disease that spreads rapidly and widely. - A rapid spread, growth, or development: *Example: an unemployment epidemic.* - Outbreaks can also be: Behavioral risk related (e.g., sexually transmitted diseases, increased risk due to malnutrition) Zoonotic -- the infectious agent is endemic to an animal population Patterns of occurrence: - **Endemic** - a communicable disease, such as influenza, measles, mumps, pneumonia, colds, small pox found permanently in a particular region or population. - **Epidemic** - when this disease is found to infect \"a lot of\" people at the same time and may spread through one or several community. - **Pandemic** - occurs when the epidemic spreads across the world The purpose of an epidemic or outbreak investigation is to identify ways to prevent further transmission of the disease. The three main objectives of an epidemic outbreak investigation are to: a. Identify the responsible etiologic agent. b. Find the source of infection by studying the occurrence of the disease among persons or in a place or time, as well as determining specific attack rates. c. Formulate recommendations to prevent further transmission **Common source** - All victims acquire the infection from the same source (e.g. a contaminated water source). - Continuous source - Common source outbreak where the exposure occurs over multiple incubation periods. - Point source- Common source outbreak where the exposure occurs in less than one incubation period **Propagated** - Transmission occurs from person to person **10 STEPS BY CDC** 1. Prepare to investigate 2. Verify the diagnosis and confirm outbreak 3. Case definition (defines a case in time, person, and place) a. Suspect case - All residents of Nursing Home A with onset of cough and fever between January 1, 2008 and February 1, 2008 b. Probable case - Meet the suspect case definition plus have pneumonia on chest X-ray. c. Confirmed case - Meet the probable case definition plus have pneumococcal infection confirmed by blood culture or other isolation of pneumococci from normally sterile site 4. Case finding 5. Perform descriptive epidemiology 6. Hypothesis generation -- the how and the why 7. Evaluate hypothesis through statistics 8. Additional environmental studies 9. Implement control/prevention measure 10. Communicate findings Why epidemics occur - Susceptible individuals travel into an endemic area where the infectious disease exists - When new I.D. is introduced by humans or animals traveling from endemic area into susceptible human population. - Preexisting infection occurs in an area of low endemicity and reaches susceptible persons as a result of new or unusual social, behavioral, sexual, or cultural practices - Host susceptibility and response are modified by natural or drug-induced immunosuppression, malnutrition, or diseases like AIDS Why some outbreaks end - No more susceptible individuals. Everybody who was susceptible got the disease. - No more exposure to the source. The individuals move away from the source of infection. - No more source of contamination. The source of contamination ends (all the contaminated food is consumed). - Individuals decrease their susceptibility. People get immunized (are vaccinated) or use preventive measures to avoid disease. - The pathogen becomes less pathogenic. Sometimes when some germs (bacteria, viruses, parasites, etc.) pass from one individual to another they change or mutate, becoming less pathogenic, or less capable of producing disease Common interventions used to control an epidemic - Control source of pathogen - Remove source of contamination - Remove people from exposure - Inactivate or neutralize pathogen - Treat infected persons - Interrupt transmission - Strerilize or disinfect environmental sources of transmission - Control vector transmission using skin repellents - improve personal sanitation - control or modify host response to exposure - immunize susceptible hosts - use prophylactic chemotherapy - modify behavior or use a barrier ***Top Public Health Issues*** A. Physical activity and nutrition B. Antibiotic resistance C. Overweight and obesity D. HIV/AIDS E. Smoking/tobacco F. Substance abuse G. Mental health H. Injury and violence I. Immunization J. Access to health care K. Cancer L. Environmental quality ***The Levels of Prevention*** I. **PRIMORDIAL Prevention** - Aims to avoid emergence and establishment of the social, economic, and cultural patterns of living that are known contribute to an elevated risk of the disease - Inhibits people from becoming high risk to prevent from developing themselves to high risk factor - Legislations, guidelines, and health policy formulations II. **PRIMARY Prevention** - Protection of health by personal and communal efforts to prevent involvement of the stage of susceptibility - Aims to prevent disease occurrence by changing the susceptibility or reducing exposure for susceptible individuals III. **SECONDARY Prevention** - This aims to reduce/slow the disease progression (early stages of the disease, i.e, pre-clinical and clinical stages), to prevent complications or to reverse communicability - Involves curing the disease at the earliest possible stage and having assumptions that pathologic conditions brought about by the agent can be reversed. IV. **TERTIARY Prevention** - Aims at producing long time dysfunction and impairment or to prolong life - This prevention assumes that pathologic conditions are irreversible especially in the stage of disability of the disease