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© 2010 Jones and Bartlett Publishers, LLC Chapter 1 Foundations of Epidemiology © 2010 Jones and Bartlett Publishers, LLC Objective: Define epidemiology Epidemiology is the study of the distribution and determinants of health-related states or events in human populations, and the application of this study to prevent and control health problems. © 2010 Jones and Bartlett Publishers, LLC Defining epidemiology The word epidemiology is based on the Greek words epi—prefix meaning on, upon, or befall; demos—root meaning the people; and logos—suffix meaning the study In other words, epidemiology is the study of what befalls the population © 2010 Jones and Bartlett Publishers, LLC Key terms in the definition Study - Epidemiology involves sound methods of scientific investigation. Methods rely on careful observation and the use of valid comparison groups to determine whether the observed health events differ from what might be expected © 2010 Jones and Bartlett Publishers, LLC Key terms in the definition Distribution Study of frequency and pattern of health events in the population Frequency – number, and number in relation to the population Pattern – the health-related state or event by person, place, and time characteristics © 2010 Jones and Bartlett Publishers, LLC Key terms in the definition Determinants Search for causes and other factors of health-related states or events © 2010 Jones and Bartlett Publishers, LLC Key terms in the definition Health-related states or events Disease states cholera, influenza, pneumonia, mental illness) Conditions associated with health physical activity, nutrition, environmental poisoning, seat belt use, and provision and use of health services Events injury, drug abuse, and suicide © 2010 Jones and Bartlett Publishers, LLC Key terms in the definition Application of this study to prevent and control health problems © 2010 Jones and Bartlett Publishers, LLC Objective: Define descriptive epidemiology Answering the who, what, when, and where questions is prerequisite to effective education, screening, prevention, and control programs © 2010 Jones and Bartlett Publishers, LLC The importance of descriptive epidemiology Descriptive epidemiology involves characterization of the distribution of health-related states or events by Person - who Place - where Time – when Clinical criteria - what © 2010 Jones and Bartlett Publishers, LLC Objective: Define analytic epidemiology Analytic epidemiology involves identifying and quantifying associations, testing hypotheses, and identifying causes of health-related states or events Explains why and how health-related states or events occur © 2010 Jones and Bartlett Publishers, LLC Objective: Identify some activities performed in epidemiology Identifying risk factors for disease, injury, and death Describing the natural history of disease Identifying individuals and populations at greatest risk for disease Identifying where the public health problem is greatest Monitoring diseases and other healthrelated events over time © 2010 Jones and Bartlett Publishers, LLC Objective: Identify some activities performed in epidemiology Evaluating the efficacy and effectiveness of prevention and treatment programs Providing information useful in health planning and decision making for establishing health programs with appropriate priorities Assisting in carrying out public health programs Being a resource person Communicating public health information © 2010 Jones and Bartlett Publishers, LLC Objective: Explain the role of epidemiology in public health practice and individual decision making Epidemiological findings contribute to preventing and controlling disease, injury, disability, and death How? By providing information leading to informed public health policy and planning, as well as individual health decision making © 2010 Jones and Bartlett Publishers, LLC Epidemiologic information for influencing public health policy and planning and individual decisions Public health surveillance Causes of disease Completing the clinical picture Program evaluation Efficacy Effectiveness See Table 1.1 of text © 2010 Jones and Bartlett Publishers, LLC Questions that need epidemiology Screening/Diagnosis What is the sensitivity/specificity of a test? Is prostate specific antigen a good test for prostate cancer? Causes Why did this patient suffer a stroke? Is obesity the cause of metabolic syndrome? © 2010 Jones and Bartlett Publishers, LLC Questions that need epidemiology Treatment Is this the best treatment for Parkinson's disease? Is my surgery as good as that of everyone else? Prognosis What are the chances of a recurrent heart attack? How long will this knee joint prosthesis last? © 2010 Jones and Bartlett Publishers, LLC Questions that need epidemiology Health promotion and protection Do current school meals harm children's future health? Will the Irish smoking ban in public places work better than the English policy? Health and disease surveillance Why are there 10 fold international differences in suicide rates? When will the next influenza pandemic occur? Health inequalities Why should life expectancy be nearly five years lower in unskilled manual workers? Do health services reduce or increase health inequalities? © 2010 Jones and Bartlett Publishers, LLC Objective: Define epidemic, endemic, and pandemic Epidemic – Health-related state or event in a defined population above the expected over a given period of time Endemic – Persistent, usual, expected health-related state or event in a defined population over a given period of time Pandemic – Epidemic affecting a large number of people, many countries, continents, or regions © 2010 Jones and Bartlett Publishers, LLC Objective: Describe common source, propagated, and mixed epidemics Common source Point Intermittent Continuous Propagated Spread from person-to-person Mixed epidemics A mixture of common source and mixed © 2010 Jones and Bartlett Publishers, LLC Common source Tend to result in more cases occurring more rapidly and sooner than host-tohost epidemics Identifying and removing exposure to the common source typically causes the epidemic to rapidly decrease © 2010 Jones and Bartlett Publishers, LLC Common source Examples anthrax, traced to milk or meat from infected animals botulism, traced to soil-contaminated food and cholera traced to fecal contamination of food and water © 2010 Jones and Bartlett Publishers, LLC Propagated Arise from infections being transmitted from one infected person to another Transmission can be through direct or indirect routes Host-to-host epidemics rise and fall more slowly than common source epidemics © 2010 Jones and Bartlett Publishers, LLC Propagated Examples tuberculosis whooping cough Influenza measles © 2010 Jones and Bartlett Publishers, LLC Mixed epidemics Occurs when a common source epidemic is followed by person-to-person contact and the disease is spread as a propagated outbreak Example – Shigellosis occurred among a group of 3000 women attending a music festival. Over the next few weeks, subsequent generations of shigella cases spread by person-to-person transmission from festival attendees. © 2010 Jones and Bartlett Publishers, LLC Disease transmission Disease transmission usually occurs by direct, person-to-person contact (e.g., STDs) fomite-borne (e.g., Hepatitis A spread by a contaminated eating utensil) vehicle-borne (e.g., HIV/AIDS spread through needle sharing drug users) vector-borne transmission (e.g., Malaria spread through mosquitoes) © 2010 Jones and Bartlett Publishers, LLC Accurate assessment requires a standard case definition A standard set of criteria, or case definition, assures that cases are consistently diagnosed, regardless of where or when they were identified and who diagnosed the case © 2010 Jones and Bartlett Publishers, LLC Objective: Define the concepts and principles of case as used in epidemiology A case is a person who has been diagnosed as having a disease, disorder, injury, or condition © 2010 Jones and Bartlett Publishers, LLC Primary case, index case The first disease case in the population is the primary case. The first disease case brought to the attention of the epidemiologist is the index case. The index case is not always the primary case. © 2010 Jones and Bartlett Publishers, LLC Secondary case Those persons who become infected and ill after a disease has been introduced into a population and who become infected from contact with the primary case © 2010 Jones and Bartlett Publishers, LLC Different levels of diagnosis Suspect An individual who has all of the signs and symptoms of a disease or condition, yet not diagnosed Confirmed All criteria met © 2010 Jones and Bartlett Publishers, LLC Different levels of diagnosis As more information (such as laboratory results) becomes available to the physician, he or she generally upgrades the diagnosis. When all criteria are met and they meet the case definition, the case is classified as a confirmed case. © 2010 Jones and Bartlett Publishers, LLC Objective: Describe the epidemiology triangle for infectious disease © 2010 Jones and Bartlett Publishers, LLC Triangle is based on the communicable disease model Shows the interaction and interdependence of agent, host, environment, and time as used in the investigation of diseases and epidemics. Agent is the cause of the disease Host is an organism, usually a human or an animal, that harbors a disease Environment includes those surroundings and conditions external to the human or animal that cause or allow disease transmission Time accounts for incubation periods, life expectancy of the host or the pathogen, and duration of the course of the illness or condition. © 2010 Jones and Bartlett Publishers, LLC Stopping an epidemic An epidemic can be stopped when one of the elements of the triangle is interfered with, altered, changed, or removed from existence, so that the disease no longer continues along its mode of transmission and routes of infection © 2010 Jones and Bartlett Publishers, LLC Some disease transmission concepts Fomites Objects such as clothing, towels, and utensils that may harbor a disease agent and are capable of transmitting it; usually used in the plural Example – transmission of cutaneous anthrax from drums to an individual © 2010 Jones and Bartlett Publishers, LLC Vector An invertebrate animal (e.g., tick, mite, mosquito, bloodsucking fly) capable of transmitting an infectious agent among vertebrates Can spread an infectious agent from an infected animal or human to other susceptible animals or humans through its waste products, bite, body fluids, or indirectly through food contamination © 2010 Jones and Bartlett Publishers, LLC Plague Yersinia pestis (the bacteria that cause plague) is found in animals throughout certain parts of the world, most commonly in rats, but occasionally in other wild animals, such as prairie dogs. Plague transmission from these infected animals generally occurs in one of three ways: Bites from infected rodent fleas (85%) Direct contact with infected tissue or bodily fluids For example, people can become directly infected with plague by handling infected rodents, rabbits, or wild carnivores that prey on these animals when plague bacteria enter through the person's skin. Inhaling infected droplets. © 2010 Jones and Bartlett Publishers, LLC Reservoir The habitat (living or nonliving) on which an infectious agent lives, grows, and multiplies and is dependent on for its survival in nature Humans often serve as both reservoir and host © 2010 Jones and Bartlett Publishers, LLC Zoonosis When an animal transmits a disease to a human Examples – Rabies, Rocky Mountain spotted fever, shigellosis © 2010 Jones and Bartlett Publishers, LLC Carrier A carrier contains, spreads, or harbors an infectious organism Example – Typhoid Mary © 2010 Jones and Bartlett Publishers, LLC Modes of disease transmission Direct transmission – direct physical contact such as touching with contaminated hands, skin-to-skin contact, kissing, or sexual intercourse Indirect transmission – occurs when pathogens or agents are transferred or carried by some intermediate item, organism, means, or process to a susceptible host, resulting in disease © 2010 Jones and Bartlett Publishers, LLC The chain of infection There is a close association between the triangle of epidemiology and the chain of infection © 2010 Jones and Bartlett Publishers, LLC Other modes of causation Infectious diseases are no longer the leading cause of death in industrialized nations so a more advanced model of the triangle of epidemiology has been proposed that better reflects the behavior, lifestyle, and chronic disease issues found in modern times © 2010 Jones and Bartlett Publishers, LLC Objective: Describe the advanced epidemiology triangle for chronic diseases and behavioral disorders © 2010 Jones and Bartlett Publishers, LLC Objective: Define the three levels of prevention used in public health and epidemiology Primary prevention (occurs prior to exposure) Immunization Sanitation Education Media campaigns Warning labels © 2010 Jones and Bartlett Publishers, LLC Active primary prevention Requires behavior change on part of subject Wearing protective devises Health promotion Lifestyle changes Community health education Ensuring healthy conditions at home, school and workplace © 2010 Jones and Bartlett Publishers, LLC Passive primary prevention Does not require behavior change Vitamin fortified foods Fluoridation of public water supplies © 2010 Jones and Bartlett Publishers, LLC Secondary prevention Occurs to reduce the progress of disease The disease already exists in the person Cancer screening – cancer already present. The goal is to detect the cancer before clinical symptoms arise in order to improve prognosis and prevent conditions from progressing and from spreading © 2010 Jones and Bartlett Publishers, LLC Tertiary prevention To reduce the limitation of disability from disease The disease has already occurred Physical therapy for stoke victims Halfway houses for recovering alcoholics Shelter homes for the developmentally disabled Fitness programs for heart attack patients © 2010 Jones and Bartlett Publishers, LLC Conclusion Epidemiology involves application of scientific methods for describing the frequency and pattern of health-related states or events Epidemiology identifies causes of healthrelated states or events and modes of transmission Epidemiology guides public health planning and decision making Epidemiology assists individuals in making informed health behavior changes © 2010 Jones and Bartlett Publishers, LLC