Foundations of Epidemiology Chapter 1 PDF

Foundations of Epidemiology Chapter 1 EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 1 Learning Objectives At the end of this chapter, students are expected to: Specify six dimensions of health Define epidemiology and explain its relationsh...

Foundations of Epidemiology Chapter 1 EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 1 Learning Objectives At the end of this chapter, students are expected to: Specify six dimensions of health Define epidemiology and explain its relationship to PH Identify selected activities performed in epidemiology Define risk factor and why it does not mean cause Differentiate among terms used to describe patterns of infectious disease occurrence Describe why a standard case definition is important in epidemiology Elaborate on the six links of the ‘Chain of Infection’ Identify the challenges of the Infectious Disease Spectrum Describe the epidemiology triangle for the control of infectious diseases EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 2 Six Dimensions of Health Ability of the human body to function properly; includes physical Physical fitness and activities of daily living. Ability to have satisfying relationships; interaction with social Social institutions and societal mores. Mental Ability to think clearly, reason objectively, and act properly. Emotional Ability to cope, adjust and adapt; self-efficacy and self-esteem. Feeling as if part of a greater spectrum of existence; personal Spiritual beliefs and choices. Comprised of external factors (i.e., one’s surroundings such as Environmental habitat or occupation) & internal factors (i.e., one’s internal structure such as genetics). EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 3  What is the Purpose of Epidemiology? to better understand the burden and risk factors of health problems in human populations and to make changes that decrease risk and improve health. A risk factor is a behavior, environmental exposure, or inherent human characteristic that increases the chance of a person developing an adverse health outcome. Risk factors can be modifiable or non-modifiable. A risk factor is insufficient to cause a disease; other contributing factors may be required before the health challenge occurs. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 4  Epidemiology can be: Applied to any aspect of human health, including: All types of diseases (infectious, chronic, genetic,..) Impairments Disabilities Injuries The utilization and outcomes of health services, and even Good health EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 5 Epidemiology can help us: Identify and understand the factors that influence the emergence, severity, and consequences of health problems. Examples include: o tobacco use and lung cancer o physical activity and heart disease o diet and longevity o lead exposure and developmental disabilities o seat belt use and motor vehicle injuries/deaths EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 6 How does Public Health relate to Epidemiology? Public health is the science and art of promoting health and extending life at the population level. There are many subfields of public health, including epidemiology, biostatistics, environmental health, health services, social and behavioral sciences, and biomedical sciences. Because of its central role in public health, epidemiology is referred to as the foundation of public health. It is the diagnostic discipline of Public Health (PH). EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 7 What is Epidemiology? (I) 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. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 8 What is Epidemiology? (II) The word epidemiology is based on the Greek words: epi – prefix meaning on, upon, or befall demos – root meaning the people logos – suffix meaning the study Epidemiology is the study of what happens to the population. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 9 Key terms in the definition (I) 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. Distribution Refers to the frequency and pattern of health events in the population Frequency – refers to the number of health events and their relationship to the size of the population Pattern – is a description of the health-related state or event by person, place, and time. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 10 Key terms in the definition (II) Determinants Denotes factors that produce (cause) a health outcome A cause is a specific event, condition, or characteristic that precedes (comes before) the health outcome Identifying causal associations is complex EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 11 Key terms in the definition (III) Health-related states or events 1. Disease states Cholera, influenza, pneumonia, mental illness 2. Conditions associated with health Physical activity, nutrition, environmental poisoning, seat belt use, and provision and use of health services 3. Events Injury, drug abuse, and suicide EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 12 Key terms in the definition (IV) Application of this study to prevent and control health problems Prevention involves measures to avoid the occurrence of disease. Control aims to prevent further spread of the disease in areas where an outbreak currently exists, Results of epidemiologic investigations can provide PH officials with information related to: Who is at the greatest risk for disease Where the disease is most common When the disease occurs most frequently What public health programs might be most effective EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 13 Why is Epidemiology Considered by Many to be the Foundation of PH? Because of its central role in carrying out the three core PH functions: 1. The assessment and monitoring of the health of communities and populations at risk to identify health problems and priorities. 2. The formulation of public policies designed to solve identified local and national health problems and priorities. 3. To assure that all populations have access to appropriate and cost-effective care, including health promotion and disease prevention services, and evaluation of the effectiveness of that care. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 14 Clinical Focus vs. Epidemiologic Focus Clinical Focus Epidemiologic Focus The collective health of people in a population of Health of the individual interest (population at risk) Identify source of exposure that caused the adverse Treating and caring for the health outcome, number of persons exposed, potential patient for further spread and interventions to prevent additional cases Treating the patient based on Uses descriptive and epidemiologic methods to scientific knowledge, provide information that ultimately help determine experience & clinical appropriate PH action to prevent and control health judgment problems EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 15 Epidemiology and the Scientific Method Epidemiology uses the scientific method to describe and analyze health-related states or events. The scientific method involves using appropriate study designs and statistical techniques for investigating an observable occurrence and acquiring new knowledge. The health problem Hypotheses Statistical testing Interpretation Dissemination of result EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 16 Selected Activities Performed in Epidemiology (I) Identifying risk factors (or protective 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 health-related events over time. Providing information useful in health planning and decision-making for establishing health programs with appropriate priorities. Assisting in carrying out public health programs. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 17 Selected Activities Performed in Epidemiology (II) Evaluating the efficacy and effectiveness of prevention and treatment programs Effectiveness refers to the ability of a program to produce the intended or expected results in the field; effectiveness differs from Efficacy, which is the ability to produce results under ideal conditions. (e.g., skiing) Efficiency refers to the ability of the program to produce the intended results with a minimum expenditure of time and resources. Being a resource person Communicating public health information EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 18  Epidemiology in PH Practice and Individual Decision Making Epidemiological findings contribute to: Preventing and controlling disease, injury, disability, and death How? By providing information that leads to informed public health policy and planning, as well as individual health decision making EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 19 Key Terms Used to Describe Basic Patterns of Infectious Disease Occurrence 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 – A worldwide epidemic Outbreak carries same meaning as epidemic but is used when cases are confined to a more limited geographic area and smaller number of people. Endemic disease levels are measured by ongoing surveillance systems. Some fluctuations usually occur. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 20 Terms Referring to Disease Patterns in Animals that May Affect Humans (I) Zoonosis is an infectious organism in vertebrate animals that can be transmitted to humans through direct contact, a fomite, or a vector. Examples: Rabies, Ebola virus, anthrax, plague & toxoplasmosis Enzootic: “Endemic” among animal populations Epizootic: “Epidemic” among animal populations These terms are defined by the American Public Health Association (APHA). Zoonotic pathogens comprise 75% of emerging infectious diseases. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 21 EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 22 Terms Referring to Disease Patterns in Animals that May Affect Humans (II) "Zoonotic transmission" or "zoonotic spillover" generally happens through: 1. Excretion (feces that comes into contact with humans) 2. Slaughter (meat consumed by humans) and 3. Vector-borne (an animal biting a human) But even under these circumstances, the infectious agent must then overcome barriers within the human body, defeating the immune system, to successfully replicate and transmit between humans themselves. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 23 Case Concepts in Epidemiology (I) A case is a person in a population who has been identified as having a particular disease, disorder, injury, or condition. 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. Standard case definition is an essential tool in a surveillance system. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 24 Case Concepts in Epidemiology (II) 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. Secondary cases: Those persons who become infected & ill after a disease has been introduced into a population & who become infected from contact with the primary case. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 25 Case Concepts in Epidemiology (III) Levels of diagnosis by physician (medical doctor): Suspect case is an individual (or a group of individuals) who has all of the signs and symptoms of a disease or condition, yet not diagnosed. Probable case: Suspect case plus supportive lab Suspect case plus epi link (e.g., contact with confirmed case) Confirmed case is when all criteria are met for the case definition. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 26 Figure 1: Diphtheria cases by year/epi-week in Nigeria, 1 May 2022 – 27 August 2023 EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 27 Case Concepts in Epidemiology (IV) Case Severity can be assessed by: § The average length of stay in a hospital § How disabling the condition is § Chances of recovery § How long the person is ill § How much care the person needs EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 28 Epidemiology Triangle for Infectious Diseases EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 29 Host Factors that Influence Susceptibility to Disease are: Sex Age Occupation Nutrition Heredity Marital status Socioeconomic status (SES) Religious and social customs Immunization history Previous history of disease EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 30 Environmental Factors that Influence Susceptibility to Disease are : Many aspects of the environment influence both an agent’s survival and growth, and a host’s contact with a disease agent. Some of the environmental factors are: Temperature Humidity Altitude Water (Example: standing water) Pollution Housing conditions (Example: crowding) Food/milk (Example: unpasteurized milk) Each disease is influenced by a particular set of factors. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 31 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. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 32 Chain of Infection For infection to occur in an individual, a process involving six related components must occur. ‫ﻣﻌرض ﻟﻠﻣرض‬ ‫ﺧزان‬ MOUTH OR NOSE DIRECT CONTACT EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 33 Etiologic Agents in the Chain of Infection There are seven categories of biological agents that can cause infectious diseases. Each has its own particular characteristics. The types of agents are: Metazoa Protozoa Fungi Bacteria Rickettsia Viruses Prions EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 34 Reservoirs in the Chain of Infection The next essential link in the chain of infection is the reservoir, the usual habitat in which the agent lives and multiplies. Depending upon the agent, the reservoir may be: Humans Animals and/or Environment When working with any disease agent, it is important to learn about its usual reservoir(s). EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 35  Human Reservoirs (I) There are two types of human reservoirs, acute clinical cases and carriers. sudden illness a. Acute clinical cases are infected with the disease agent and become ill. Because they are ill, their contacts and activities may be limited They are also more likely to be diagnosed and treated than carriers are. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 36 Human Reservoirs (II) they have the disease but do not have any sympotoms b. Carriers are people who harbor infectious agents but are not ill. Carriers may present more risk for disease transmission than acute clinical cases, because their contacts are unaware of their infection, and their activities are not restricted by illness. Depending on the disease, any of the following types of carriers may be important: Incubatory carriers Inapparent infections (also called subclinical cases) Convalescent carriers Chronic carriers EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 37 Types of Human Carriers time period onset illness between exposure a. Incubatory carriers: People who are going to become ill but begin transmitting their infection before their symptoms start. Example: HIV Infection b. Inapparent infections: People with inapparent infections never develop an illness but are able to transmit their infection to others. Example: COVID-19 among children? c. Convalescent carriers are people who continue to be infectious during and even after their recovery from illness. Example: Salmonella patients d. Chronic carriers are people who continue to harbor infections for a year or longer after their recovery Example: Typhoid Mary EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 38  Types of Animal Reservoirs Animal reservoirs of infectious agents can be described in the same way as human reservoirs. They may be: Acute clinical cases, or Carriers Depending upon the disease, different carrier phases may be important in transmission. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 39 Environmental Reservoirs Plants, soil and water may serve as the reservoir of infection for a variety of diseases. Most fungal agents (mycoses) live and multiply in the soil. Examples: The agents that cause tetanus, anthrax and botulism are widely distributed in soil. The agent of Legionnaire’s Disease lives in water, including hot water heaters. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 40 Portal of Exit in the Chain of Infection Portal of exit is the route by which the disease agent may escape from the human or animal reservoir. While many disease agents have only one portal of exit, others may leave by various portals. The portals most commonly associated with human and animal diseases are: Respiratory (most important portal and the most difficult to control). Genitourinary Alimentary Skin (Superficial lesions, Percutaneous) from needles or scar Transplacental EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 41 Mode of Transmission in the Chain of Infection (I) A mode of transmission is necessary to bridge the gap between the portal of exit from the reservoir and the portal of entry into the host. The two basic modes are direct and indirect. a. Direct transmission occurs immediately. Many diseases are transmitted by direct physical contact with humans, animals, or environmental reservoirs. Example: Sexually transmitted diseases EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 42 Mode of Transmission in the Chain of Infection (II) b. Indirect transmission may occur through animate (alive) or inanimate mechanisms. Animate mechanisms involve vectors. living thing that does not have a spinal cord Vector: An invertebrate animal (e.g., tick, mite, mosquito, bloodsucking fly) capable of transmitting an infectious agent among vertebrates Example: Flies may transmit infectious agents such as shigella in a purely mechanical way, by walking on feces and then on food. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 43  Mode of Transmission in the Chain of Infection (III) non living - like water,air,food, or other objects Inanimate mechanisms: when disease agents are spread by environmental vehicles (non-living intermediary) or by air, this is referred to as indirect transmission by inanimate mechanisms. Anything may be a vehicle, including objects (e.g., elevator buttons, stethoscope), food, water, milk, or biological products. Food is a common vehicle for salmonella infections Water is the usual vehicle in cholera outbreaks Surgical instruments and implanted medical devices may be the vehicles of staphylococcal infections EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 44 Transmission of Pathogens Through the Air (I) Individuals infected with a pathogen during the infectious stage of the disease (the source) can generate particles containing the pathogen, along with water and respiratory secretions. Such particles are described as potentially infectious particles. These potentially infectious particles are carried by expired airflow, exit the infectious person’s mouth/nose through breathing, talking, singing, spitting, coughing, or sneezing, and enter the surrounding air. From this point, these particles are known as infectious respiratory particles or IRPs. IRPs exist in various sizes (from sub-microns to millimeters in diameter). Many environmental factors influence how IRPs travel through the air (temperature, humidity, sunlight, airflow, …) and whether they remain infective upon reaching other individuals. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 45 Transmission of Pathogens Through the Air (II) ‘Through the air’ can be used in a general way to characterize an infectious disease where the main mode of transmission involves the pathogen traveling through or being suspended in the air. ‘Transmission through the air’ can describe the mode of transmission of IRPs through the air. Under the umbrella of ‘through the air,’ two descriptors can be used: ‘Airborne transmission/inhalation’ can occur when IRP have traveled either short or long distances. Direct deposition occurs when the IRP is expelled through the air following a short- range semi-ballistic trajectory and then directly deposited on another person's exposed facial mucosal surfaces. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 46 47  Portal of Entry in the Chain of Infection The portal of entry into the host is usually the same as the portal of exit from the reservoir. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 48 Susceptible Host in the Chain of Infection (I) The last essential component in the chain of infection is the susceptible host. Susceptibility is affected by: Genetic factors Genes do seem to play a role in the progression of HIV disease General resistance factors Example: intact skin, cilia in our respiratory tracts Specific acquired immunity Has the greatest influence on host susceptibility. This immunity is specific to a particular disease agent, and it may be acquired naturally or artificially. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 49 Specific Acquired Immunity (I) a. Natural immunity may be acquired by experiencing an infection, which is called “active natural immunity.” Many diseases confer immunity after a single infection, but many others do not. Example: Infection with measles or chickenpox confer lifelong immunity. Example: Infection with Influenza and salmonella do not confer immunity and therefore may recur. Another mechanism of natural immunity is the transfer of antibodies from the mother to the newborn child, via the placenta and/or breast milk. This is called “passive natural immunity,” and it diminishes after varying lengths of time. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 50  Specific Acquired Immunity (II) b. Artificial immunity may be acquired through the use of vaccines, toxoids and immune globulins. – Active immunity: Receiving a vaccine or toxoid stimulates “active” immunity, since the recipient responds by producing his/her own antibodies. – Passive immunity: Receiving an antitoxin or immune globulin confers “passive” immunity, essentially by borrowing the antibodies of other people. Passive immunity lasts for only a short time, while active immunity usually lasts much longer, even for a lifetime. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 51 Specific Acquired Immunity: Conclusion Acquired Immunity Natural Artificial Active Passive Active Passive Antibodies developed in Antibodies received Antibodies developed in Antibodies received response to an infection from mother response to a vaccination from a medicine EPIDEMIOLOGY PBH 366 Z.U DUBAI CAMPUS 52  Stopping Transmission of Infectious Diseases To stop the spread of infectious diseases in a community, one or more of the links in the ‘Chain of Infection’ must be broken. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 53 The Infectious Disease Spectrum (I) So, the factors that work together to cause the transmission of infectious agents is very complex. The impact of disease agents on human host populations is also a bit complex. If a large number of individuals are equally exposed to an infectious agent, they do not all respond in the same manner. In fact, there may be a broad range of responses: Some do not become infected at all Some become infected but develop no symptoms Some become infected and develop mild or moderate symptoms Some become infected and develop severe symptoms Some die as a result of their infection EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 54 The Infectious Disease Spectrum (II) Part of this variation is due to the capacity of the agent to produce disease. Infection of a healthy adult population with salmonella is likely to result in mostly inapparent or mild cases, with only a few people with more severe symptoms and very few deaths. On the other end of the spectrum, infections with rabies almost always result in severe illness and death. Part of the variation is due to differing levels of resistance of the hosts Example: Herd Immunity EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 55 The Infectious Disease Spectrum (III) The existence of the infectious disease spectrum can make it challenging to find out the extent of transmission in a particular population. Most cases with inapparent or mild symptoms will never be discovered or reported, since these people will not seek health care. So, when moderate or severe cases are reported, they may represent the “tip of the iceberg.” Another challenge is that many diseases share the same signs and symptoms, so laboratory studies are important to identify the specific disease agent in any epidemiological investigation. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 56 Test Your Knowledge (VI) 10. Which of the following is an example of a vehicle of transmission? a. Towels of infected person b. Door handles touched by an infected person c. Utensils ( cup, plate… etc.) used by an infected person d. All the above EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 57 Levels of Prevention (I) 1. Primary prevention: is preventing the disease or disorder before it happens. Example: Immunization, hand washing active prevention ex.wearing a. Active primary prevention a face mask by myself "behavioral chnage" Requires behavior change on part of subject: o Wearing protective devices o Health promotion o Lifestyle changes o Ensuring healthy conditions at home, school, and workplace EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 58 Levels of Prevention (II) b. Passive primary prevention passive the goverment provides it Does not require behavior change Examples: o Vitamin-fortified foods o Fluoridation of public water supplies EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 59  Levels of Prevention (III) 2. Secondary prevention: Aims to reduce the progress of disease by early disease detection The disease already exists in the person. Example: o 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. EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 60 Levels of Prevention (IV) 3. Tertiary prevention To reduce the limitation of disability from disease by providing rehabilitation services. The goal is to help the disabled or diseased or injured individuals avoid wasteful use of health care services and not become dependent on healthcare practitioners. Example: Physical therapy for stroke victims EPIDEMIOLOGY PBH 366 ZU DUBAI CAMPUS 61