Module 1_ Epidemiology Introduction and Review PDF

1 The lesson presented here has been condensed from the self-study course “Principles of Epidemiology in Public Health Practice” (Course SS1978) to serve as a review of key epidemiological terms and concepts appropriate for this course. 1. Introduction Epidemiology is considered the basic science...

1 The lesson presented here has been condensed from the self-study course “Principles of Epidemiology in Public Health Practice” (Course SS1978) to serve as a review of key epidemiological terms and concepts appropriate for this course. 1. Introduction Epidemiology is considered the basic science of public health. Epidemiology is: a quantitative basic science built on a working knowledge of probability and statistics; a method of causal reasoning based on sound research methods; a tool for public health action to promote and protect the public’s health. As a public health discipline, epidemiology is instilled with the spirit that epidemiologic information should be used to promote and protect the public’s health. The word epidemiology comes from the Greek words epi, meaning “on or upon,” demos, meaning “people,” and logos, meaning “the study of.” Many definitions have been proposed, but the following definition captures the underlying principles and the public health spirit of epidemiology: “Epidemiology is 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.” 2  Figure 1. Study of Epidemiology Epidemiology is concerned with the frequency and pattern of health events in a population. Frequency includes not only the number of such events in a population, but also the rate or risk of disease in the population. The rate (number of events divided by size of the population) is critical to epidemiologists because it allows valid comparisons across different populations. 1.2 Uses of Epidemiology Identify the cause of the disease Identify the risk factors Identify control and prevention measures Assesses community health Collects a complete clinical picture Guides an individual’s health decisions 3 Pattern refers to the occurrence of health-related events by time, place, and personal characteristics. Time characteristics include annual occurrence, seasonal occurrence, and daily or even hourly occurrence during an epidemic. Place characteristics include geographic variation, urban-rural differences, and location of worksites or schools. Personal characteristics include demographic factors such as age, race, sex, marital status, and socioeconomic status, as well as behaviors and environmental exposures. 4 2.1 The Epidemiologic Approach A systematic approach for determining: Who? (person) What? (clinical information) When? (time) Where? (place) Why? (cause/risk factor) How? (Mechanism of disease/pathogenesis) A case definition is a set of standard criteria for deciding whether a person has a particular disease or other health-related condition. By using a standard case definition, we ensure that every case is diagnosed in the same way, regardless of when or where it occurred, or who identified it. We can then compare the number of cases of the disease that occurred in one time or place with the number that occurred at another time or another place. With a standard case definition, when we find a difference in disease occurrence, we know it is likely to be a real difference rather than the result of differences in how cases were diagnosed. A case definition consists of clinical criteria and, sometimes, limitations on time, place, and person. The clinical criteria usually include confirmatory laboratory tests, if available, or combinations of symptoms (subjective complaints), signs (objective physical findings), and other findings. 5 A case definition may have several sets of criteria, depending on how certain the diagnosis is. For example, during an outbreak of measles, we might classify a person with a fever and rash as having a suspect, probable, or confirmed case of measles, depending on what additional evidence of measles was present. In other situations, we temporarily classify a case as suspect or probable until laboratory results are available. When we receive the laboratory report, we then reclassify the case as either confirmed or “not a case,” depending on the lab results. In the midst of a large outbreak of a disease caused by a known agent, we may permanently classify some cases as suspect or probable, because it is unnecessary and wasteful to run laboratory tests on every patient with a consistent clinical picture and a history of exposure. Case definitions should not rely on laboratory culture results alone, since organisms are sometimes present causing disease. Case definitions may also vary according to the purpose for classifying the occurrences of a disease. For example, health officials need to know as soon as possible if anyone has symptoms of plague or foodborne botulism so that they 6 can begin planning what actions to take. For such rare but potentially fatal communicable diseases, where it is important to identify every possible case, health officials use a sensitive, or “loose” case definition. On the other hand, if investigators want to be certain that any person included in the investigation really had the disease, the investigator will prefer a specific or “strict” case definition. A disadvantage of a strict case definition is an underestimate of the total number of cases. 7 3.1 Numbers and Rates A basic task of a health department is counting cases in order to measure and describe morbidity. When physicians diagnose a case of a reportable disease they send a report of the case to their local health department. These reports are legally required to contain information on time (when the case occurred), place (where the patient lived), and person (the age, race, and sex of the patient). The health department combines the reports and summarizes the information by time, place, and person. From these summaries, the health department determines the extent and patterns of disease occurrence in the area, and identifies clusters or outbreaks of disease. A simple count of cases, however, does not provide all the information a health department needs. To compare the occurrence of a disease at different locations or during different times, a health department converts the case counts into rates, which relate the number of cases to the size of the population where they occurred. 8 4.1 Descriptive Epidemiology In descriptive epidemiology, we organize and summarize data according to time, place, and person. Compiling and analyzing data by time, place, and person is desirable for several reasons. First, the investigator becomes intimately familiar with the data and with the extent of the public health problem being investigated. Second, this provides a detailed description of the health of a population that is easily communicated. Third, such analysis identifies the populations that are at the greatest risk of acquiring a particular disease. This information provides important clues to the causes of the disease, and these clues can be turned into testable hypotheses. Figure 2. Data is organized and summarized according to time, place, and person. 9 4.1.1 Time Disease rates change over time. Some of these changes occur regularly and can be predicted. For example, the seasonal increase of influenza cases with the onset of cold weather is a pattern that is familiar to everyone. By knowing when flu outbreaks will occur, health departments can time their flu shot campaigns effectively. Other disease rates make unpredictable changes. By examining events that precede a disease rate increase or decrease, we may identify causes and appropriate actions to control or prevent further occurrence of the disease. Depending on what event we are describing, we may be interested in a period of years or decades, or we may limit the period to days, weeks, or months when the number of cases reported is greater than normal. Figure 3. An example of an epidemic curve. 10 Secular (long-term) Trends Graphing the annual cases or rate of a disease over a period of years shows long-term or secular trends in the occurrence of the disease. We commonly use these trends to suggest or predict the future incidence of a disease. We also use them in some instances to evaluate programs or policy decisions or to suggest what caused an increase or decrease in the occurrence of a disease, particularly if the graph indicates when related events took place. Seasonality By graphing the occurrence of a disease by week or month over the course of a year or more we can show its seasonal pattern, if any. Seasonal patterns may suggest hypotheses about how the infection is transmitted, what behavioral factors increase risk, and other possible contributors to the disease or condition. Epidemic Period To show the time course of a disease outbreak or epidemic, we use a specialized graph called an epidemic curve. By convention, we use a histogram for epidemic curves. The shape and other features of an epidemic curve can suggest hypotheses about the time and source of exposure, the mode of transmission, and the causative agent. 11 4.1.2 Place We describe a health event by place to gain insight into the geographical extent of the problem. For place, we may use place of residence, birthplace, place of employment, school district, hospital unit, etc., depending on which may be related to the occurrence of the health event. Similarly, we may use large or small geographic units: country, state, county, census tract, street address, map coordinates, or some other standard geographical designation. Sometimes, we may find it useful to analyze data according to place categories such as urban or rural, domestic or foreign, and institutional or noninstitutional. Not all analyses by place will be equally informative. Figure 4. Place was very important to John Snow in determining the source of an 1854 cholera outbreak in London. 12 By analyzing data by place, we can also get an idea of where the agent that causes a disease normally lives and multiplies, what may carry or transmit it, and how it spreads. When we find that the occurrence of a disease is associated with a place, we can infer that factors that increase the risk of the disease are present either in the persons living there or in the environment, or both. For example, diseases that are passed from one person to another spread more rapidly in urban areas than in rural ones, mainly because the greater crowding in urban areas provides more opportunities for susceptible people to come into contact with someone who is infected. On the other hand, diseases that are passed from animals to humans often occur in greater numbers in rural and suburban areas because people in those areas are more likely to come into contact with disease-carrying animals, ticks, and the like. Although we can show data by place in a table, it is often better to show it pictorially on a map. On a map, we can use different shadings, colors, or line patterns to indicate how a disease or health event has different numbers or rates of occurrence in different areas. For a rare disease or outbreak, we often find it useful to prepare a spot map, in which we mark with a dot or an X the relation of each case to a place that is potentially relevant to the health event being investigated—such as where each case lived or worked. We may also label other sites on a spot map, such as where we believe cases may have been exposed, to show the orientation of cases within the area mapped. 4.1.3 Person In descriptive epidemiology, when we organize or analyze data by “person” there are several person categories available to us. We may use inherent characteristics of people (for example, age, race, sex), their acquired characteristics (immune or marital status), their activities (occupation, leisure activities, use of medications/tobacco/drugs), or the conditions under which they live (socioeconomic status, access to medical care). These categories 13 determine to a large degree who is at the greatest risk of experiencing some undesirable health condition, such as becoming infected with a particular disease organism. In analyzing data by person, we often must try a number of different person categories before we find which are the most useful and enlightening. Age and sex are most critical; we almost always analyze data according to these. Often, we analyze data into more than one category simultaneously; for example, we may look at age and sex simultaneously to see if the sexes differ in how they develop a condition that increases with age. Age Age is probably the single most important “person” attribute because almost every health-related event or state varies with age. A number of factors that also vary with age are behind this association: susceptibility, opportunity for exposure, latency or incubation period of the disease, and physiologic response (which affects, among other things, disease development). When we analyze data by age, we try to use age groups that are narrow enough to detect any age-related patterns that may be present in the data. In an initial breakdown by age, we commonly use five-year age intervals: 0 to 4 years, 5 to 9, 10 to 14, and so on. Larger intervals, such as 0 to 19 years, 20 to 39, etc., can conceal variations related to age, which we need to know to identify the true population at risk. Sometimes, even the commonly used five-year age groups can hide important differences. Sex In general, males have higher rates of illness and death than females do for a wide range of diseases. For some diseases, this sex-related difference is because of genetic, hormonal, anatomic, or other inherent differences between the sexes. These inherent differences affect their susceptibility or physiologic responses. 14 Ethnic and Racial Groups In examining epidemiologic data, we are interested in any group of people who have lived together long enough to acquire common characteristics, either biologically or socially. Several terms are commonly used to identify such groups: race, nationality, religion, or local reproductive or social groups, such as tribes and other geographically or socially isolated groups. Differences that we observe in racial, ethnic, or other groups may reflect differences in their susceptibility or in their exposure, or they may reflect differences in other factors that bear more directly on the risk of diseases, such as socioeconomic status and access to health care. Socioeconomic Status Socioeconomic status is difficult to quantify. It is made up of many variables such as occupation, family income, educational achievement, living conditions, and social standing. The variables that are easiest to measure may not reflect the overall concept. Nevertheless, we commonly use occupation, family income, and educational achievement, while recognizing that these do not measure socioeconomic status precisely. The frequency of many adverse health conditions increases with decreasing socioeconomic status. 15 5.1 Analytical Epidemiology Comparison groups, which provide baseline data, are a key feature of analytic epidemiology. When we find that persons with a particular characteristic are more likely than those without the characteristic to develop a certain disease, then the characteristic is said to be associated with the disease. Identifying factors that are associated with disease helps us identify populations at increased risk of disease; we can then target public health prevention and control activities. We use analytic epidemiology to quantify the association between exposures and outcomes and to test hypotheses about causal relationships. It is sometimes said that epidemiology can never prove that a particular exposure caused a particular outcome. Epidemiology may, however, provide sufficient evidence for us to take appropriate control and prevention measures. Epidemiologic studies fall into two categories: experimental and observational. In an experimental study, we determine the exposure status for each individual (clinical trial) or community (community trial); we then follow the individuals or communities to detect the effects of the exposure. In an observational study, which is more common, we simply observe the exposure and outcome status of each study participant. Three types of observational studies are the cohort study, Cross-sectional, and the case-control study. A cohort study is similar in concept to the experimental study. We categorize subjects on the basis of their exposure and then observe them to see 16 if they develop the health conditions we are studying. This differs from an experimental study in that, in a cohort study, we observe the exposure status rather than determine it. After a period of time, we compare the disease rate in the exposed group with the disease rate in the unexposed group. The length of follow-up varies, ranging from a few days for acute diseases to several decades for cancer, cardiovascular disease, and other chronic diseases. The Framingham study is a well-known cohort study that has followed over 5,000 residents of Framingham, Massachusetts since the early 1950s to establish the rates and risk factors for heart disease. In a cross sectional study, exposure and outcome are assessed simultaneously from each study subject. Examples include surveys asking individuals questions about their exposure and disease status on the questionnaire. This is also used in outbreak investigations where party attendees may be asked what food they consumed and if they fell ill afterwards. In a case-control study, we enroll a group of people with disease (“cases”) and a group without disease (“controls”) and compare their patterns of previous exposures. The key to a case-control study is to identify an appropriate control, or comparison, group because it provides our measure of the expected amount of exposure. In summary, the purpose of an epidemiologic study is to quantify the relationship between exposure and a health outcome. The hallmark of an epidemiologic study is the presence of at least two groups, one of which serves as a comparison group. In an experimental study, the investigator determines the exposure of the study subjects; in an observational study, the subjects determine their own exposure. In an observational cohort study, subjects first are enrolled on the basis of their exposure, then are followed to document the occurrence of disease. In cross-sectional study, exposure and outcome are assessed simultaenously. In an observational case-control study, subjects first are enrolled according to whether they have the disease or not, then are questioned or tested to determine their prior exposure. 17 5.2 The Epidemiologic Triad: Agent, Host, and Environment The epidemiologic triad is the traditional model of infectious disease causation. It has three components: an external agent, a susceptible host, and an environment that brings the host and agent together. In this model, the environment influences the agent, the host, and the route of transmission of the agent from a source to the host. Figure 5. The epidemiologic triad 18 5.2.1 Agent Agent originally referred to an infectious microorganism—virus, bacterium, parasite, or other microbes. Generally, these agents must be present for disease to occur. That is, they are necessary but not always sufficient to cause disease. As epidemiology has been applied to noninfectious conditions, the concept of agent in this model has been broadened to include chemical and physical causes of disease. This model does not work well for some noninfectious diseases, because it is not always clear whether a particular factor should be classified as an agent or as an environmental factor. 5.2.2 Host Host factors are intrinsic factors that influence an individual’s exposure, susceptibility, or response to a causative agent. Age, race, sex, socioeconomic status, behaviors (smoking, drug abuse, lifestyle, sexual practices, and contraception, eating habits) are just some of the many host factors which affect a person’s likelihood of exposure. Age, genetic composition, nutritional and immunologic status, anatomic structure, presence of disease or medications, and psychological makeup are some of the host factors which affect a person’s susceptibility and response to an agent. 5.2.3 Environmental Factors Environmental factors are extrinsic factors that affect the agent and the opportunity for exposure. Generally, environmental factors include physical factors such as geology, climate, and physical surroundings (e.g., a nursing home, hospital); biologic factors such as insects that transmit the agent; and socioeconomic factors such as crowding, sanitation, and the availability of 19 health services. Agent, host, and environmental factors interrelate in a variety of complex ways to produce disease in humans. Their balance and interactions are different for different diseases. When we search for causal relationships, we must look at all three components and analyze their interactions to find practical and effective prevention and control measures. 20 6.1 Natural History and Spectrum of Disease Natural history of disease refers to the progress of a disease process in an individual over time, in the absence of intervention. The process begins with exposure to or accumulation of factors capable of causing disease. Without medical intervention, the process ends with recovery, disability, or death. Most diseases have a characteristic natural history (which is poorly understood for many diseases), although the time frame and specific manifestations of disease may vary from individual to individual. With a particular individual, the usual course of a disease may be halted at any point in the progression by preventive and therapeutic measures, host factors, and other influences. The natural history begins with the appropriate exposure to or accumulation of factors sufficient to begin the disease process in a susceptible host. Usually, a period of subclinical or inapparent pathologic changes follows exposure, ending with the onset of symptoms. For infectious diseases, this period is usually called the incubation period; for chronic diseases, this period is usually called the latency period. This period may be as brief as seconds for hypersensitivity and toxic reactions to as long as decades for certain chronic diseases. Even for a single disease, the characteristic incubation period has a range. Although disease is inapparent during the incubation period, some pathologic changes may be detectable with laboratory, radiographic, or other screening methods. Most screening programs attempt to identify the disease process during this phase of its natural history, since early intervention may be more effective than treatment at a later stage of disease progression. The onset of symptoms marks 21 the transition from subclinical to clinical disease. Most diagnoses are made during the stage of clinical disease. In some people, however, the disease process may never progress to clinically apparent illness. In others, the disease process may result in a wide spectrum of clinical illnesses, ranging from mild to severe or fatal. Three terms are used to describe an infectious disease according to the various outcomes that may occur after exposure to its causative agent: Infectivity refers to the proportion of exposed persons who become infected. Pathogenicity refers to the proportion of infected persons who develop clinical disease. Virulence refers to the proportion of persons with clinical disease who become severely ill or die. The natural history and spectrum of disease present challenges to the clinician and to the public health worker. Because of the clinical spectrum, cases of illness diagnosed by clinicians in the community often represent only the “tip of the iceberg.” Many additional cases may be too early to diagnose or may remain asymptomatic. For the public health worker, the challenge is that persons with inapparent or undiagnosed infections may nonetheless be able to transmit them to others. Such persons who are infectious but have subclinical disease are called carriers. Frequently, carriers are persons with incubating disease or inapparent infection. On the other hand, carriers may also be persons who appear to have recovered from their clinical illness. 22 7.1 Chain of Infection The traditional model (epi triad) illustrates that infectious diseases result from the interaction of agent, host, and environment. More specifically, transmission occurs when the agent leaves its reservoir or host through a portal of exit, is conveyed by some mode of transmission, and enters through an appropriate portal of entry to infect a susceptible host. 7.1.1 Reservoir The reservoir of an agent is the habitat in which an infectious agent normally lives, grows, and multiplies. Reservoirs include humans, animals, and the environment. The reservoir may or may not be the source from which an agent is transferred to a host. For example, the reservoir of Clostridium botulinum is soil, but the source of most botulism infections is improperly canned food containing C. botulinum spores. 7.1.2 Carrier A carrier is a person without apparent disease who is nonetheless capable of transmitting the agent to others. Carriers may be asymptomatic carriers, who never show symptoms during the time they are infected, or may be incubatory or convalescent carriers, who are capable of transmission before or after they 23 are clinically ill. A chronic carrier is one who continues to harbor an agent for an extended time (months or years) following the initial infection. Carriers commonly transmit disease because they do not recognize they are infected and consequently take no special precautions to prevent transmission. Symptomatic persons, on the other hand, are usually less likely to transmit infection widely because their symptoms increase their likelihood of being diagnosed and treated, thereby reducing their opportunity for contact with others. 7.1.3 Animal Reservoirs Infectious diseases that are transmissible under normal conditions from animals to humans are called zoonoses. In general, these diseases are transmitted from animal to animal, with humans as incidental hosts. Another group of diseases with animal reservoirs are those caused by viruses transmitted by insects and caused by parasites that have complex life cycles, with different reservoirs at different stages of development. Such diseases include St. Louis encephalitis and malaria (both requiring mosquitos) and schistosomiasis (requiring freshwater snails). Lyme disease is a zoonotic disease of deer incidentally transmitted to humans by the deer tick. Figure 6. Animal reservoirs 24 7.1.4 Environmental Reservoirs Plants, soil, and water in the environment are also reservoirs for some infectious agents. Many fungal agents, such as those causing histoplasmosis, live and multiply in the soil. The primary reservoir of Legionnaires’ bacillus appears to be pools of water, including those produced by cooling towers and evaporative condensers. 7.1.5 Portal of Exit Portal of exit is the path by which an agent leaves the source host. The portal of exit usually corresponds to the site at which the agent is localized. Thus, tubercle bacilli and influenza viruses exit the respiratory tract, schistosomes through urine, cholera, and vibrio in feces, Sarcoptes scabiei in scabies skin lesions, and enterovirus 70, an agent of hemorrhagic conjunctivitis, in conjunctival secretions. Some blood-borne agents can exit by crossing the placenta (rubella, syphilis, toxoplasmosis), while others exit by way of the skin (percutaneously) through cuts or needles (hepatitis B) or blood-sucking arthropods (malaria). 7.1.6 Modes of Transmission After an agent exits its natural reservoir, it may be transmitted to a susceptible host in numerous ways. These modes of transmission are classified as: 25 Direct Indirect Direct contact Airborne Droplet spread Vehicleborne Vectorborne Mechanical Biologic In direct transmission, there is essentially immediate transfer of the agent from a reservoir to a susceptible host by direct contact or droplet spread. Direct contact occurs through kissing, skin-to-skin contact, and sexual intercourse. Direct contact refers also to contact with soil or vegetation harboring infectious organisms. Droplet spread refers to spray with relatively large, short-range aerosols produced by sneezing, coughing, or even talking. Droplet spread is classified as direct because transmission is by direct spray over a few feet before the droplets fall to the ground. 26 In indirect transmission, an agent is carried from a reservoir to a susceptible host by suspended air particles or by animate (vector) or inanimate (vehicle) intermediaries. Most vectors are arthropods such as mosquitoes, fleas, and ticks. These may carry the agent through purely mechanical means. In mechanical transmission, the agent does not multiply or undergo physiologic changes in the vector. This is in contrast to instances in which an agent undergoes part of its life cycle inside a vector before being transmitted to a new host. When the agent undergoes changes within the vector, the vector is serving as both an intermediate host and a mode of transmission. This type of indirect transmission is a biologic transmission. Vehicles that may indirectly transmit an agent include food, water, biologic products (blood), and fomites (inanimate objects such as handkerchiefs, bedding, or surgical scalpels). As with vectors, vehicles may passively carry an agent or may provide an environment in which the agent grows, multiplies, or produces toxin—as improperly canned foods may provide an environment in which C. botulinum produces toxin. Airborne transmission is by particles that are suspended in the air. There are two types of these particles: dust and droplet nuclei. Airborne dust includes infectious particles blown from the soil by the wind as well as material that has settled on surfaces and become resuspended by air currents. Droplet nuclei are the residue of dried droplets. The nuclei are less than 5 μ (microns) in size and may remain suspended in the air for long periods, may be blown over great distances, and are easily inhaled into the lungs and exhaled. This makes them an important means of transmission for some diseases. Tuberculosis, for example, is believed to be transmitted more often indirectly, through droplet nuclei, than directly, through droplet spread. 27 7.1.7 Portal of Entry An agent enters a susceptible host through a portal of entry. The portal of entry must provide access to tissues in which the agent can multiply or a toxin can act. Often, organisms use the same portal to enter a new host that they use to exit the source host. For example, the influenza virus must exit the respiratory tract of the source host and enter the respiratory tract of the new host. The route of transmission of many enteric (intestinal) pathogenic agents is described as “fecal-oral” because the organisms are shed in feces, carried on inadequately washed hands, and then transferred through a vehicle (such as food, water, or cooking utensil) to the mouth of a new host. Other portals of entry include the skin (hookworm), mucous membranes (syphilis, trachoma), and blood (hepatitis B). 28 7.1.8 Host The final link in the chain of infection is a susceptible host. Susceptibility of a host depends on genetic factors, specified acquired immunity, and other general factors which alter an individual’s ability to resist infection or to limit pathogenicity. An individual’s genetic makeup may either increase or decrease susceptibility. General factors which defend against infection include the skin, mucous membranes, gastric acidity, cilia in the respiratory tract, the cough reflex, and nonspecific immune response. General factors that may increase susceptibility are malnutrition, alcoholism, and disease or therapy which impairs the nonspecific immune response. Specific acquired immunity refers to protective antibodies that are directed against a specific agent. Individuals gain protective antibodies in two ways: development of antibodies in response to infection, vaccine, or toxoid called active immunity; acquisition of their mothers’ antibodies before birth through the placenta or receiving injections of antitoxins or immune globulin called passive immunity. Note that the chain of infection may be interrupted when an agent does not find a susceptible host. This may occur if a high proportion of individuals in a population are resistant to an agent. These persons limit spread to the relatively few who are susceptible by reducing the probability of contact between infected and susceptible persons. This concept is called herd immunity. The degree of herd immunity necessary to prevent or abort an outbreak varies by disease. In theory, herd immunity means that not everyone in a community needs to be resistant (immune) to prevent disease spread and occurrence of an outbreak. In practice, herd immunity has not prevented outbreaks of measles and rubella in populations with immunity levels as high as 85 to 90%. One problem is that, in highly immunized populations, the relatively few susceptible persons are often clustered in population subgroups, usually defined by socioeconomic or cultural 29 factors. If the agent is introduced into one of these subgroups, an outbreak may occur. 30 8.1 Level of Disease Occurrence The amount of a particular disease that is usually present in a community is the baseline level of the disease. This level is not necessarily the preferred level, which should in fact be zero; rather it is the observed level. Theoretically, if no intervention occurred and if the level is low enough not to deplete the pool of susceptible persons, the disease occurrence should continue at the baseline level indefinitely. Thus, the baseline level is often considered the expected level of the disease. Different diseases, in different communities, show different patterns of expected occurrence: (a) a persistent level of occurrence with a low to moderate disease level is referred to as an endemic level; (b) a persistently high level of occurrence is called a hyper-endemic level; (c) an irregular pattern of occurrence, with occasional cases occurring at irregular intervals is called sporadic. Occasionally, the level of disease rises above the expected level. When the occurrence of a disease within an area is clearly in excess of the expected level for a given time period, it is called an epidemic. Public health officials often use the term outbreak, which means the same thing, because it is less provocative to the public. When an epidemic spreads over several countries or continents, affecting a large number of people, it is called a pandemic. Epidemics occur when an agent and susceptible hosts are present in adequate numbers, and the agent can effectively be conveyed from a source to the susceptible hosts. More specifically, an epidemic may result from the following: A recent increase in the amount or virulence of the agent 31  The recent introduction of the agent into a setting where it has not been before An enhanced mode of transmission so that more susceptibles are exposed Some change in the susceptibility of the host response to the agent Factors that increase host exposure or involve introduction through new portals of entry 8.1.1 Epidemic Patterns We sometimes classify epidemics by how they spread through a population, as shown below: Common-source ○ Point ○ Intermittent ○ Continuous Propagated Other A common-source outbreak is one in which a group of persons is exposed to a common noxious influence, such as an infectious agent or a toxin. If the group is exposed over a relatively brief period, so that everyone who becomes ill develops disease at the end of one incubation period, then the common-source outbreak is further classified as a point-source outbreak. When the number of cases in a point-source epidemic is plotted over time, the resulting epidemic curve classically has a steep upslope and a more gradual downslope (a so-called “log-normal distribution”). In some common-source outbreaks, cases may be exposed over a period of days, weeks, or longer, with the exposure being either intermittent or continuous. 32 When we plot the cases of a continuous common-source outbreak over time, the range of exposures and range of incubation periods tend to dampen and widen the peaks of the epidemic curve. Similarly, when we plot an intermittent common-source outbreak we often find an irregular pattern that reflects the intermittent nature of the exposure. An outbreak that does not have a common source but instead spreads gradually from person to person is called a propagated outbreak. Usually, transmission is by direct person-to-person contact. Transmission may also be vehicleborne, such as the transmission of hepatitis B or HIV by sharing needles, or vectorborne, such as the transmission of yellow fever by mosquitoes. In a propagated epidemic, cases occur over more than one incubation period. In theory, the epidemic curve of a propagated epidemic would have a successive series of peaks reflecting increasing numbers of cases in each generation. The epidemic usually wanes after a few generations, either because the number of susceptibles falls below some critical level, or because intervention measures become effective. Finally, some epidemics are neither common-source, in its usual sense, nor propagated from person to person. Outbreaks of zoonotic or vectorborne disease may result from sufficient prevalence of infection in host species, sufficient presence of vectors, and sufficient human-vector interaction. 33 9.1 Epidemiology in Public Health Practice Epidemiology is a tool that is essential for carrying out four fundamental functions: public health surveillance, disease investigation, analytic studies, and program evaluation. 9.1.1 Public Health Through public health surveillance, a health department systematically collects, analyzes, interprets, and disseminates health data on an ongoing basis. By knowing the ongoing pattern of disease occurrence and disease potential, a health department can effectively and efficiently investigate, prevent, and control disease in the community. The most common source of surveillance data is reports of disease cases received from health care providers, who are required to report patients with certain diseases. Each state mandates a list of reportable diseases. In addition, surveillance data may come from laboratory reports, surveys, disease registries, death certificates, and public health program data such as immunization coverage. It may also come from investigations by the health department of cases or clusters of cases reported to it. Most health departments use simple surveillance systems. They monitor individual morbidity and mortality case reports, record a limited amount of 34 information on each case, and look for patterns by time, place, and person. Unfortunately, with some reportable diseases, a health department may receive reports of only 10% to 25% of the cases that actually occur. Nevertheless, health departments have found that even a simple surveillance system can be invaluable in detecting problems and guiding public health action. Figure 7. Public health surveillance data flow for state reportable and national notifiable conditions. 35 9.1.2 Sources of Data Mortality reports ○ Vital records Morbidity reports ○ Laboratory reports ○ Case reports/questionnaire Outbreak investigations Animal health data Environmental data Demographic data Figure 8. Tennessee’s department of health regions. 36 9.1.3 Regulations for Diseases Reportable by Law Specifies diseases or conditions Specifies who is responsible for reporting Specifies what information is required Specifies to whom and how quickly to report Specifies control measures Resource This webpage provides a list of reportable diseases effective 01/01/20 TN Department of Health. (2020, January 1). Reportable Diseases. Department of Health. https://www.tn.gov/health/cedep/reportable-diseases.html 9.1.4 Types of Surveillance Passive ○ Provider-initiated, routine reporting by health care providers based on a known set of rules and regulations Active ○ Health department-initiated, periodic visits to health care providers to obtain required information Limitations of Notifiable Disease Surveillance Underreporting Lack of representativeness Lack of timeliness Inconsistency of case definitions 37 Reasons for Incomplete Reporting Lack of knowledge of the reporting requirement Negative attitude towards reporting Time-consuming Compromises patient-provider relationship Concern for confidentiality Availability and utilization of appropriate diagnostic laboratories Availability of effective disease control measures 9.1.5 Disease Investigation Surveillance and case investigation sometimes are sufficient to identify causes, modes of transmission, and appropriate control and prevention measures. Investigators initially use descriptive epidemiology to examine clusters of cases or outbreaks of disease. They examine incidence of the disease and its distribution by time, place, and person. They calculate rates and identify parts of the population that are at higher risk than others. When they find a strong association between exposure and disease, the investigators may implement control measures immediately. More often, investigators find that descriptive studies, like case investigations, generate hypotheses that they can then test with analytic studies. Epidemiologists must be familiar with all aspects of the analytic study, including its design, conduct, analysis, and interpretation. In addition, the epidemiologist must be able to communicate the findings as well. Study design includes determining the appropriate study design, writing justifications and protocols, calculating sample sizes, deciding on criteria for subject selection (e.g., choosing controls), designing questionnaires, and 38 numerous other tasks that are part of the study plan. To conduct a study requires securing appropriate clearances and approvals, abstracting records, tracking down and interviewing subjects, collecting and handling specimens, and managing the data. Analysis begins with describing the characteristics of the subjects and progresses to calculating rates, creating comparative tables (e.g., two-by-two tables), computing measures of association (e.g., risk ratios and odds ratios), tests of statistical significance (e.g., chi-square), confidence intervals, and the like. Many epidemiologic studies require more advanced analytic techniques such as stratified analysis, regression, and modeling. Finally, interpretation involves putting the findings of the study into perspective and making appropriate recommendations. Steps in an Epi Investigation Health Department notified of illness complaint Initial Assessment (Fact-Finding) – 1. Prepare for fieldwork – 2. Establish the existence of an outbreak – 3. Verify diagnosis Prepare for Investigation (Evidence Gathering) – 4a. Develop case definition – 4b. Identify cases – Clinical symptoms – Lab results Collect food/environmental samples Obtain health status of workers Facility survey/inspection Obtain clinical samples Investigation – 5. Descriptive epidemiology Data management 39  Capture data Interview cases and controls Characterize data (person, place, time) – 6. Formulate hypotheses (study design) – 7. Analytical calculations (test hypotheses) – 8. Refine hypotheses and execute additional studies – 9. Implement prevention and control measures – 10. Communicate the findings Figure 9. An example of how to stop a foodborne outbreak. Source: CDC. 40 9.1.6 Establish the Existence of an Outbreak To determine if an outbreak exists (i.e., whether the observed number of cases exceeds the expected number), the expected number of cases for the area in the given time frame must be determined. If the current number of reported cases exceeds the expected number, further investigation is needed. 9.1.7 Verify the Diagnosis Epidemiologists identify as accurately as possible the nature of the disease. First, to ensure the problem has been properly diagnosed and that it really is what it is reported to be. ○ Review clinical findings and laboratory results for people affected ○ Verify laboratory findings ○ Visit and interview several people who became ill Second, for outbreaks involving infectious or toxic-chemical agents, to be certain that the increase in diagnosed cases is not the result of a mistake in the laboratory. 9.1.8 Develop Case Definition Epidemiologists establish a case definition: a standard set of criteria for deciding whether a person should be classified as having the disease or condition under study. Usually includes the following components: 1. Clinical information about the disease 41  2. Characteristics about the people who are affected 3. Information about the location or place 4. A specification of the time during which the outbreak occurred 9.1.9 Identify Cases The following information is collected on a case report form: 1. Identifying information 2. Demographic information Details to characterize population at risk 3. Clinical information Allows the creation of an epidemic curve and a description of the spectrum of illness 4. Risk factor information Helps to tailor the investigation to the specific disease in question 9.1.10 Case Definition Standard criteria for case classification Composed of clinical criteria (signs and symptoms) and may include lab results Can have limitations by person, place, and time ○ Ensures every case diagnosed the same ○ Allows for comparison among cases 9.1.11 Define Cases Investigators often classify cases as one of the following: 42  Confirmed: usually has laboratory verification Probable: usually has clinical features without lab verification Suspect: usually has had some clinical features 9.1.12 Descriptive Epidemiology Characterize the outbreak by time, place, and person Benefits ○ Allows you to become familiar with the data, especially what is and is not reliable ○ Provides a comprehensive description of the outbreak ○ Allows you to develop a causal hypothesis based on what is known about the disease 9.1.13 Characterizing by Person Determine the populations at risk by characterizing the outbreak by person. Define populations by: ○ Personal characteristics (Examples: age, race, sex, or medical status) ○ Exposures (Examples: occupation, leisure activities, use of medications, tobacco, drugs) Age and sex are usually assessed first, because they are often the characteristics most strongly related to exposure and to the risk of disease. 9.1.14 Line Listing Next, selected critical items are abstracted into a table called a “line listing”. 43  Each column represents an important variable, such as age and sex Each row represents a different case, by number This simple format allows the investigator to scan key information on every case and update it easily. Figure 10. An example of a line listing. 9.1.15 Characterizing by Time: Epidemic Curve Epidemic curve or “epi curve” = a graph of the number of cases by their date of onset. Simple visual display of outbreak’s magnitude and time trend Shows course of epidemic 44  May enable estimation of probable time period of exposure May enable inferences to be drawn about the epidemic pattern How to Draw an Epidemic Curve Know the time of onset for each person Number of cases is plotted on the y-axis Time is plotted on the x-axis The unit of time is based on incubation period and length of time over which cases are distributed. Select a unit that is one-fourth to one-third as long as the incubation period. Figure 11. Epidemic curve 45 9.1.16 Epidemic Curve: Interpreting the Shape The first step in interpreting an epidemic curve is to consider its overall shape, which will be determined by the pattern of the epidemic (e.g., whether it has a common-source or person-to-person transmission), the period of time over which susceptible people are exposed, and the minimum, average, and maximum incubation periods for the disease. Point-source epidemic ○ Shape: a steep upslope, a peak, and a gradual downslope ○ Interpretation: people are exposed to the same source over a relatively brief period Continuous common-source epidemic ○ Shape: curve will have a plateau instead of a peak ○ Interpretation: people are exposed to the same source over an extended period Propagated epidemic ○ Shape: a series of progressively taller peaks ○ Interpretation: person-to-person spread 46 Figure 12. An example of the classic epidemic curve of a propagated epidemic: measles cases by date of onset, Aberdeen, South Dakota, October 15–January 16 1971. 9.1.17 Characterizing by Place: Spot Map Assessment of an outbreak by place provides information on the geographic extent of a problem A spot map of cases in a community may show clusters or patterns that reflect water supplies, wind currents, or proximity to a restaurant or grocery store. 47 Figure 13. John Snow and Broad Street Pump map. 48  If the size of the overall population varies between the areas under comparison, a spot map, because it shows numbers of cases, can be misleading. This is a weakness of spot maps. Reflection Question: Discuss the data in the spot map on the right. What are some possible interpretations? Figure 14. Dead crow sightings, 2000. Source: CT DPH Mosquito Management Program 2000 Annual Report. 49 9.1.18 Formulate Hypotheses Hypotheses may be based on: ○ Interviews with affected people ○ Consultation with health officials in community ○ Descriptive epidemiology—person, place, and time It should incorporate the known characteristics of the agent It should be testable 9.1.19 Evaluate Hypotheses Two Approaches 1. Compare hypotheses with the established facts. This method is used when the evidence is so strong that the hypothesis does not need to be tested. 2. Use analytic epidemiology to test hypotheses by using a comparison group to quantify relationships between various exposures and the disease. 9.1.18 Refine Hypotheses and Carry Out Additional Studies Laboratory and environmental studies While epidemiology can implicate vehicles and guide appropriate public health action, laboratory evidence can clinch the findings Environmental studies often help explain why an outbreak occurred and may be very important in some settings 50 9.1.20 Implementing Control and Prevention Measures Control measures, which can be implemented early, should be aimed at specific links in the chain of infection, the agent, the source, or the reservoir In some situations, control measures are directed at interrupting transmission or exposure ○ Limit airborne spread ○ Use the method of “cohorting” by putting infected people together in a separate area Some control measures are directed at reducing susceptibility, such as travel immunizations 9.1.21 Communicate Findings The final task in an investigation is to communicate the findings to others who need to know. This communication usually takes two forms: 1. An oral briefing 2. A written report 9.1.22 Outbreak Scenario HD first notified of seven persons with culture-confirmed Salmonella typhimurium Initial fact-finding discovered that five of seven had eaten a common meal served at a church for Thanksgiving Church-sponsored meal served approximately 600 people from 4–7 p.m. 51 Evidence Gathering No remaining food items...the menu was somewhat fixed except for desserts, which were numerous. No servers reported ill. Patrons were served by the ladies of the church who were wearing gloves. No facility inspection. Ladies of the church prepared the food at home and then transported the food to the church. Patrons either ate at the church or were given take-out meals to be consumed later. Investigation Person, place, time Case definition: persons who ate the church meal and experienced diarrhea within 24–72 hours Hypothesis: contaminated food item—undercooked turkey 59 interviews of persons eating the church meal for food history (case-control study design: if individuals were recruited based on who was ill and who was NOT and each group was asked about their food choices at the event; so for each person you know that attended and was ill (Cases) , there should be a person attended the event but was NOT ill afterwards.) (Cross-sectional study design: if all attendees were sent a questionnaire to report if they were ill or not after the meal and what food they ate at the event.) 52 Descriptive Calculations Number Interviewed Attendees 59 (100%) Male 26 (44%) Sex Female 33 (66%) Median age 59 years Number of ill (cases) 17 (29%) Diarrhea 17 (100%) Nausea 16 (89%) Symptoms (present) Stomach cramps 16 (89%) Vomiting 6 (33%) Bloody stools 1 (6%) Mean 45.82 hours Incubation Median 41.00 hours 53 Figure 15.Epi curve (point source) 54 Analytical Calculations Menu Item OR (95% CI) P VALUE Turkey 4.17 (2.61, 8.41) 0.03 Ham 0.63 (0.26, 1.56) 0.46 Dressing 0.63 (0.27, 1.43) 0.48 Gravy 2.29 (0.85, 6.18) 0.14 Mashed potatoes 1.22 (0.41, 3.56) 0.99 Peas 0.48 (0.16, 1.45) 0.27 Green beans 2.64 (0.98, 7.14) 0.07 Slaw 1.36 (0.56, 3.33) 0.69 Note: Turkey was positively associated with the illness and was statistically significant (OR was above 1, 95% CI does not include 1, and p value was less than 0.05. Gravy, Mashed potatoes, green beans, and Slaw were each positively associated with the illness but were not statistically significant. Ham, Dressing, and Peas were each negatively associated with the illness (OR was below 1) were not statistically significant (95% CI includes 1, and p value was higher than 0.05). Recommendations and Control Measures It was noted that one of the turkeys that arrived at the church was not thoroughly cooked. This turkey was cooked further at the church. Provided education on food safety Investigation Reports The epidemiologist provides a written report that follows the usual scientific format of introduction, background, methods, results, discussion, and recommendations. ○ Serves as a record of performance ○ A document for potential legal issues ○ A reference for a similar situation in the future 55  Public record Patient information cannot be released without the patient's signed release Implications For Public Health By knowing how an agent exits and enters a host, and what its modes of transmission are, we can determine appropriate control measures. In general, we should direct control measures against the link in the infection chain that is most susceptible to interference, unless practical issues dictate otherwise. For some diseases, the most appropriate intervention may be directed at controlling or eliminating the agent at its source. In the hospital setting, patients may be treated and/or isolated, with appropriate “enteric precautions,” “respiratory precautions,” “universal precautions,” and the like for different exit pathways. In the community, soil may be decontaminated or covered to prevent escape of the agent. Sometimes, we direct interventions at the mode of transmission. For direct transmission, we may provide treatment to the source host or educate the source host to avoid the specific type of contact associated with transmission. In the hospital setting, since most infections are transmitted by direct contact, hand-washing is the single most important way to prevent diseases from spreading. For vehicle borne transmission, we may decontaminate or eliminate the vehicle. For fecal-oral transmission, we may also try to reduce the risk of contamination in the future by rearranging the environment and educating the persons involved in better personal hygiene. For airborne transmission, we may modify ventilation or air pressure, and filter or treat the air. For vectorborne transmission, we usually attempt to control (i.e., reduce or eradicate) the vector population. Finally, we may apply measures that protect portals of entry of a 56 susceptible potential host or reduce the susceptibility of the potential host. For example, a dentist’s mask and gloves are intended to protect the dentist from a patient’s blood, secretions, and droplets, as well to protect the patient from the dentist. Prophylactic antibiotics and vaccination are strategies to improve a potential host’s defenses. 57