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C S H MPREPARATION GUIDE

VOLUME 3Area III Safety, Health, and Environmental ApplicationsSection E: EpidemiologyStudy Notes, Questions, and Answer Key

Prepared bySteven J. Geigle, M.A., CSHM

Published by OSHA Training Network

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The information in this preparation guide has been compiled from texts recommended by ISHM for study, and represents the best current information on the various subjects. No guarantee, warranty of other representation is made as to the absolute correctness or sufficiency of any information contained in this preparation guide. OSHA Training Network assumes no responsibility in connection therewith; nor can it be assumed that all acceptable safety measures are contained in the preparation guide or that other or additional measures may not be required under particular or exceptional circumstances.

As this preparation guide will continue to be updated and revised on a periodic basis, contributions and comments from readers are invited. Additional volumes to this preparation guide will be produced and made available in the future.

Disclaimer: OSHA Training Network (OTN) cannot warrant that the use of this preparation guide will result in certification from the Institute for Safety and Health Management (ISHM). While the content is representative of the knowledge required of a safety and health manager, the successful completion of the CSHM examination is depends on many factors including the applicant's academic background, safety management experience and individual study for the examination. This information is for educational purposes only and does not replace any regulations promulgated by state of federal government agencies.

AREA III E: EPIDEMIOLOGY

Epidemiology

Philip S. Brachman, M.D. Rollins School of Public Health, Emory University Reprinted with permission of the Author

General Concepts

Definitions

Epidemiology is the study of the determinants, occurrence, and distribution of health and disease in a defined population. Infection is the replication of organisms in host tissue, which may cause disease. A carrier is an individual with no overt disease who harbors infectious organisms. Dissemination is the spread of the organism in the environment.

Chain of Infection

There are three major links in disease occurrence: the etiologic agent, the method of transmission (by contact, by a common vehicle, or via air or a vector), and the host.

Epidemiologic Methods

Epidemiologic studies may be (1) descriptive, organizing data by time, place, and person; (2) analytic, incorporating a case-control or cohort study; or (3) experimental. Epidemiology utilizes an organized approach to problem solving by: (1) confirming the existence of an epidemic and verifying the diagnosis; (2) developing a case definition and collating data on cases; (3) analyzing data by time, place, and person; (4) developing a hypothesis; (5) conducting further studies if necessary; (6) developing and implementing control and prevention measures; (7) preparing and distributing a public report; and (8) evaluating control and preventive measures.

INTRODUCTION

This chapter reviews the general concepts of epidemiology, which is the study of the determinants, occurrence, distribution, and control of health and disease in a defined population. Epidemiology is a descriptive science and includes the determination of rates, that is, the quantification of disease occurrence within a specific population. The most commonly studied rate is the attack rate: the number of cases of the disease divided by the population among whom the cases have occurred. Epidemiology can accurately describe a disease and many factors concerning its occurrence before its cause is identified. For example, Snow described many aspects of the epidemiology of cholera in the late 1840s, fully 30 years before Koch described the bacillus and Semmelweis described puerperal fever in detail in 1861 and recommended appropriate control and prevention measures a number of years before the streptococcal agent was fully described. One goal of epidemiologic studies is to define the parameters of a disease, including risk factors, in order to develop the most effective measures for control. This chapter includes a discussion of the chain of infection, the three main epidemiologic methods, and how to investigate an epidemic (Table 9-1).

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Proper interpretation of disease-specific epidemiologic data requires information concerning past as well as present occurrence of the disease. An increase in the number of reported cases of a disease that is normal and expected, representing a seasonal pattern of change in host susceptibility, does not constitute an epidemic. Therefore, the regular collection, collation, analysis, and reporting of data concerning the occurrence of a disease is important to properly interpret short-term changes in occurrence.

A sensitive and specific surveillance program is important for the proper interpretation of disease occurrence data. Almost every country has a national disease surveillance program that regularly collects data on selected diseases. The quality of these programs varies, but, generally, useful data are collected that are important in developing control and prevention measures. There is an international agreement that the occurrence of three diseasescholera, plague, and yellow feverwill be reported to the World Health Organization in Geneva, Switzerland. In the United States, the Centers for Disease Control and Prevention (CDC), U.S. Public Health Service, and the state health officers of all 50 states have agreed to report the occurrence of 51 diseases weekly and of another 10 diseases annually from the states to the CDC. Many states have regulations or laws that mandate reporting of these diseases and often of other diseases of specific interest to the state health department.

The methods of case reporting vary within each state. Passive reporting is one of the main methods. In such a case, physicians or personnel in clinics or hospitals report occurrences of relevant diseases by telephone, postcard, or a reporting form, usually at weekly intervals. In some instances, the report may be initiated by the public health or clinical laboratory where the etiologic agent is identified. Some diseases, such as human rabies, must be reported by telephone as soon as diagnosed. In an active surveillance program, the health authority regularly initiates the request for reporting. The local health department may call all or some health care providers at regular intervals to inquire about the occurrence of a disease or diseases. The active system may be used during an epidemic or if accurate data concerning all cases of a disease are desired. The health care provider usually makes the initial passive report to a local authority, such as a city or county health department. This unit collates its data and sends a report to the next highest health department level, usually the state health department.

The number of cases of each reportable disease are presented weekly, via computer linkage, by the state health department to the CDC. Data are analyzed at each level to develop needed information to assist public health authorities in disease control and prevention. For some diseases, such as hepatitis, the CDC requests preparation of a separate case reporting form containing more specific details.

In addition, the CDC prepares and distributes routine reports summarizing and interpreting the analyses and providing information on epidemics and other appropriate public health matters. Most states and some county health departments also prepare and distribute their own surveillance reports. The CDC publishes Morbidity and Mortality Weekly Report, which is available for a small fee from the Massachusetts Medical Society. The CDC also prepares more detailed surveillance reports for specific diseases, as well as an annual summary report, all of which can also be obtained through the Massachusetts Medical Society.

Infection is the replication of organisms in the tissue of a host; when defined in terms of infection, disease is overt clinical manifestation. In an inapparent (subclinical) infection, an immune response can occur without overt clinical disease. A carrier (colonized individual) is a person in whom organisms are present and may be


multiplying, but who shows no clinical response to their presence. The carrier state may be permanent, with the organism always present; intermittent, with the organism present for various periods; or temporary, with carriage for only a brief period. Dissemination is the movement of an infectious agent from a source directly into the environment; when infection results from dissemination, the source, if an individual, is referred to as a dangerous disseminator.

Infectiousness is the transmission of organisms from a source, or reservoir (see below), to a susceptible individual. A human may be infective during the preclinical, clinical, postclinical, or recovery phase of an illness. The incubation period is the interval in the preclinical period between the time at which the causative agent first infects the host and the onset of clinical symptoms; during this time the agent is replicating. Transmission is most likely during the incubation period for some diseases such as measles; in other diseases such as shigellosis, transmission occurs during the clinical period. The individual may be infective during the convalescent phase, as in diphtheria, or may become an asymptomatic carrier and remain infective for a prolonged period, as do approximately 5% of persons with typhoid fever.

The spectrum of occurrence of disease in a defined population includes sporadic (occasional occurrence); endemic (regular, continuing occurrence); epidemic (significantly increased occurrence); and pandemic (epidemic occurrence in multiple countries).

Chain of Infection

The chain of infection includes the three factors that lead to infection: the etiologic agent, the method of transmission, and the host (Fig. 9-1). These links should be characterized before control and prevention measures are proposed. Environmental factors that may influence disease occurrence must be evaluated.

FIGURE 9-1 Summary of important aspects involved in the chain of any infection. Etiologic Agent

The etiologic agent may be any microorganism that can cause infection. The pathogenicity of an agent is its ability to cause disease; pathogenicity is further characterized by describing the organism's virulence and invasiveness. Virulence refers to the severity of infection, which can be expressed by describing the morbidity (incidence of disease) and mortality (death rate) of the infection. An example of a highly virulent organism is Yersinia pestis, the agent of plague, which almost always causes severe disease in the susceptible host.


The invasiveness of an organism refers to its ability to invade tissue. Vibrio cholerae organisms are noninvasive, causing symptoms by releasing into the intestinal canal an exotoxin that acts on the tissues. In contrast, Shigella organisms in the intestinal canal are invasive and migrate into the tissue.

No microorganism is assuredly avirulent. An organism may have very low virulence, but if the host is highly susceptible, as when therapeutically immunosuppressed, infection with that organism may cause disease. For example, the poliomyelitis virus used in oral polio vaccine is highly attenuated and thus has low virulence, but in some highly susceptible individuals it may cause paralytic disease.

Other factors should be considered in describing the agent. The infecting dose (the number of organisms necessary to cause disease) varies according to the organism, method of transmission, site of entrance of the organism into the host, host defenses, and host species. Another agent factor is specificity; some agents (for example, Salmonella typhimurium) can infect a broad range of hosts; others have a narrow range of hosts. S typhi, for example, infects only humans. Other agent factors include antigenic composition, which can vary within a species (as in influenza virus or Streptococcus species); antibiotic sensitivity; resistance transfer plasmids (see Ch. 5); and enzyme production.

The reservoir of an organism is the site where it resides, metabolizes, and multiplies. The source of the organism is the site from which it is transmitted to a susceptible host, either directly or indirectly through an intermediary object. The reservoir and source can be different; for example, the reservoir for S typhi could be the gallbladder of an infected individual, but the source for transmission might be food contaminated by the carrier. The reservoir and source can also be the same, as in an individual who is a permanent nasal carrier of S aureus and who disseminates organisms from this site. The distinction can be important when considering where to apply control measures.

Method of Transmission

The method of transmission is the means by which the agent goes from the source to the host. The four major methods of transmission are by contact, by common vehicle, by air or via a vector.

In contact transmission the agent is spread directly, indirectly, or by airborne droplets. Direct contact transmission takes place when organisms are transmitted directly from the source to the susceptible host without involving an intermediate object; this is also referred to as person-to-person transmission. An example is the transmission of hepatitis A virus from one individual to another by hand contact. Indirect transmission occurs when the organisms are transmitted from a source, either animate or inanimate, to a host by means of an inanimate object. An example is transmission of Pseudomonas organisms from one individual to another by means of a shaving brush. Droplet spread refers to organisms that travel through the air very short distances, that is, less than 3 feet from a source to a host. Therefore, the organisms are not airborne in the true sense. An example of a disease that may be spread by droplets is measles.

Common-vehicle transmission refers to agents transmitted by a common inanimate vehicle, with multiple cases resulting from such exposure. This category includes diseases in which food or water as well as drugs and parenteral fluids are the vehicles of infection. Examples include food-borne salmonellosis, waterborne shigellosis, and bacteremia resulting from use of intravenous fluids contaminated with a gram-negative organism.

The third method of transmission, airborne transmission, refers to infection spread by droplet nuclei or dust. To be truly airborne, the particles should travel more than 3 feet through the air from the source to the host. Droplet nuclei are the residue from the evaporation of fluid from droplets, are light enough to be transmitted more than 3 feet from the source, and may remain airborne for prolonged periods. Tuberculosis is primarily an airborne disease; the source may be a coughing patient who creates aerosols of droplet nuclei that contain tubercle bacilli. Infectious agents may be contained in dust particles, which may become resuspended and transmitted to hosts. An example occurred in an outbreak of salmonellosis in a newborn nursery in which Salmonella-contaminated dust in a vacuum cleaner bag was resuspended when the equipment was used repeatedly, resulting in infections among the newborns.


The fourth method of transmission is vector borne transmission, in which arthropods are the vectors. Vector transmission may be external or internal. External, or mechanical, transmission occurs when organisms are carried mechanically on the vector (for example, Salmonella organisms that contaminate the legs of flies). Internal transmission occurs when the organisms are carried within the vector. If the pathogen is not changed by its carriage within the vector, the carriage is called harborage (as when a flea ingests plague bacilli from an infected individual or animal and contaminates a susceptible host when it feeds again; the organism is not changed while in the flea). The other form of internal transmission is called biologic. In this form, the organism is changed biologically during its passage through the vector (for example, malaria parasites in the mosquito vector).

An infectious agent may be transmitted by more than one route. For example, Salmonella may be transmitted by a common vehicle (food) or by contact spread (human carrier). Francisella tularensis may be transmitted by any of the four routes.

Host

The third link in the chain of infection is the host. The organism may enter the host through the skin, mucous membranes, lungs, gastrointestinal tract, or genitourinary tract, and it may enter fetuses through the placenta. The resulting disease often reflects the point of entrance, but not always: meningococci that enter the host through the mucous membranes may nonetheless cause meningitis. Development of disease in a host reflects agent characteristics (see above) and is influenced by host defense mechanisms, which may be nonspecific or specific.

Nonspecific defense mechanisms include the skin, mucous membranes, secretions, excretions, enzymes, the inflammatory response, genetic factors, hormones, nutrition, behavioral patterns, and the presence of other diseases. Specific defense mechanisms or immunity may be natural, resulting from exposure to the infectious agent, or artificial, resulting from active or passive immunization (see Ch. 8).

The environment can affect any link in the chain of infection. Temperature can assist or inhibit multiplication of organisms at their reservoir; air velocity can assist the airborne movement of droplet nuclei; low humidity can damage mucous membranes; and ultraviolet radiation can kill the microorganisms. In any investigation of disease, it is important to evaluate the effect of environmental factors. At times, environmental control measures are instituted more on emotional grounds than on the basis of epidemiologic fact. It should be apparent that the occurrence of disease results from the interaction of many factors (Table 9-2). Some of these factors are outlined here.


Epidemiologic Methods

The three major epidemiologic techniques are descriptive, analytic, and experimental. Although all three can be used in investigating the occurrence of disease, the method used most is descriptive epidemiology. Once the basic epidemiology of a disease has been described, specific analytic methods can be used to study the disease further, and a specific experimental approach can be developed to test a hypothesis.

Descriptive Epidemiology

In descriptive epidemiology, data that describe the occurrence of the disease are collected by various methods from all relevant sources. The data are then collated by time, place, and person. Four time trends are considered in describing the epidemiologic data. The secular trend describes the occurrence of disease over a prolonged period, usually years; it is influenced by the degree of immunity in the population and possibly nonspecific measures such as improved socioeconomic and nutritional levels among the population. For example, the secular trend of tetanus in the United States since 1920 shows a gradual and steady decline.

The second time trend is the periodic trend. A temporary modification in the overall secular trend, the periodic trend may indicate a change in the antigenic characteristics of the disease agent. For example, the change in antigenic structure of the prevalent influenza A virus every 2 to 3 years results in periodic increases in the occurrence of clinical influenza caused by lack of natural immunity among the population. Additionally, a lowering of the overall immunity of a population or a segment thereof (known as herd immunity) can result in an increase in the occurrence of the disease. This can be seen with some immunizable diseases when periodic decreases occur in the level of immunization in a defined population. This may then result in an increase in the


number of cases, with a subsequent rise in the overall level of herd immunity. The number of new cases then decreases until the herd's immunity is low enough to allow transmission to occur again and new cases then appear.

The third time trend is the seasonal trend. This trend reflects seasonal changes in disease occurrence following changes in environmental conditions that enhance the ability of the agent to replicate or be transmitted. For example, food-borne disease outbreaks occur more frequently in the summer, when temperatures favor multiplication of bacteria. This trend becomes evident when the occurrence of salmonellosis is examined on a monthly basis (Fig. 9-2).

FIGURE 9-2 An example of a disease showing a seasonal trend. Reported human Salmonella isolations, by 4-week average, in the United States from 1968 to 1980.

The fourth time trend is the epidemic occurrence of disease. An epidemic is a sudden increase in occurrence due to prevalent factors that support transmission.

A description of epidemiologic data by place must consider three different sites: where the individual was when disease occurred; where the individual was when he or she became infected from the source; and where the source became infected with the etiologic agent. Therefore, in an outbreak of food poisoning, the host may become clinically ill at home from food eaten in a restaurant. The vehicle may have been undercooked chicken, which became infected on a poultry farm. These differences are important to consider in attempting to prevent additional cases.

The third focus of descriptive epidemiology is the infected person. All pertinent characteristics should be noted: age, sex, occupation, personal habits, socioeconomic status, immunization history, presence of underlying disease, and other data.

Once the descriptive epidemiologic data have been analyzed, the features of the epidemic should be clear enough that additional areas for investigation are apparent.

Analytic Epidemiology

The second epidemiologic method is analytic epidemiology, which analyzes disease determinants for possible causal relations. The two main analytic methods are the case-control (or case-comparison) method and the


cohort method. The case-control method starts with the effect (disease) and retrospectively investigates the cause that led to the effect. The case group consists of individuals with the disease; a comparison group has members similar to those of the case group except for absence of the disease. These two groups are then compared to determine differences that would explain the occurrence of the disease. An example of a case-control study is selecting individuals with meningococcal meningitis and a comparison group matched for age, sex, socioeconomic status, and residence, but without the disease, to see what factors may have influenced the occurrence in the group that developed disease.

The second analytic approach is the cohort method, which prospectively studies two populations: one that has had contact with the suspected causal factor under study and a similar group that has had no contact with the factor. When both groups are observed, the effect of the factor should become apparent. An example of a cohort approach is to observe two similar groups of people, one composed of individuals who received blood transfusions and the other of persons who did not. The occurrence of hepatitis prospectively in both groups permits one to make an association between blood transfusions and hepatitis; that is, if the transfused blood was contaminated with hepatitis B virus, the recipient cohort should have a higher incidence of hepatitis than the nontransfused cohort.

The case-control approach is relatively easy to conduct, can be completed in a shorter period than the cohort approach, and is inexpensive and reproducible; however, bias may be introduced in selecting the two groups, it may be difficult to exclude subclinical cases from the comparison group, and a patient's recall of past events may be faulty. The advantages of a cohort study are the accuracy of collected data and the ability to make a direct estimate of the disease risk resulting from factor contact; however, cohort studies take longer and are more expensive to conduct.

Another analytic method is the cross-sectional study, in which a population is surveyed over a limited period to determine the relationship between a disease and variables present at the same time that may influence its occurrence.

Experimental Epidemiology

The third epidemiologic method is the experimental approach. A hypothesis is developed and an experimental model is constructed in which one or more selected factors are manipulated. The effect of the manipulation will either confirm or disprove the hypothesis. An example is the evaluation of the effect of a new drug on a disease. A group of people with the disease is identified, and some members are randomly selected to receive the drug. If the only difference between the two is use of the drug, the clinical differences between the groups should reflect the effectiveness of the drug.

Epidemic Investigation

An epidemic investigation describes the factors relevant to an outbreak of disease; once the circumstances related to the occurrence of disease are defined, appropriate control and prevention measures can be identified. In an epidemic investigation, data are collected, collated according to time, place, and person, and analyzed and inferences are drawn.

In the investigation, the first action should be to confirm the existence of the epidemic by noting from past surveillance data the number of cases suspected and comparing this with the number of cases initially reported. Additionally, the investigator should discuss the occurrence of the disease with physicians or others who have seen or reported cases after examining patients and reviewing laboratory and hospital records. These diagnoses should then be verified. A case definition should be developed to differentiate patients who represent actual cases, those who represent suspected or presumptive cases, and those who should be omitted from further study. Additional cases may be sought or additional patient data obtained, and a rough case count made.

This initial phase consists basically of collecting data, which then must be organized according to time, place, and person. The population at risk should be identified and a hypothesis developed concerning the occurrence of the disease. If appropriate, specimens should be collected and transported to the laboratory. More specific studies may be indicated. Additional data from these studies should be analyzed and the hypothesis confirmed


or altered. After analysis, control and prevention measures should be developed and, as far as possible, implemented. A report containing this information should be prepared and distributed to those involved in investigating the outbreak and in implementing control and/or prevention measures. Continued surveillance activities may be appropriate to evaluate the effectiveness of the control and prevention measures.

In the United States, the CDC assists state health departments by providing epidemiologic and laboratory support services on request. Its assistance supports disease investigations and diagnostic laboratory activities and includes various training programs conducted in the states and at the CDC. A close working relationship exists between the CDC and state health departments. Additionally, physicians frequently consult with CDC personnel on a variety of health-related problems and attend public health training programs.

The use of epidemiology to characterize a disease before its etiology has been identified is exemplified by the initial studies of acquired immune deficiency syndrome (AIDS). The first cases came to the attention of the CDC late in 1981 when an increase was observed in requests for pentamidine for treatment of Pneumocystis carinii pneumonia. This initiated specific surveillance activities and epidemiologic studies that provided important information about this newly diagnosed disease.

Initial symptoms include fever, loss of appetite, weight loss, extreme fatigue, and enlargement of lymph nodes. A severe immune deficiency then develops, which appears to be associated with opportunistic infections. These infections include P carinii pneumonia, diagnosed in 52 percent of cases; Kaposi sarcoma in 26 percent of cases; and both P carinii pneumonia and Kaposi sarcoma in 7 percent of cases. The remaining 15 percent of AIDS patients have other parasitic, fungal, bacterial, or viral infections associated with immunodeficiencies. Among the first 2,640 cases reported to the CDC, there were 1,092 deaths, a case-fatality rate of 41 percent. Approximately 95 percent of the cases were male; 70 percent were 20 to 49 years of age at the time of diagnosis. Approximately 40 percent of the cases were reported from New York City, 12 percent from San Francisco, 8 percent from Los Angeles, and the remainder from 32 other states. Cases were reported from at least 16 other countries. Among the 90 percent of patients who were categorized according to possible risk factors, those at highest risk were homosexuals or bisexuals (70 percent), intravenous drug abusers ( 17 percent), Haitian entrants into the United States (9.5 percent), and persons with hemophilia (1 percent).

Analysis of these initial data, collected before the etiologic agent of AIDS was identified, supported the hypothesis that transmission occurred primarily by sexual contact, receipt of contaminated blood or blood products, or contact with contaminated intravenous needles. Spread through casual contact did not seem likely. The epidemiologic data indicated that AIDS was an infectious disease. It has now been determined that AIDS results from infection with a retrovirus of the human T cell leukemia/lymphoma virus family, which has been designated human immunodeficiency virus type I (HIV-l). The initial hypotheses have been proven as shown by analysis of data subsequently collected.

REFERENCES

Beaglehole R, Bonita R, Kjellstrom T: Basic Epidemiology. World Health Organization, Geneva, Switzerland, 1993 Benenson A: Control of Communicable Disease Manual 16th Ed., American Public Health Association, Washington, DC, 1995 Bennett JV, Brachman PS: Hospital Infections. 3rd Ed., Little, Brown, Boston, 1992 Evans AS, Brachman PS: Bacterial Infections of Humans. Epidemiology and Control.2nd Ed. Plenum New York, 1991 Fox JP, Hall CE, Elveback LR: Epidemiology, Man and Disease. Macmillan, New York, 1970 Hennekens CH, Buring JE: Epidemiology in Medicine. Little, Brown, Boston, 1987 Langmuir AD: The surveillance of communicable diseases of national importance. N Engl J Med 268: 182, 1963 Lilienfeld DE, Stolley PD: Foundations of Epidemiology. 3rd Ed. Oxford University Press, New York, 1994 MacMahon B, Pugh TF: Epidemiology Principles and Methods. Little, Brown, Boston, 1970 Mandell GL, Douglas RG, Jr, Bennett JE: Principles and Practice of Infectious Diseases, 3rd Ed. Churchill Livingstone, New York, 1990


Smith DM, Haupt BJ: Hospital discharge data used as feedback in planning research and education for primary care. Public Health Rep 98:457, 1983 World Health Organization: The surveillance of communicable diseases. WHO Chron 22:439, 1968

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EpidemiologyFrom Wikipedia, the free encyclopedia.

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 control of health problems (Last 2001). Epidemiology is the scientific study of factors affecting the health and illness of individuals and populations, and, in this capacity, it serves as the foundation and logic of interventions made in the interest of the public’s health. The acting epidemiologist works on issues from the practical, such as outbreak investigation, environmental exposure, and health promotion, to the theoretical, including the development of statistical, mathematical, philosophical, and biological theory. To this end, epidemiologists employ a range of study designs from the observational to experimental with the purpose of revealing the unbiased relationships between exposures such as nutrition, HIV, stress, or chemicals to outcomes such as disease, wellness, and health indicators.

Epidemiological studies are generally categorized as descriptive, analytic (aiming to examine associations, commonly hypothesized causal relationships), and experimental (a term often equated with clinical or community trials of treatments and other interventions).

Epidemiologists work in a variety of settings. Some epidemiologists work "in the field", i.e., in the community, commonly in a public health service, and are at the forefront of investigating and combating disease outbreaks.

Epidemiology as causal inference

Although epidemiology is sometimes viewed as a collection of statistical tools used to elucidate the associations of exposures to health outcomes, a deeper understanding of this science is that of discovering causal relationships. This conceptualization of epidemiology is difficult to grasp because our internal notions of causation are often poorly developed, frequently being predicated on the notion of a one-to-one relationship. For example, almost everyone would agree that gravity causes a dropped ball to fall towards the ground, but would most agree that drinking one glass of milk a day will cause weight loss? Even very heavy smokers know that their vice causes lung cancer, but only 10% of life-long smokers will get lung cancer. How can this be?The answer is complex and delves into the philosophical notions of causality, induction, deduction, logic and other dense topics. It is nearly impossible to say with perfect accuracy how even the most simple physical systems will behave, much less the complex field of epidemiology that draws on biology, sociology, mathematics, statistics, anthropology, psychology, and policy. However, for the epidemiologist the key is in the term inference. As epidemiologists, we use gathered data and a broad range of bio-medical and psycho-social theories in an iterative way to generate or expand theory, to test hypotheses, and to make educated, informed assertions about which relationships are causal and exactly how they are causal.

Epidemiology as advocacy

An alternative aspect of an epidemiologist’s duty is to advocacy for the health of populations, bearing in mind the outpost perspective they have over factors that affect a whole population. In this capacity the epidemiologist is not limited by the strict requirements for scientific accuracy. This of course does not mean that the epidemiologist can advocate for whatever position they please independent of the data, but presentation of the data can take more artistic modes to engender behavior or perspective change. For example, consider these two alternative admonishments against smoking:


1. smoking has been consistently linked to health problems such as lung cancer and coronary heart disease in several large prospective studies, this link has been deemed causal by a complex process of induction, consensus, and modeling.

2. Smoking will kill you. Although statement one is more accurate, statement two has an air of finality and explicit causation that may help to reduce the rate of smoking, albeit scientific and philosophically questionable.The best epidemiologist as advocate will consider the broader intellectual landscape beyond the epidemiology and public health literature to render judgment on a course of action for a population, in this manner they are employing a different analytical framework than the Cartesian framework that is more common in scientific epidemiology. However, it is rare for one person to wield the skills and embody the features required to be a leader in both the scientific and advocacy aspects of epidemiology.

Measures1. Measures of occurrence

1. Incidence measures 1. Incidence density (also known as Incidence rate) (Szklo & Nieto, 2000) 2. Hazard rate 3. Cumulative incidence

2. Prevalence measures 1. Point prevalence 2. Period prevalence

2. Measures of association 1. Relative measures

1. Risk ratio 2. Rate ratio 3. Odds ratio 4. Hazard ratio

2. Absolute measures 1. Risk/rate/incidence difference 2. Attributable risk

1. Attributable risk in exposed 2. Percent attributable risk 3. Levin’s attributable risk

History of epidemiology

Dr. John Snow is famous for the suppression of an 1854 outbreak of cholera in London's Soho district. He identified the cause of the outbreak as a public water pump on Broad Street and had the handle removed, thus ending the outbreak. (It has been questioned as to whether the epidemic was already in decline when Snow took action.) This has been perceived as a major event in the history of public health and can be regarded as the founding event of the science of epidemiology.

Other pioneers include Danish physician P.A. Schleisner, who in 1849 related his work on the prevention of the epidemic of tetanus neonatorum on the Vestmanna Islands in Iceland. Another important pioneer was Hungarian physician Ignaz Semmelweis, who in 1847 brought down infant mortality at Vienna hospital by instituting a disinfection procedure. His findings were published in 1850, but his work was ill received by this colleagues, who discontinued the procedure. Disinfection did not become widely practiced until British surgeon Joseph Lister "discovered" antiseptics in 1865 in light of the work of Louis Pasteur.

In the early 20th century, mathematical methods were introduced into epidemiology by Ronald Ross, Anderson Gray McKendrick and others. Another breakthrough was the 1956 publication of the results of a British doctor's study, which lent statistical support to the suspicion that tobacco smoking was linked to lung cancer.

References Last JM (2001). "A dictionary of epidemiology", 4th edn, Oxford: Oxford University Press.


Nutter FW Jr (1999) "Understanding the Interrelationships Between Botanical, Human, and Veterinary Epidemiology: The Ys and Rs of It All. Ecosystem Health 5 (3): 131-140".

Szklo MM & Nieto FJ (2002). "Epidemiology: beyond the basics", Aspen Publishers, Inc.

Analyzing MultipleRisk Factors

The Event

Listed below are some of the initial steps that take place in an outbreak investigation. This section will provide you with a basis of information to complete this module.

Background Information

Recently, a national political action committee (PAC) held a fundraising event at a Hotel & Convention Center in Washington, D.C. to select which candidate they would support by providing contributions to the candidate's campaign.

The candidate would be selected by a popular vote that was scheduled to occur at the completion of the annual meeting, which took place during August 28--31. Upon arrival at the meeting, members of the PAC attended a banquet to recognize the accomplishments of the group during the previous calendar year.

Within 24 hours of the banquet, a number of diners had become ill with diarrhea and at least one of the following symptoms:

Headache Fever

Abdominal pain

Nausea

Vomiting

At this point, the local public health department has been notified and is onsite to conduct an investigation.

Identifying an Outbreak

Often, health departments are alerted of a potential outbreak by a health-care provider or the general public. It is your responsibility to identify whether the event is truly an outbreak or a series of unrelated cases.

Establishing a Case Definition

A case definition is a "set of standard criteria for deciding whether a person has a particular disease or health-related condition, by specifying clinical criteria and limitations on time, place, and person" (1). In the


beginning of an investigation, the case definition can be fairly general, but as information is gathered, the epidemiologist may use a more specific description.

Initial Case Definition

An illness with diarrhea and at least one of the following symptoms: headache, fever, abdominal pain, nausea, and/or vomiting. Onset of illness or symptoms occurred during August 28--31.

Identification of Cases

Often, the initial cases identified in an investigation represent only a fraction of the affected population. Therefore, it is important to detect and identify other cases through interviews, questionnaires, surveys of health care facilities, newspaper articles, and other means.

During the next part of the outbreak investigation, the team obtained an occupancy list of all of the guests who were registered at the hotel.

Using an adapted version of the foodborne outbreak investigation questionnaire developed by CDC, which contained questions pertaining to demographics, clinical information, exposure information, and a 72-hour meal history, the team divided the list and began to collect data in an effort to identify affected cases and systematically organize key information.

The team established that 105 of the 435 hotel guests had become acutely ill and exhibited many of the symptoms listed in the initial case definition. All of these persons were members of the PAC.

Formulating Hypotheses

The next step in an investigation is to develop an initial hypothesis, based upon the information collected during early interviews and other information provided by health-care facilities or medical institutions that treated the patients.

Initial Hypothesis

Onset of diarrheal illness of an unknown origin occurred during August 28--31. The potential exposure to the unknown agent could have occurred at the PAC banquet at Hotel A in Washington, D.C.

Evaluating the Hypothesis

In order to identify the source of the epidemic, investigators must evaluate the relationships between exposures and the disease in question. The following information will be helpful in evaluating the hypothesis presented on the following pages.

Additional Information

A descriptive analysis of the information gathered from the interviews and questionnaires identified the following characteristics of the distribution of the disease in question.

Person - members of the PAC Place - the PAC banquet on August 28 Time - the banquet occurred at 6:00 p.m., and the first onset of illness surfaced at 6:00 a.m. on August

29


This information suggests that attendance at the banquet was the common exposure, and the reported symptoms sound like an outbreak of gastroenteritis, which is commonly caused by food contamination.

Next, we will discuss methods to organize the data in an effort to begin to make epidemiologic associations.

Potential Exposures

The team was able to determine from the interviews that the potential exposures in this outbreak include all the items served at the banquet:

Turkey Gravy

Ham

Mashed potatoes

Green beans

Chocolate sorbet

Strawberry sorbet

We need to discover, each of the potential exposures, which is associated with becoming ill.

Risk Factors

All of the potential exposures cited in the scenario so far could be classified as risk factors, which can be defined as "an aspect of personal behavior, lifestyle, or environmental exposure...which, on the basis of epidemiologic evidence, is known to be associated with a health-related condition considered important to prevent".(1)

Since we are trying to determine what factors are associated with the illness, we must analyze the data in order to measure the frequencies with which a potential exposure can be associated with the illness. Measures of association may be expressed in several ways.

Attack Rates

One of the most commonly used measures expresses how many diseases are a consequence of a certain factor. It can be expressed as an attack rate, which measures the proportion of people in a well-defined population that develop an illness of interest during a limited period of time.

These rates can be calculated by dividing the number of new cases among the population during the limited time period (X) by the population at risk at the beginning of the period (Y). This number is then multiplied by (100) to obtain a percentage. The formula can be expressed as:


As you can see, attack rates measure the probability or risk for becoming a case. So the greater the difference between attack rates for those exposed to a particular risk factor and those not exposed, the higher the probability that the risk factor caused the outbreak.

Let's look at our previous example to calculate the attack rate for the entire population who attended the PAC banquet:

X = the number of ill persons who attended the banquet = 105Y = the total number of persons who attended the banquet = 235

Risk Ratio

Before we can make any conclusions, we must also calculate the attack rates for the unexposed population (i.e., the population that did not eat the menu item). In the table below, we have provided you with the attack rate of both the exposed and unexposed population so that we will be able to calculate the risk ratio. The risk ratio compares the amount of disease risk in the exposed population to the amount of risk in the unexposed population. The formula can be expressed as:

Risk ratios can be evaluated by using the following criteria:

RR = 1.0 indicates identical risk in both groups RR > 1.0 indicates an increased risk for the exposed group compared to the unexposed group

RR < 1.0 indicates a decreased risk for the exposed group compared to the unexposed group

Selecting a Reference Group

Selection of an appropriate reference or comparison group is critical to determining whether an association exists between the exposure and illness. Typically, the reference group is the population of individuals that has had no exposure to the risk factor being analyzed. Ideally, the exposed and unexposed populations should be similar, with the exception of the exposure. Thus, if no direct association exists between the exposure and the illness, the attack rates will be relatively similar.

Unfortunately, the analysis of food-specific attack rates did not clearly point to any one of the menu items as the source of the outbreak.

However, one of the epidemiologists pointed out that eight persons arrived late to the banquet and were able to eat only dessert. All of these persons became ill.

Of these eight dessert-only diners:


four ate chocolate sorbet four ate strawberry sorbet

In addition, the outbreak investigation team realized through their analysis of the data that ten persons who ate the main course of the meal did not eat dessert. Of these ten non-dessert diners, only two became ill.

Analyses of Multiple Risk Factors

"It is important to note that when several risk factors are being considered simultaneously, the non-exposed group should be defined as those with none of the risk factors under evaluation".(2) In this case, the reference group is the population of individuals that were not exposed to (did not eat) the risk factors being analyzed.

We have established the group that ate no dessert as the "risk-free" reference group, as they have many of the same characteristics of the exposed population but did not eat any of the dessert items.

Previously, when the risk ratios were calculated for the two dessert items, they were 1.32 for chocolate sorbet and 1.07 for strawberry sorbet. There is a difference between these figures and those calculated above (2.25 for chocolate sorbet and 2.00 for strawberry sorbet). There are several reasons for this discrepancy listed below.

Two high-risk populations were compared because the comparison (unexposed) group included all those who ate the other implicated food item (minus those who ate both items).

The resulting attack rate for the comparison (unexposed) group was relatively similar to the attack rate for the exposed population.

Therefore, the risk ratios were relatively low and did not appear to be statistically significant.

Knowing when to refocus and reanalyze like this is not easy; it is something that investigators generally learn from judgment and experience. It is also essential to stay open and alert to clues. A key clue was that among a small number of people who ate nothing but dessert, all became ill, and among a small number who ate no dessert, few became ill.

Further Investigation

After a thorough analysis of the data, the investigation team hypothesized that the primary contributing factors to the illness were associated with eating chocolate sorbet or strawberry sorbet.

Early in the investigation, stool specimens had been taken from several persons. In addition, the team also took stool samples from all of the kitchen staff on duty during the preparation of the food for the banquet. All of the samples were sent to the local health department lab to be cultured for enteric pathogens.

Revising the HypothesisOnce you have characterized the outbreak by time, place, and person and analyzed the data to determine risk factors associated with the disease, it is helpful to revisit the initial hypothesis and to determine if any changes need to be made on the basis of the information gathered so far. Much of this information can be derived from descriptive epidemiology and the analyses of the individual risk factors.

For instance, we are able to deduce that the vehicles for transmission of the illness were chocolate sorbet and strawberry sorbet. Unfortunately, at this point the causative agent (such as a microorganism, chemical substance, or form of radiation, whose presence, excessive presence, or relative absence is essential for the occurrence of a disease) is still unknown, and additional data will therefore need to be gathered after the revised hypothesis is developed.


Our revised hypothesis could read "The illness was caused by consumption of either chocolate sorbet and/or strawberry sorbet consumed at the Political Action Committee Banquet at 6 p.m. on August 28.

When the results were returned from the lab, the results indicated that:

No kitchen staff tested positive for any enteric pathogens, Stool cultures from the early onset cases tested positive for Salmonella Typhimurium,

Cultures from the leftover chocolate sorbet and strawberry sorbet yielded the identical strain of Salmonella to the persons tested at the banquet.

Based upon this new information, the investigation team revised their case definition to read:

An illness with diarrhea (multiple loose bowel movements within a 24-hour period) and at least one of the following symptoms: headache, fever, abdominal pain, nausea, and/or vomiting

OR

A positive stool culture for Salmonella Typhimurium

AND

Onset of illness or symptoms occurred during August 28--31 and the patient attended the PAC banquet.

In addition, local sanitarians also conducted an environmental analysis of the kitchen facility and found no indication of Salmonella Typhimurium on any of the equipment or utensils that were used in the storage, preparation, and service of the meal. In addition, they also found that the kitchen staff practiced good hygiene and safe food-handling practices.

Understanding the Outbreak

Many outbreak investigations are not straightforward, and this one is no exception. Another aspect of evaluating a hypothesis is to review reference material on the disease or illness to determine whether the situation is similar to other reported incidents. Often, the Emerging Infectious Diseases (EID) journal is a valuable resource for finding incidents, trends, and specific disease information. (EID is available at: http://www.cdc.gov/ncidod/EID/.

Some of the areas that will help with the investigation are:

Typical signs and symptoms Modes of transmission

Exposures

In the example of the PAC Banquet, the majority of ill persons interviewed met the established symptoms of Salmonella Typhimurium, such as:

Diarrhea, fever, and abdominal cramps Incubation period of 6 -- 72 hours after infection


Illness usually lasts 4 -- 7 days

Transmitted through contaminated food, water, or contact with infected animals

However, the fact that Salmonella Typhimurium is not typically found in the ingredients used in chocolate sorbet and strawberry sorbet led the investigation team to consider the possibility of deliberate tampering.

When the investigators first suspected intentional food tampering, they contacted local law enforcement officials, who in turn notified the Federal Bureau of Investigation (FBI).

Information obtained in the ensuing investigation revealed that a supporter of one of the candidates owned the company that supplied desserts to many of the food service facilities in the area. This person later admitted to trying to sabotage the election by inoculating Salmonella Typhimurium into the desserts in an attempt to alter the results of the popular vote for funding. The saboteur and colleagues skipped the banquet and had planned to show up for the vote while others were ill.

If you recall, there were two persons reported that they did not eat the dessert. As it turns out, when asked again, they indicated that they forgot to mention taking a "few bites" from their spouses' desserts.

Also, four hotel employees who were not employed in the kitchen, were identified as having Salmonella Typhimurium. These employees became ill after they ate desserts that had been left over from the banquet.

In the end, all of the cases were accounted for and associated with either chocolate sorbet or strawberry sorbet.

Summary

Many outbreak investigations are not straightforward, and therefore it is important to look for clues that will help you identify what factors are associated with the illness in question. To accomplish this, you should select an appropriate reference group and analyze the data for measures of association. The following is a brief description of the steps involved in this process.

Selecting a Reference GroupSelection of an appropriate reference or comparison group is critical to determining whether an association exists between the exposure and illness. Typically, the reference group is the population of individuals that has had no exposure to the risk factor being analyzed. Ideally, the exposed and unexposed populations should be similar, with the exception of the exposure. Thus, if no direct association exists between the exposure and the illness, the attack rates will be relatively similar.

Calculating Rates and RatiosTo determine what factors are associated with the illness, we must analyze the data to measure the frequencies with which a potential exposure can be associated with the illness. There are several ways to express measures of association in cohort studies.

Attack rates are calculated by dividing the number of new cases among the population during the limited time period (X) by the population at risk at the beginning of the period (Y). This number is then multiplied by (100) to express in a percentage. The formula can be expressed as:


Risk ratios compare the amount of risk associated with an event such as a disease or death in the exposed population to the amount of risk in the unexposed population. As a result, they are calculated by dividing the attack rate in the exposed population by the attack rate in the unexposed population. The formula can be expressed as:

Analyzing Multiple Risk FactorsWhen several risk factors are being considered simultaneously, it is often necessary to establish an experimental control, where there is an absence of exposure to all of the risk factors being analyzed. By establishing an experimental control, a uniform denominator is created and the resulting risk ratios can be compared. The following can assist with comparison of risk ratios.

RR = 1.0 indicates identical risk in both groups RR > 1.0 indicates an increased risk for the exposed group compared to the unexposed group

RR < 1.0 indicates a decreased risk for the exposed group compared to the unexposed group

Identifying the correct association will the illness does not always occur on the first attempt and knowing when to refocus and reanalyze often requires experience and judgment and might require advanced analysis. Often, investigations are an iterative process in which a hypothesis is established, tested, and revised several times before association(s) can be established. Overall, it is also essential to stay open and be alert to clues

__________________________________________________________________________________________

Glossary

AGE-ADJUSTED MORTALITY RATE. A mortality rate statistically modified to eliminate the effect of different age distributions in the different populations.

AGENT. A factor, such as a microorganism, chemical substance, or form of radiation, whose presence, excessive presence, or (in deficiency diseases) relative absence is essential for the occurrence of a disease.

AGE-SPECIFIC MORTALITY RATE. A mortality rate limited to a particular age group. The numerator is the number of deaths in that age group; the denominator is the number of persons in that age group in the population.

ANALYTIC EPIDEMIOLOGY. The aspect of epidemiology concerned with the search for health-related causes and effects. Uses comparison groups, which provide baseline data, to quantify the association between exposures and outcomes, and test hypotheses about causal relationships.

ANALYTIC STUDY. A comparative study intended to identify and quantify associations, test hypotheses, and identify causes. Two common types are cohort study and case-control study.

APPLIED EPIDEMIOLOGY. The application or practice of epidemiology to address public health issues.

ASSOCIATION. Statistical relationship between two or more events, characteristics, or other variables.


ATTACK RATE. A variant of an incident rate, applied to a narrowly defined population observed for a limited period of time, such as during an epidemic.

ATTRIBUTABLE PROPORTION. A measure of the public health impact of a causative factor; proportion of a disease in a group that is exposed to a particular factor which can be attributed to their exposure to that factor.

B

BAR CHART. A visual display of the size of the different categories of a variable. Each category or value of the variable is represented by a bar.

BIAS. Deviation of results or inferences from the truth, or processes leading to such systematic deviation. Any trend in the collection, analysis, interpretation, publication, or review of data that can lead to conclusions that are systematically different from the truth.

BIOLOGIC TRANSMISSION. The indirect vector-borne transmission of an infectious agent in which the agent undergoes biologic changes within the vector before being transmitted to a new host.

BOX PLOT. A visual display that summarizes data using a ``box and whiskers'' format to show the minimum and maximum values (ends of the whiskers), interquartile range (length of the box), and median (line through the box).

C

CARRIER. A person or animal without apparent disease that harbors a specific infectious agent and is capable of transmitting the agent to others. The carrier state may occur in an individual with an infection that is inapparent throughout its course (known as asymptomatic carrier), or during the incubation period, convalescence, and postconvalescence of an individual with a clinically recognizable disease. The carrier state may be of short or long duration (transient carrier or chronic carrier).

CASE. In epidemiology, a countable instance in the population or study group of a particular disease, health disorder, or condition under investigation. Sometimes, an individual with the particular disease.

CASE-CONTROL STUDY. A type of observational analytic study. Enrollment into the study is based on presence (``case'') or absence (``control'') of disease. Characteristics such as previous exposure are then compared between cases and controls.

CASE DEFINITION. A set of standard criteria for deciding whether a person has a particular disease or health-related condition, by specifying clinical criteria and limitations on time, place, and person.

CASE-FATALITY RATE. The proportion of persons with a particular condition (cases) who die from that condition. The denominator is the number of incident cases; the numerator is the number of cause-specific deaths among those cases.

CAUSE OF DISEASE. A factor (characteristic, behavior, event, etc.) that directly influences the occurrence of disease. A reduction of the factor in the population should lead to a reduction in the occurrence of disease.


CAUSE-SPECIFIC MORTALITY RATE. The mortality rate from a specified cause for a population. The numerator is the number of deaths attributed to a specific cause during a specified time interval; the denominator is the size of the population at the midpoint of the time interval.

CENSUS. The enumeration of an entire population, usually with details being recorded on residence, age, sex, occupation, ethnic group, marital status, birth history, and relationship to head of household.

CHAIN OF INFECTION. A process that begins when an agent leaves its reservoir or host through a portal of exit, and is conveyed by some mode of transmission, then enters through an appropriate portal of entry to infect a susceptible host.

CLASS INTERVAL. A span of values of a continuous variable which are grouped into a single category for a frequency distribution of that variable.

CLUSTER. An aggregation of cases of a disease or other health-related condition, particularly cancer and birth defects, which are closely grouped in time and place. The number of cases may or may not exceed the expected number; frequently the expected number is not known.

COHORT. A well-defined group of people who have had a common experience or exposure, who are then followed up for the incidence of new diseases or events, as in a cohort or prospective study. A group of people born during a particular period or year is called a birth cohort.

COHORT STUDY. A type of observational analytic study. Enrollment into the study is based on exposure characteristics or membership in a group. Disease, death, or other health-related outcomes are then ascertained and compared.

COMMON SOURCE OUTBREAK. An outbreak that results from a group of persons being exposed to a common noxious influence, such as an infectious agent or toxin. If the group is exposed over a relatively brief period of time, so that all cases occur within one incubation period, then the common source outbreak is further classified as a point source outbreak. In some common source outbreaks, persons may be exposed over a period of days, weeks, or longer, with the exposure being either intermittent or continuous.

CONFIDENCE INTERVAL. A range of values for a variable of interest, e.g., a rate, constructed so that this range has a specified probability of including the true value of the variable. The specified probability is called the confidence level, and the end points of the confidence interval are called the confidence limits.

CONFIDENCE LIMIT. The minimum or maximum value of a confidence interval.

CONTACT. Exposure to a source of an infection, or a person so exposed.

CONTAGIOUS. Capable of being transmitted from one person to another by contact or close proximity.

CONTINGENCY TABLE. A two-variable table with cross-tabulated data.

CONTROL. In a case-control study, comparison group of persons without disease.

CRUDE MORTALITY RATE. The mortality rate from all causes of death for a population.

CUMULATIVE FREQUENCY. In a frequency distribution, the number or proportion of cases or events with a particular value or in a particular class interval, plus the total number or proportion of cases or events with smaller values of the variable.


CUMULATIVE FREQUENCY CURVE. A plot of the cumulative frequency rather than the actual frequency for each class interval of a variable. This type of graph is useful for identifying medians, quartiles, and other percentiles.

D

DEATH-TO-CASE RATIO. The number of deaths attributed to a particular disease during a specified time period divided by the number of new cases of that disease identified during the same time period.

DEMOGRAPHIC INFORMATION. The ``person'' characteristics--age, sex, race, and occupation--of descriptive epidemiology used to characterize the populations at risk.

DENOMINATOR. The lower portion of a fraction used to calculate a rate or ratio. In a rate, the denominator is usually the population (or population experience, as in person-years, etc.) at risk.

DEPENDENT VARIABLE. In a statistical analysis, the outcome variable(s) or the variable(s) whose values are a function of other variable(s) (called independent variable(s) in the relationship under study).

DESCRIPTIVE EPIDEMIOLOGY. The aspect of epidemiology concerned with organizing and summarizing health-related data according to time, place, and person.

DETERMINANT. Any factor, whether event, characteristic, or other definable entity, that brings about change in a health condition, or in other defined characteristics.

DIRECT TRANSMISSION. The immediate transfer of an agent from a reservoir to a susceptible host by direct contact or droplet spread.

DISTRIBUTION. In epidemiology, the frequency and pattern of health-related characteristics and events in a population. In statistics, the observed or theoretical frequency of values of a variable.

DOT PLOT. A visual display of the actual data points of a noncontinuous variable.

DROPLET NUCLEI. The residue of dried droplets that may remain suspended in the air for long periods, may be blown over great distances, and are easily inhaled into the lungs and exhaled.

DROPLET SPREAD. The direct transmission of an infectious agent from a reservoir to a susceptible host by spray with relatively large, short-ranged aerosols produced by sneezing, coughing, or talking.

E

ENDEMIC DISEASE. The constant presence of a disease or infectious agent within a given geographic area or population group; may also refer to the usual prevalence of a given disease within such area or group.

ENVIRONMENTAL FACTOR. An extrinsic factor (geology, climate, insects, sanitation, health services, etc.) which affects the agent and the opportunity for exposure.

EPIDEMIC. The occurrence of more cases of disease than expected in a given area or among a specific group of people over a particular period of time.


EPIDEMIC CURVE. A histogram that shows the course of a disease outbreak or epidemic by plotting the number of cases by time of onset.

EPIDEMIC PERIOD. A time period when the number of cases of disease reported is greater than expected.

EPIDEMIOLOGIC TRIAD. The traditional model of infectious disease causation. Includes three components: an external agent, a susceptible host, and an environment that brings the host and agent together, so that disease occurs.

EPIDEMIOLOGY. 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.

EVALUATION. A process that attempts to determine as systematically and objectively as possible the relevance, effectiveness, and impact of activities in the light of their objectives.

EXPERIMENTAL STUDY. A study in which the investigator specifies the exposure category for each individual (clinical trial) or community (community trial), then follows the individuals or community to detect the effects of the exposure.

EXPOSED (GROUP). A group whose members have been exposed to a supposed cause of disease or health state of interest, or possess a characteristic that is a determinant of the health outcome of interest.

F

FREQUENCY DISTRIBUTION. A complete summary of the frequencies of the values or categories of a variable; often displayed in a two column table: the left column lists the individual values or categories, the right column indicates the number of observations in each category.

FREQUENCY POLYGON. A graph of a frequency distribution with values of the variable on the x-axis and the number of observations on the y-axis; data points are plotted at the midpoints of the intervals and are connected with a straight line.

G

GRAPH. A way to show quantitative data visually, using a system of coordinates.

H

HEALTH. A state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity.

HEALTH INDICATOR. A measure that reflects, or indicates, the state of health of persons in a defined population, e.g., the infant mortality rate.

HEALTH INFORMATION SYSTEM. A combination of health statistics from various sources, used to derive information about health status, health care, provision and use of services, and impact on health.


HIGH-RISK GROUP. A group in the community with an elevated risk of disease.

HISTOGRAM. A graphic representation of the frequency distribution of a continuous variable. Rectangles are drawn in such a way that their bases lie on a linear scale representing different intervals, and their heights are proportional to the frequencies of the values within each of the intervals.

HOST. A person or other living organism that can be infected by an infectious agent under natural conditions.

HOST FACTOR. An intrinsic factor (age, race, sex, behaviors, etc.) which influences an individual's exposure, susceptibility, or response to a causative agent.

HYPERENDEMIC DISEASE. A disease that is constantly present at a high incidence and/or prevalence rate.

HYPOTHESIS. A supposition, arrived at from observation or reflection, that leads to refutable predictions. Any conjecture cast in a form that will allow it to be tested and refuted.

HYPOTHESIS, NULL. The first step in testing for statistical significance in which it is assumed that the exposure is not related to disease.

HYPOTHESIS, ALTERNATIVE. The hypothesis, to be adopted if the null hypothesis proves implausible, in which exposure is associated with disease.

I

IMMUNITY, ACTIVE. Resistance developed in response to stimulus by an antigen (infecting agent or vaccine) and usually characterized by the presence of antibody produced by the host.

IMMUNITY, HERD. The resistance of a group to invasion and spread of an infectious agent, based on the resistance to infection of a high proportion of individual members of the group. The resistance is a product of the number susceptible and the probability that those who are susceptible will come into contact with an infected person.

IMMUNITY, PASSIVE. Immunity conferred by an antibody produced in another host and acquired naturally by an infant from its mother or artificially by administration of an antibody-containing preparation (antiserum or immune globulin).

INCIDENCE RATE. A measure of the frequency with which an event, such as a new case of illness, occurs in a population over a period of time. The denominator is the population at risk; the numerator is the number of new cases occurring during a given time period.

INCUBATION PERIOD. A period of subclinical or inapparent pathologic changes following exposure, ending with the onset of symptoms of infectious disease.

INDEPENDENT VARIABLE. An exposure, risk factor, or other characteristic being observed or measured that is hypothesized to influence an event or manifestation (the dependent variable).

INDIRECT TRANSMISSION. The transmission of an agent carried from a reservoir to a susceptible host by suspended air particles or by animate (vector) or inanimate (vehicle) intermediaries.

INDIVIDUAL DATA. Data that have not been put into a frequency distribution or rank ordered.


INFECTIVITY. The proportion of persons exposed to a causative agent who become infected by an infectious disease.

INFERENCE, STATISTICAL. In statistics, the development of generalizations from sample data, usually with calculated degrees of uncertainty.

INTERQUARTILE RANGE. The central portion of a distribution, calculated as the difference between the third quartile and the first quartile; this range includes about one-half of the observations in the set, leaving one-quarter of the observations on each side.

L

LATENCY PERIOD. A period of subclinical or inapparent pathologic changes following exposure, ending with the onset of symptoms of chronic disease.

M

MEAN, ARITHMETIC. The measure of central location commonly called the average. It is calculated by adding together all the individual values in a group of measurements and dividing by the number of values in the group.

MEAN, GEOMETRIC. The mean or average of a set of data measured on a logarithmic scale.

MEASURE OF ASSOCIATION. A quantified relationship between exposure and disease; includes relative risk, rate ratio, odds ratio.

MEASURE OF CENTRAL LOCATION. A central value that best represents a distribution of data. Measures of central location include the mean, median, and mode. Also called the measure of central tendency.

MEASURE OF DISPERSION. A measure of the spread of a distribution out from its central value. Measures of dispersion used in epidemiology include the interquartile range, variance, and the standard deviation.

MEDIAN. The measure of central location which divides a set of data into two equal parts.

MEDICAL SURVEILLANCE. The monitoring of potentially exposed individuals to detect early symptoms of disease.

MIDRANGE. The halfway point or midpoint in a set of observations. For most types of data, it is calculated as the sum of the smallest observation and the largest observation, divided by two. For age data, one is added to the numerator. The midrange is usually calculated as an intermediate step in determining other measures.

MODE. A measure of central location, the most frequently occurring value in a set of observations.

MORBIDITY. Any departure, subjective or objective, from a state of physiological or psychological well-being.


MORTALITY RATE. A measure of the frequency of occurrence of death in a defined population during a specified interval of time.

MORTALITY RATE, INFANT. A ratio expressing the number of deaths among children less than one year of age reported during a given time period divided by the number of births reported during the same time period. The infant mortality rate is usually expressed per 1,000 live births.

MORTALITY RATE, NEONATAL. A ratio expressing the number of deaths among children from birth up to but not including 28 days of age divided by the number of live births reported during the same time period. The neonatal mortality rate is usually expressed per 1,000 live births.

MORTALITY RATE, POSTNEONATAL. A ratio expressing the number of deaths among children from 28 days up to but not including 1 year of age during a given time period divided by the number of lives births reported during the same time period. The postneonatal mortality rate is usually expressed per 1,000 live births.

N

NATURAL HISTORY OF DISEASE. The temporal course of disease from onset (inception) to resolution.

NECESSARY CAUSE. A causal factor whose presence is required for the occurrence of the effect (of disease).

NOMINAL SCALE. Classification into unordered qualitative categories; e.g., race, religion, and country of birth as measurements of individual attributes are purely nominal scales, as there is no inherent order to their categories.

NORMAL CURVE. A bell-shaped curve that results when a normal distribution is graphed.

NORMAL DISTRIBUTION. The symmetrical clustering of values around a central location. The properties of a normal distribution include the following: (1) It is a continuous, symmetrical distribution; both tails extend to infinity; (2) the arithmetic mean, mode, and median are identical; and, (3) its shape is completely determined by the mean and standard deviation.

NUMERATOR. The upper portion of a fraction.

O

OBSERVATIONAL STUDY. Epidemiological study in situations where nature is allowed to take its course. Changes or differences in one characteristic are studied in relation to changes or differences in others, without the intervention of the investigator.

ODDS RATIO. A measure of association which quantifies the relationship between an exposure and health outcome from a comparative study; also known as the cross-product ratio.

ORDINAL SCALE. Classification into ordered qualitative categories; e.g., social class (I, II, III, etc.), where the values have a distinct order, but their categories are qualitative in that there is no natural (numerical) distance between their positive values.


OUTBREAK. Synonymous with epidemic. Sometimes the preferred word, as it may escape sensationalism associated with the word epidemic. Alternatively, a localized as opposed to generalized epidemic.

P

PANDEMIC. An epidemic occurring over a very wide area (several countries or continents) and usually affecting a large proportion of the population.

PATHOGENICITY. The proportion of persons infected, after exposure to a causative agent, who then develop clinical disease.

PERCENTILE. The set of numbers from 0 to 100 that divide a distribution into 100 parts of equal area, or divide a set of ranked data into 100 class intervals with each interval containing 1/100 of the observations. A particular percentile, say the 5th percentile, is a cut point with 5 percent of the observations below it and the remaining 95% of the observations above it.

PERIOD PREVALENCE. The amount a particular disease present in a population over a period of time.

PERSON-TIME RATE. A measure of the incidence rate of an event, e.g., a disease or death, in a population at risk over an observed period to time, that directly incorporates time into the denominator.

PIE CHART. A circular chart in which the size of each ``slice'' is proportional to the frequency of each category of a variable.

POINT PREVALENCE. The amount of a particular disease present in a population at a single point in time.

POPULATION. The total number of inhabitants of a given area or country. In sampling, the population may refer to the units from which the sample is drawn, not necessarily the total population of people.

PREDICTIVE VALUE POSITIVE. A measure of the predictive value of a reported case or epidemic; the proportion of cases reported by a surveillance system or classified by a case definition which are true cases.

PREVALENCE. The number or proportion of cases or events or conditions in a given population.

PREVALENCE RATE. The proportion of persons in a population who have a particular disease or attribute at a specified point in time or over a specified period of time.

PROPAGATED OUTBREAK. An outbreak that does not have a common source, but instead spreads from person to person.

PROPORTION. A type of ratio in which the numerator is included in the denominator. The ratio of a part to the whole, expressed as a ``decimal fraction'' (e.g., 0.2), as a fraction (1/5), or, loosely, as a percentage (20%).

PROPORTIONATE MORTALITY. The proportion of deaths in a specified population over a period of time attributable to different causes. Each cause is expressed as a percentage of all deaths, and the sum of the causes must add to 100%. These proportions are not mortality rates, since the denominator is all deaths, not the population in which the deaths occurred.


PUBLIC HEALTH SURVEILLANCE. The systematic collection, analysis, interpretation, and dissemination of health data on an ongoing basis, to gain knowledge of the pattern of disease occurrence and potential in a community, in order to control and prevent disease in the community.

R

RACE-SPECIFIC MORTALITY RATE. A mortality rate limited to a specified racial group. Both numerator and denominator are limited to the specified group.

RANDOM SAMPLE. A sample derived by selecting individuals such that each individual has the same probability of selection.

RANGE. In statistics, the difference between the largest and smallest values in a distribution. In common use, the span of values from smallest to largest.

RATE. An expression of the frequency with which an event occurs in a defined population.

RATE RATIO. A comparison of two groups in terms of incidence rates, person-time rates, or mortality rates.

RATIO. The value obtained by dividing one quantity by another.

RELATIVE RISK. A comparison of the risk of some health-related event such as disease or death in two groups.

REPRESENTATIVE SAMPLE. A sample whose characteristics correspond to those of the original population or reference population.

RESERVOIR. The habitat in which an infectious agent normally lives, grows and multiplies; reservoirs include human reservoirs, animal reservoirs, and environmental reservoirs.

RISK. The probability that an event will occur, e.g. that an individual will become ill or die within a stated period of time or age.

RISK FACTOR. An aspect of personal behavior or lifestyle, an environmental exposure, or an inborn or inherited characteristic that is associated with an increased occurrence of disease or other health-related event or condition.

RISK RATIO. A comparison of the risk of some health-related event such as disease or death in two groups.

S

SAMPLE. A selected subset of a population. A sample may be random or non-random and it may be representative or non-representative.

SCATTER DIAGRAM. A graph in which each dot represents paired values for two continuous variables, with the x-axis representing one variable and the y-axis representing the other; used to display the relationship between the two variables; also called a scattergram.


SEASONALITY. Change in physiological status or in disease occurrence that conforms to a regular seasonal pattern.

SECONDARY ATTACK RATE. A measure of the frequency of new cases of a disease among the contacts of known cases.

SECULAR TREND. Changes over a long period of time, generally years or decades.

SENSITIVITY. The ability of a system to detect epidemics and other changes in disease occurrence. The proportion of persons with disease who are correctly identified by a screening test or case definition as having disease.

SENTINEL SURVEILLANCE. A surveillance system in which a pre-arranged sample of reporting sources agrees to report all cases of one or more notifiable conditions.

SEX-SPECIFIC MORTALITY RATE. A mortality rate among either males or females.

SKEWED. A distribution that is asymmetrical.

SPECIFICITY. The proportion of persons without disease who are correctly identified by a screening test or case definition as not having disease.

SPORADIC. A disease that occurs infrequently and irregularly.

SPOT MAP. A map that indicates the location of each case of a rare disease or outbreak by a place that is potentially relevant to the health event being investigated, such as where each case lived or worked.

STANDARD DEVIATION. The most widely used measure of dispersion of a frequency distribution, equal to the positive square root of the variance.

STANDARD ERROR (OF THE MEAN). The standard deviation of a theoretical distribution of sample means about the true population mean.

SUFFICIENT CAUSE. A causal factor or collection of factors whose presence is always followed by the occurrence of the effect (of disease).

SURVEILLANCE. see PUBLIC HEALTH SURVEILLANCE

SURVIVAL CURVE. A curve that starts at 100% of the study population and shows the percentage of the population still surviving at successive times for as long as information is available. May be applied not only to survival as such, but also to the persistence of freedom from a disease, or complication or some other endpoint.

T

TABLE. A set of data arranged in rows and columns.

TABLE SHELL. A table that is complete except for the data.

TRANSMISSION OF INFECTION. Any mode or mechanism by which an infectious agent is spread through the environment or to another person.


TREND. A long-term movement or change in frequency, usually upwards or downwards.

U

UNIVERSAL PRECAUTIONS. Recommendations issued by CDC to minimize the risk of transmission of bloodborne pathogens, particularly HIV and HBV, by health care and public safety workers. Barrier precautions are to be used to prevent exposure to blood and certain body fluids of all patients.

V

VALIDITY. The degree to which a measurement actually measures or detects what it is supposed to measure.

VARIABLE. Any characteristic or attribute that can be measured.

VARIANCE. A measure of the dispersion shown by a set of observations, defined by the sum of the squares of deviations from the mean, divided by the number of degrees of freedom in the set of observations.

VECTOR. An animate intermediary in the indirect transmission of an agent that carries the agent from a reservoir to a susceptible host.

VEHICLE. An inanimate intermediary in the indirect transmission of an agent that carries the agent from a reservoir to a susceptible host.

VIRULENCE. The proportion of persons with clinical disease, who after becoming infected, become severely ill or die.

VITAL STATISTICS. Systematically tabulated information about births, marriages, divorces, and deaths, based on registration of these vital events.

Y

YEARS OF POTENTIAL LIFE LOST. A measure of the impact of premature mortality on a population, calculated as the sum of the differences between some predetermined minimum or desired life span and the age of death for individuals who died earlier than that predetermined age.

Z

ZOONOSES. An infectious disease that is transmissible under normal conditions from animals to humans.

The definitions given are valid as they are used in this publication but different definitions may be used in other contexts. A Dictionary of Epidemiology, Second Edition, edited by J.M. Last for the International Epidemiological Association and published by Oxford University Press, 1988, was helpful in providing a number of the definitions.


Questions and Answer Key

1. Which of the following is not one of the three types of epidemiological data? (Plog, 138)

a. Descriptiveb. Retrospectivec. Prospectived. Proactive

2. These epidemiological studies identify a change or difference in the prevalence of a disease in a subgroup of the population: (Plog, 138)

a. Descriptive studiesb. Retrospective studiesc. Prospective studiesd. Proactive studies

3. These epidemiological studies reveal a relationship between a chemical and a certain effect caused by exposure that occurred months or years before the initiation of data collection: (Plog, 138)

a. Descriptive or cross sectional studiesb. Retrospective or control group studiesc. Prospective or cohort studiesd. Proactive or reactive studies

4. These epidemiological studies can define more precisely the time relationship and the magnitude of risk: (Plog, 138)


5. These epidemiological studies are present and future continuing studies that measure health effects as the exposures occur in work areas: (Plog, 138)


6. Analysis that reveals the relationship between time of occurrence of an adverse effect and age at the time of first exposure: (Plog, 138)

a. Descriptive analysisb. Retrospective analysisc. Prospective analysisd. Epidemiological analysis


7. What should the removal of a chemical from the environment be followed by: (Plog, 138)

a. documentation that the chemical has been safely removedb. evidence of a decline in the frequency of the effectc. assurance that the chemical will not be reintroducedd. public relations campaign

8. What should the removal of a chemical from the environment be followed by: (Plog, 138)

a. documentation that the chemical has been safely removedb. evidence of a decline in the frequency of the effectc. assurance that the chemical will not be reintroducedd. public relations campaign

9. Which of the following is not considered one of the three primary reasons for underreporting of work-associated infections? (Plog, 406)

a. employee fear of reportingb. issues of liabilityc. rewards for underreportingd. employer refusal to report

10. Surveys have revealed that the most common routes of exposure to be all of the following, EXCEPT: (Plog, 406)

a. percutaneous inoculationb. inhalation of aerosols generated by work practicesc. ingestion of contaminated foodd. contact between hands and contaminated material

11. Surveys have revealed that the most common routes of exposure to be all of the following, EXCEPT: (Plog, 406)

a. subcutaneous absorptionb. inhalation of aerosols generated by accidentsc. ingestion of contaminated materiald. contact between mucous membranes and contaminated material

12. Studies indicate that people engaged in these activities have acquired the greatest number of infections: (Plog, 406)

a. constructionb. healthcarec. researchd. agriculture

13. Which one of the following is not one of the most frequently reported laboratory-acquired infections, according to the Pike studies? (Plog, 406)

a. brucellosisb. Q feverc. hepatitisd. HIV


14. Which one of the following is not one of the most frequently reported laboratory-acquired infections, according to the Pike studies? (Plog, 406)


15. Which of the following infections is an emerging virus found in the workplace? (Plog, 406)


16. This method of evaluating the nature and severity of hazards reviews literature of a population-based research that may provide information about adverse health effects not yet noticed in a small work force: (Plog, 454)

a. Process or operation analysisb. Variability of responsec. Epidemiological and risk assessmentd. Interviews and maintenance activities

17. According to Leon Gordis, each of the following is an important objective of the science of epidemiology, EXCEPT: (Gordis, 4)

a. To provide a foundation for developing regulationsb. To discover the etiology of an occupational disease,c. Determine the extent to which the disease if found in the workplaced. Evaluate the degree to which occupational disease impacts productivity

18. According to Leon Gordis, each of the following is an important objective of the science of epidemiology, EXCEPT: (2) , (3) to study the natural history and prognosis of a disease, (4) , and (5) provide a foundation for developing regulations. (Gordis, 4)

a. To study the natural history and prognosis of a diseaseb. To discover the etiology of an occupational disease,c. Determine the extent to which the animals mirror human etiologyd. Evaluate existing and new therapeutic and preventive measures


Area III. Topic E. Epidemiology

1. d. Proactive2. a. Descriptive studies3. b. Retrospective or control group studies4. c. Prospective studies5. c. Prospective studies6. d. Epidemiological analysis7. b. evidence of a decline in the frequency of the

effect8. b. evidence of a decline in the frequency of the

effect9. c. rewards for underreporting10. d. contact between hands and contaminated

material

11. a. subcutaneous absorption12. c. research13. d. HIV14. d. HIV15. c. Epidemiological and risk assessment16. d. Evaluate the degree to which occupational

disease impacts productivity17. a. To determine effects of toxins on control

populations of animals18. c. Determine the extent to which the animals

mirror human etiology

c s h m - certisafety.com · web viewoutbreak. synonymous with epidemic. sometimes the preferred...

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