Epidemiology Module 2

Making individual decisions

Many individuals may not realize that they use epidemiologic information to make daily decisions affecting their health. When persons decide to quit smoking, climb the stairs rather than wait for an elevator, eat a salad rather than a cheeseburger with fries for lunch, or use a condom, they may be influenced, consciously or unconsciously, by epidemiologists' assessment of risk. Since World War II, epidemiologists have provided information related to all those decisions. In the 1950s, epidemiologists reported the increased risk of lung cancer among smokers. In the 1970s, epidemiologists documented the role of exercise and proper diet in reducing the risk of heart disease. In the mid-1980s, epidemiologists identified the increased risk of HIV infection associated with certain sexual and drug-related behaviors. These and hundreds of other epidemiologic findings are directly relevant to the choices people make every day, choices that affect their health over a lifetime.

Searching for causes

Much epidemiologic research is devoted to searching for causal factors that influence one's risk of disease. Ideally, the goal is to identify a cause so that appropriate public health action might be taken. One can argue that epidemiology can never prove a causal relationship between an exposure and a disease, since much of epidemiology is based on ecologic reasoning. Nevertheless, epidemiology often provides enough information to support effective action. Examples date from the removal of the handle from the Broad St. pump following John Snow's investigation of cholera in the Golden Square area of London in 1854,5 to the withdrawal of a vaccine against rotavirus in 1999 after epidemiologists found that it increased the risk of intussusception, a potentially life-threatening condition.15 Just as often, epidemiology and laboratory science converge to provide the evidence needed to establish causation. For example, epidemiologists were able to identify a variety of risk factors during an outbreak of pneumonia among persons attending the American Legion Convention in Philadelphia in 1976, even though the Legionnaires' bacillus was not identified in the laboratory from lung tissue of a person who had died from Legionnaires' disease until almost 6 months later.16

experiment

researcher is manipulating something

methodolgy

tools for identifying and categorizing knowledge

Dr. John Snow

used non-medical means to discover source of cholera outbreak in 1854.

exposure

variable that may predict/cause an outcome

independent variable

variable that predicts/causes an outcome

Miasma

was considered to be a poisonous vapor or mist filled with particles from decomposed mater that caused illness, was identifiable by its foul smell. Found to not be real.

John Graunt

1662- a London haberdasher and councilman who published a landmark analysis of mortality data in 1662- publication was the first to quantify patterns of birth, death, and disease occurrence, noting disparities between males and females, high infant mortality, urban/rural differences, and seasonal variations

John Snow

1854- anesthesiologist was conducting a series of investigations in London that warrant his being considered the "father of field epidemiology." Twenty years before the development of the microscope, he conducted studies of cholera outbreaks both to discover the cause of disease and to prevent its recurrence. Because his work illustrates the classic sequence from descriptive epidemiology to hypothesis generation to hypothesis testing (analytic epidemiology) to application.

William Farr

1880- built upon Graunt's work by systematically collecting and analyzing Britain's mortality statistics, considered the father or modern vital statistics and surveillance, developed many of the best practices used today in vital statistics and disease classification, concentrated his efforts on collecting vital statistics, assembling and evaluating data, and reporting to responsible health authorities and the general public

Hippocrates

400 B.C.- attempted to explain disease occurrence from a rational rather than a supernatural viewpoint- essay entitled "On Airs, Waters, and Places" he suggest environmental and host factors such as behaviors might influence the development of disease

The source of Cholera

Because Snow believed that water was a source of infection for cholera, he marked the location of water pumps on his spot map, then looked for a relationship between the distribution of households with cases of cholera and the location of pumps. He noticed that more case households clustered around Pump A, the Broad Street pump, than around Pump B or C. When he questioned residents who lived in the Golden Square area, he was told that they avoided Pump B because it was grossly contaminated, and that Pump C was located too inconveniently for most of them. From this information, Snow concluded that the Broad Street pump (Pump A) was the primary source of water and the most likely source of infection for most persons with cholera in the Golden Square area. He noted with curiosity, however, that no cases of cholera had occurred in a two-block area just to the east of the Broad Street pump. Upon investigating, Snow found a brewery located there with a deep well on the premises. Brewery workers got their water from this well, and also received a daily portion of malt liquor. Access to these uncontaminated rations could explain why none of the brewery's employees contracted cholera.

Users and Assessing the Community's health

Epidemiology and the information generated by epidemiologic methods have been used in many ways.9 Some common uses are described below. Assessing the community's health Public health officials responsible for policy development, implementation, and evaluation use epidemiologic information as a factual framework for decision making. To assess the health of a population or community, relevant sources of data must be identified and analyzed by person, place, and time (descriptive epidemiology). • What are the actual and potential health problems in the community? • Where are they occurring? • Which populations are at increased risk? • Which problems have declined over time? • Which ones are increasing or have the potential to increase? • How do these patterns relate to the level and distribution of public health services available? More detailed data may need to be collected and analyzed to determine whether health services are available, accessible, effective, and efficient. For example, public health officials used epidemiologic data and methods to identify baselines, to set health goals for the nation in 2000 and 2010, and to monitor progress toward these goals.10-12

herd immunity

Global eradication was possible because of

1980's

In the 1980s, epidemiology was extended to the studies of injuries and violence. In the 1990s, the related fields of molecular and genetic epidemiology (expansion of epidemiology to look at specific pathways, molecules and genes that influence risk of developing disease) took root. Meanwhile, infectious diseases continued to challenge epidemiologists as new infectious agents emerged (Ebola virus, Human Immunodeficiency virus (HIV)/ Acquired Immunodeficiency Syndrome (AIDS)), were identified (Legionella, Severe Acute Respiratory Syndrome (SARS)), or changed (drug-resistant Mycobacterium tuberculosis, Avian influenza). Beginning in the 1990s and accelerating after the terrorist attacks of September 11, 2001, epidemiologists have had to consider not only natural transmission of infectious organisms but also deliberate spread through biologic warfare and bioterrorism. Today, public health workers throughout the world accept and use epidemiology regularly to characterize the health of their communities and to solve day-to-day problems, large and small.

19th and 20th centuries

In the mid- and late-1800s, epidemiological methods began to be applied in the investigation of disease occurrence. At that time, most investigators focused on acute infectious diseases. In the 1930s and 1940s, epidemiologists extended their methods to noninfectious diseases. The period since World War II has seen an explosion in the development of research methods and the theoretical underpinnings of epidemiology. Epidemiology has been applied to the entire range of health-related outcomes, behaviors, and even knowledge and attitudes. The studies by Doll and Hill linking lung cancer to smoking6and the study of cardiovascular disease among residents of Framingham, Massachusetts7 are two examples of how pioneering researchers have applied epidemiologic methods to chronic disease since World War II. During the 1960s and early 1970s health workers applied epidemiologic methods to eradicate naturally occurring smallpox worldwide.8 This was an achievement in applied epidemiology of unprecedented proportions.

Cholera outbreak in London

Snow conducted one of his now famous studies in 1854 when an epidemic of cholera erupted in the Golden Square of London. He began his investigation by determining where in this area persons with cholera lived and worked.

Snow's Second Investigation

Snow's second investigation reexamined data from the 1854 cholera outbreak in London. During a cholera epidemic a few years earlier, Snow had noted that districts with the highest death rates were serviced by two water companies: the Lambeth Company and the Southwark and Vauxhall Company. At that time, both companies obtained water from the Thames River at intake points that were downstream from London and thus susceptible to contamination from London sewage, which was discharged directly into the Thames. To avoid contamination by London sewage, in 1852 the Lambeth Company moved its intake water works to a site on the Thames well upstream from London. Over a 7-week period during the summer of 1854, Snow compared cholera mortality among districts that received water from one or the other or both water companies.

Epidemiology

The study of the distribution and determinants of disease or health status in a population.

observational epidemiology

Uses information from individuals or small groups, determines the strength between a particular variable and disease.

Results of Snow's Study

This study, demonstrating a higher death rate from cholera among households served by the Southwark and Vauxhall Company in the mixed districts, added support to Snow's hypothesis. It also established the sequence of steps used by current-day epidemiologists to investigate outbreaks of disease. Based on a characterization of the cases and population at risk by time, place, and person, Snow developed a testable hypothesis. He then tested his hypothesis with a more rigorously designed study, ensuring that the groups to be compared were comparable. After this study, efforts to control the epidemic were directed at changing the location of the water intake of the Southwark and Vauxhall Company to avoid sources of contamination. Thus, with no knowledge of the existence of microorganisms, Snow demonstrated through epidemiologic studies that water could serve as a vehicle for transmitting cholera and that epidemiologic information could be used to direct prompt and appropriate public health action.

The Broad Street Pump

To confirm that the Broad Street pump was the source of the epidemic, Snow gathered information on where persons with cholera had obtained their water. Consumption of water from the Broad Street pump was the one common factor among the cholera patients. After Snow presented his findings to municipal officials, the handle of the pump was removed and the outbreak ended. The site of the pump is now marked by a plaque mounted on the wall outside of the appropriately named John Snow Pub.

Completing the clinical pictures

When investigating a disease outbreak, epidemiologists rely on health-care providers and laboratorians to establish the proper diagnosis of individual patients. But epidemiologists also contribute to physicians' understanding of the clinical picture and natural history of disease. For example, in late 1989, a physician saw three patients with unexplained eosinophilia (an increase in the number of a specific type of white blood cell called an eosinophil) and myalgias (severe muscle pains). Although the physician could not make a definitive diagnosis, he notified public health authorities. Within weeks, epidemiologists had identified enough other cases to characterize the spectrum and course of the illness that came to be known as eosinophilia-myalgia syndrome.13 More recently, epidemiologists, clinicians, and researchers around the world have collaborated to characterize SARS, a disease caused by a new type of coronavirus that emerged in China in late 2002.14 Epidemiology has also been instrumental in characterizing many non-acute diseases, such as the numerous conditions associated with cigarette smoking — from pulmonary and heart disease to lip, throat, and lung cancer.

paradigms

a set of assumptions, theories, and perspectives that make up a way of understanding social reality

research paradigms

define how the world works, how knowledge is extracted from this world, and how one is to think, write, and talk about this knowledge

risk factors

exposures that increase or decrease the likelihood of developing certain disorders, conditions, or diseases

epistemology

how we acquire and process knowledge

ontology

inquiry into how we define and experience reality