Section 1: Definition of Epidemiology

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Lesson 1: Introduction to Epidemiology

In fact, epidemiology is often described as the basic science of public health, and for good reason. First, epidemiology is a quantitative discipline that relies on a working knowledge of probability, statistics, and sound research methods. Second, epidemiology is a method of causal reasoning based on developing and testing hypotheses grounded in such scientific fields as biology, behavioral sciences, physics, and ergonomics to explain health-related behaviors, states, and events. However, epidemiology is not just a research activity but an integral component of public health, providing the foundation for directing practical and appropriate public health action based on this science and causal reasoning.(2)

A. Distribution

Epidemiology is concerned with the frequency and pattern of health events in a population:

a. Frequency refers not only to the number of health events such as the number of cases of meningitis or diabetes in a population, but also to the relationship of that number to the size of the population. The resulting rate allows epidemiologists to compare disease occurrence across different populations.

b. Pattern refers to the occurrence of health-related events by time, place, and person. Time patterns may be annual, seasonal, weekly, daily, hourly, weekday versus weekend, or any other breakdown of time that may influence disease or injury occurrence. Place patterns include geographic variation, urban/rural differences, and location of work sites or schools. Personal characteristics include demographic factors which may be related to risk of illness, injury, or disability such as age, sex, marital status, and socioeconomic status, as well as behaviors and environmental exposures.

Characterizing health events by time, place, and person are activities of descriptive epidemiology, discussed in more detail later in this lesson.

B. Determinants

Determinant: any factor, whether event, characteristic, or other definable entity, that brings about a change in a health condition or other defined characteristic.

Epidemiology is also used to search for determinants, which are the causes and other factors that influence the occurrence of disease and other health-related events. Epidemiologists assume that illness does not occur randomly in a population, but happens only when the right accumulation of risk factors or determinants exists in an individual. To search for these determinants, epidemiologists use analytic epidemiology or epidemiologic studies to provide the “Why” and “How” of such events. They assess whether groups with different rates of disease differ in their demographic characteristics, genetic or immunologic make-up, behaviors, environmental exposures, or other so-called potential risk factors. Ideally, the findings provide sufficient evidence to direct prompt and effective public health control and prevention measures.

C. Application

Epidemiology is not just “the study of” health in a population; it also involves applying the knowledge gained by the studies to community-based practice. Like the practice of medicine, the practice of epidemiology is both a science and an art. To make the proper diagnosis and prescribe appropriate treatment for a patient, the clinician combines medical (scientific) knowledge with experience, clinical judgment, and understanding of the patient. Similarly, the epidemiologist uses the scientific methods of descriptive and analytic epidemiology as well as experience, epidemiologic judgment, and understanding of local conditions in “diagnosing” the health of a community and proposing appropriate, practical, and acceptable public health interventions to control and prevent disease in the community.

Challenge 1: Below are three key terms taken from the definition of epidemiology, followed by a list of activities that an epidemiologist might perform. Match the term to the activity that best describes it. You should match only one term per activity.

Distribution

Determinants

Application

1. ____ Recommend that close contacts of a child recently reported with Hepatitis A receive a an injection of gamma-globulin.

2. Salmonella and those without.

____ Compare food histories between persons with a food poisoning caused by

3. ____ Graph the number of cases of congenital HIV infections by year for the country

4. ____ Compare frequency of brain cancer among anatomists with frequency in general population

5 ____ Tabulate the frequency of clinical signs, symptoms, and laboratory findings among children with chickenpox in Bridgeport, CT.

6. ____ Mark on a map the residences of all children suffering from with leukemia within 2 miles of a hospital incinerator.

Section 2: Historical Evolution of Epidemiology

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Read the following passage and answer the questions below.

In the mid-1800s, an anesthesiologist named John Snow 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, Snow 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, two of his investigations will be described in detail.

Snow conducted one of his now famous studies in 1854 when an epidemic of cholera erupted in the Golden Square of London.(5) He began his investigation by determining where in this area persons with cholera lived and worked. He marked each residence on a map of the area, as shown in Figure 1.1. Today, this type of map, showing the geographic distribution of cases, is called a spot map.

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

Figure 1.1 Spot map of deaths from cholera in Golden Square area, London, 1854 (redrawn from original)

Source: Snow J. Snow on cholera. London: Humphrey Milford: Oxford University Press; 1936.

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 Figure 1.1 Spot map of deaths from cholera in Golden Square area, London, 1854 (redrawn from original) 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.

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.

Briefly explain (in 2-3 sentences) how epidemiologist John Snow discovered the origin of the cholera outbreak in 1854, London.

Look at the map. From this information, Snow was able to deduce the primary source of contaminated water and the most likely source of infection for most persons with cholera in the Golden Square area. How did he make this determination?

He noted with curiosity, however, that no cases of cholera had occurred in one area in the Golden Square area. 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. Where was the brewery?

Section 4: Core Epidemiologic Functions

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The following are Core Functions used by epidemiologists to solve epidemics. An example of each is given below.

Public health surveillance

Field investigation

Analytic studies

Evaluation

E. Linkages

F. Policy development

A. Public health surveillance Reviewing reports of test results for Chlamydia trachomatis from public health clinics

B. Field investigation Interviewing persons infected with Chlamydia to identify their sex partners

C. Analytic studies Comparing persons with symptomatic versus asymptomatic Chlamydia infection to iden- tify predictors

D. Evaluation Conducting an analysis of patient flow at the public health clinic to determine waiting times for clinic patients

E. Linkages Meeting with directors of family planning clinics and college health clinics to discuss Chlamydia testing and reporting

F. Policy development

Developing guidelines/criteria about which patients coming to the clinic should be screened (tested) for Chlamydia infection

Challenge 3: Your goal is to go back to Section 2 above, read the description of John Snow’s research on the cholera epidemic. Now identify an example of each of these core epidemiological functions that Snow used in his study. Choose 3 of the 6 Core Functions.