Epidemiology Chapter 1

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Goals of Public Health

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To promote the health of the population through organized Community efforts , focusing on preventing illness in the community instead of the individual.

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Definition of epidemiology

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Epi means on or upon, and demos means the common people, and logy means study. Therefore the definition is the study of that which falls upon the common people. Also defined as:

The study of the distribution and determinants of disease frequency in human populations and the application of this study to control health problems

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Disease frequency

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How often a disease arises in a population

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Counting, which is a key activity of epidemiologists, includes three steps

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1) developing a definition of disease 2) instituting a mechanism for counting cases of disease within a specified population

3) determining the size of that population

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Sources of scientific knowledge in Public Health

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1: the basic Sciences such as pathology and toxicology 2: the clinical or medical Sciences such as internal medicine and Pediatrics 3: the Public Health Sciences such as epidemiology, Environmental Health Science, health education, and Behavioral Science.

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Disease distribution

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Disease distribution refers to the analysis of disease patterns according to the characteristics of person, place, and time. In other words, who is getting the disease, where is it occurring, and how is it changing over time?

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Disease determinants

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Disease determinants are factors that bring about a change in a person's health, that is, factors that either cause a healthy individual to become sick or cause a sick person to recover.

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Hypothesis

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A tentative explanation for an observation, phenomenon or scientific problem that can be tested by further investigation

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Disease Control

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Epidemiologists accomplish Disease Control through epidemiologic research and through surveillance.

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The purpose of surveillance

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To monitor aspects of disease occurrences that are pertinent to effective control

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The first epidemiologist

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John Graunt, by summarizing the bills of mortality for his 1662 publication entitled " natural and political observations mentioned in a following index, and made upon the bills of mortality"

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Bills of mortality

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A weekly count of people who died that had been conducted by the parish Clerks of London since 1592 because of concern about the plague.

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James Lind

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In the mid 1700, James Lind conducted one of the earliest experimental studies on the treatment of scurvy, a common disease and cause of death at the time

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The Hallmarks of designed studies are

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1: the comparison of at least two groups of individuals, an experimental group and a control group 2: the active manipulation of the factor or agent under study by the investigator, that is the investigator assigns individuals either to receive or not to receive a preventive or therapeutic measure.

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William Farr

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William Farr made many important advances in the field of epidemiology in the mid- 1800s and is now considered one of the founders of modern epidemiology.

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One of Farr's most important contributions:

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Calculations that combined registration data on births, marriages, and deaths as the numerator and census data on the population size as the denominator.

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John Snow

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Mid 1800s major contributor to epidemiological methods. A British physician. He was interested in the cause and spread of Cholera. He used data to support an Innovative hypothesis that cholera was an infectious disease spread by fecal contamination of drinking water.

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Farr made several practical and methodological contributions to the field of epidemiology. Name three.

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1: he constantly strove to ensure that the collected data were accurate and complete. 2: he devised a categorization system for the causes of death so that these data could be reduced to a usable form. The system that he devised is the antecedent of the modern International Classification of Diseases. 3: Farr made a number of important contributions to the analysis of data, including the invention of the "standardized mortality rate".

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Snow's 1849 pamphlet

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The Mode of Communication of Cholera.

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Snow's investigation of the Broad Street epidemic is noteworthy for at least three reasons :

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1: He was able to form a hypothesis implicating the Broad Street pump after he met the geographic distribution of the cholera deaths and studied that distribution in relation to the surrounding public water pumps. 2: he collected data on the drinking water habits of unaffected as well as affected individuals, which allowed him to make a comparison that would support or refute his hypothesis. 3: the results of his investigation were so convincing that they led to immediate action to curb the disease, namely the pump handle was removed.

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Three reasons why Snow's investigations are considered a nearly perfect model for epidemiologic research:

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1: snow organized his observations logically so that meaningful inferences could be derived from them. 2: he recognized that a natural experiment had occurred in the sub districts of London that would enable him to gather unquestionable proof either for or against his hypothesis. 3: he conducted a quantitative analysis of the data contrasting the occurrence of Cholera deaths in relation to the drinking water company.

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Name four innovations that made the streptomycin tuberculosis trial innovative.

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1: the use of randomization to assign patients in the trial. 2: the placement of restrictions on the type of patient eligible for the trial. 3: the data collection methods helped to ensure that the results would be free of bias. 4: the investigators considered the ethical issues involved in conducting the trial, including whether it was ethical to withhold the streptomycin treatment from the control group.

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Modern epidemiologists consider the 1950's Doll and Hill study on smoking and lung cancer to be...

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A classic exemplar for the investigation of a given outcome and an array of exposures. No previous research paper lays out the essentials of the case-controlled method with such understanding and meticulous care.

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The epitome of successful epidemiologic research, and the prototype and model of the cohort study is the...

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The Framingham study

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Define Public Health

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Public health is a multidisciplinary field whose goal is to promote the health of populations through organized Community efforts.

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Define epidemiology:

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Epidemiology is the study of the distribution and determinants of disease frequency in human populations and the application of this study to control health problems. Disease refers to a broad array of Health - related States and events including diseases, injuries, disabilities, and death

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Define population

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A population is a group of people with a common characteristic.

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Define disease frequency

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A measure of disease frequency quantifies how often a disease arises in a population. It's calculation involves establishing the disease definition, developing a mechanism for counting the diseased cases (the numerator), and determining the size of the underlying population (the denominator).

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Define disease distribution

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Disease distribution refers to the pattern of disease according to the characteristics of person (who is getting the disease ) place (where is it occurring? ) and time (how is it changing over time? ).

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Define disease determinant

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Disease determinants are factors that cause a healthy person to become sick or cause a sick person to recover.

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Define Disease Control

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Disease Control is the ultimate aim of epidemiology, and refers to the reduction or elimination of disease occurrence. It is accomplished through epidemiologic research and surveillance.

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What is the primary difference between public health and Medicine?

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Public Health focuses on preventing disease in communities, and Medicine focuses on treating diseases at the individual level.

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What are the main objectives of epidemiology?

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The main objectives of epidemiology are to study the natural course of disease, to determine the extent of disease in a population, to identify patterns and Trends in disease occurrence, to identify the causes of disease, and to evaluate the effectiveness of measures that prevent and treat disease.

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How do epidemiologists quantify the disease frequency in a population?

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Epidemiologists quantify the frequency of disease by developing a definition of the disease, instituting a mechanism for counting cases of disease within the population, and determining the size of that population. It is only when the number of cases are related to the size of the population that we know the true frequency of disease.

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John Graunt

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John Graunt summarized the patterns of mortality in 17th century London and discovered the regularity of deaths and births.

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John snow

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John Snow conducted one of the first observational studies in the neighborhoods of nineteenth-century London and discovered that contaminated drinking water was the cause of Cholera.

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Richard Doll &Austin Bradford Hill

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Conducted groundbreaking studies on cigarette smoking and lung cancer in the 1950s.

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James Lind

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Conducted one of the earliest experimental studies which was on the treatment of scurvy among Sailors. Using sound experimental principles, he found that the consumption of oranges and Lemons with the most effective remedies for scurvy in this population.

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William Farr

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William far was the compiler of Statistical Abstract in Great Britain from 1839 through 1880. In this capacity he pioneered many activities encompassed by modern epidemiology, including the calculation of mortality rates using census data for denominators.

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Nosology

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1: classifying of diseases: the branch of medicine concerned with the classification and description of known diseases. 2: classified list of diseases: a completed classification of known diseases.

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Epidemiologic premise

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Human disease:

1:Does not occur at random.

2:Has causal and preventative factors.

Such factors can be identified through a systematic investigation of different populations in different places or different times.

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What epidemiology does:

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1: measurement of disease frequency. (Quantification of the existence or occurrence of disease)

2: distribution of disease (who is getting disease in the population, where and when is disease occurring.)

3: determinants of disease (the final piece. This is derived from other components.

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Epidemiology is a comparative discipline

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It Is by making comparisons of the disease occurrences in different groups that we can develop and test hypotheses about the causes of disease

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Objective of epidemiology:

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To determine or judge whether an association between an exposure and the disease is causal. ( once it is determined that an exposure or characteristic is associated with increased disease, we must determine if the association is causal. If the association is by pure chance, then altering that exposure won't have any effect on the burden of disease)

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Epidemiology questions

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1: how many? 2: compared to what? 3: is it real?

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Purposes of epidemiology:

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Describe, explain, predict, control

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Koch's postulates

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1) the organism must be present in every case of the disease.

2) the organism must be able to be isolated and grown in pure culture.

3) the organism must, when inoculated into a susceptible animal, cause this specific disease.

4) the organism must be recovered from the infected animal, and again identified.

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The web of causation

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1) incorporates concepts of multifactorial causes.

2) implies that the cases of disease could be prevented by cutting only a 'few strands of the web'

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Models of causal relationships:

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1) necessary and sufficient. An example would be the necessary Force to break a bone is also sufficient to break the bone.

2) necessary but not sufficient. An example would be the breaking of a bone in a person with osteoporosis. The force may be necessary to break the bone, but not sufficient to break it unless osteoporosis is also present.

3) sufficient but not necessary. An example of this would be exposure to radon gas is sufficient to cause lung cancer, but it's not necessary to cause lung cancer

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In a causal pie, a sufficient cause is:

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A set of conditions without any one of which the disease would not have been able to occur.

In other words, it is the whole causal pie.

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In a causal pie a component cause:

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Any one of the set of conditions which are necessary for the completion of a sufficient cause. This is considered a piece of the causal pie.

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In a causal pie:

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The completion of a sufficient cause is synonymous with occurrence of disease, although not necessarily a diagnosis.

Component causes can act far apart in time.

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In a causal pie, a necessary cause is:

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A compliment cause that is a member of every sufficient causal pie.

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A component cause can be one of these two:

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1) the positive presence of a causative exposure

2) the lack of a preventitive exposure

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Blocking of any component in a causal pie can be done by

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1) preventing a causative exposure

2) providing a preventitive exposure

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When blocking of a component is obtained in a causal pie, the result is:

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A prevention of disease by that particular pathway

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Hill's guidelines of causality

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1) temporal relationship

2) strength of Association

3) dose - response

4) consistency

5) biologic plausibility, coherence

6) specificity

7) experiment

8) analogy

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Temporality causal criteria

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This is the most straightforward. Exposure to the causal Factor must happen before the disease starts. This is the only one of Hill's criteria that everyone agrees with.

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Association as a causal criteria

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If the outcome is very much more common in individuals who are exposed versus those who are not exposed, then it is much more likely to be causal. This describes a strong Association with a potential cause of disease

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The strength of Association:

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The larger the measure of Association the greater the likelihood that exposure is causing the disease. An example is that the risk of lung cancer for heavy smokers is 20 times the risk for non-smokers. This is an example of a strong Association between a potential cause and a disease

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Strong associations:

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These are unlikely to be due to bias and confounding.

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Weak associations

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These may also be causal, but it is harder to rule out bias and confounding.

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Dose Response as a causal criteria:

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This describes the biological phenomena in which increasing exposure results in increasingly higher risk of disease. An example is that lung cancer death rates increase with the number of cigarettes that an individual smokes.

Some exposures do not reach the criteria of a dose-response because of the "threshold affect". This describes a does under which exposure results in no adverse outcome.

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Consistency as a causal criteria:

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If many different studies produce the same or similar results then that is considered evidence that it is more likely to represent a causal relationship. If other investigations utilize different populations, places, times, circumstances, and investigators , and they get the same or similar results, it represents a high degree of consistency.

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Biological plausibility or coherence as a criteria for causality

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It's a plus if the association makes biological sense, but it is not required. If it does not make sense it could be that we just don't yet understand the biology involved.

Biological plausibility implies that Association does not conflict with current knowledge of natural history and biology of disease.

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Specificity as a causal criteria

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The idea is that a single exposure should cause a single disease. There are many exceptions. An example is that exposure to cigarette smoking can cause cancer of many types ( lungs, breast, etc), COPD, and intrauterine growth retardation.

If specificity is present, it is evidence of causality, but its absence does not preclude causation.

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Experiment as a causal criteria

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An investigator controlled change in exposure, such as prevention or treatment or removal, resulting in less disease.

An example is fluoridated drinking water and the effect it has on Dental caries

Experiments provide nice evidence when it can be gotten, but there are ethical issues.

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Analogy as a causal criteria

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This is the weakest criteria. This is described as a similar relationship observed with another exposure and or disease. An example is that both thalidomide and rubella can both lead to birth defects.

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How do we actually assess whether a risk factor is indeed causal?

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1) first, evaluate each individual study that we find in the literature

When a study is being evaluated, the focus should be on determining the validity of the study and its methods.

Regarding the collection of data, is a temporal relationship demonstrated?

Regarding analysis of data, how is the strength of Association? Is there a dose-response? Finally, how is the data being interpreted?

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Useful criteria in evaluating a study:

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What is the purpose of the research? What is the study design?

Who are the study subjects?

What are the definitions of outcomes and exposures?

What are potential sources of bias and how are they addressed?

What is the statistical analysis, e.g. measures of association?

Are the conclusions Justified?

Are there alternate explanations?

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Essentially, when a study is being evaluated, we are asking:

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Was this study well-conducted? How much credence can we place in the results that were found in this individual study?

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Once each individual study in the literature it has been evaluated then:

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Those that have been well-conducted can be considered as a whole. Are the studies consistent? Are the association's coherent? Does the literature include experimental data? Does the literature include analogous studies?

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Temporal relationship

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Exposure comes before outcome

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Strength of Association

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The measure of Association is large

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Dose response

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The risk increases with increasing levels of exposure

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Consistency

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Several studies using different populations and or methods find the same or similar relationship

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Biologic plausibility or coherence

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The association makes sense with what we know of the biology of the outcome

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Specificity

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There is a one-to-one relationship between the exposure and the disease. One exposure causes one specific disease.

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Experiment

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Animal studies or human experimental studies show this to be a causal relationship

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Analogy

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A similar relationship can be seen with another exposure and disease

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What factors comprise the epidemiological triangle?

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Environment, host, agent