The objectives of epidemiology include the following:
- to identify the etiology or cause of disease
- to determine the extent of disease
- to study the progression of disease
- to evaluate preventive and therapeutic measures for a disease or condition
- to develop public health policy
Causality in Epidemiology
One objective of epidemiology is to identify the cause of a disease, with a desire to prevent or modify the severity of the condition. Consider Table 1. Would you agree that this table accurately portrays the true causes of death in the U.S. population? Why or why not?
As you may have noticed, the causes of death in Table 1 are all related to modifiable factors. The percentages do not total 100, but if these results are accurate, a large percentage of deaths can be postponed. The opportunity to prevent or ameliorate disease is an exciting component of epidemiologic study.
Epidemiologists follow pre-determined procedures in deciding whether to attribute a particular factor as a cause of a disease or condition. In the late 19th century, a German microbiologist, Robert Koch, devised a scheme for deciding whether or not a paricular microbe caused a disease.
- Infectious Disease Model
- Koch's Postulates: One organism leads to one disease. (one-to-one)
- A specific organism must be always be observed in association with the disease. (regular presence)
- The organism must be isolated from an infected host and grown in pure culture in the laboratory. (exclusive presence)
- When organisms from the pure culture are inoculated into a susceptible host organism, it must cause the disease. (sufficient cause)
- The infectious organism must be re-isolated from the diseased organism and grown in pure culture.
Do you see any problem with applying Koch's postulates to determine the cause of all diseases? Consider asthma or lung cancer: can one micro-organism be isolated as causing the development of these conditions?
- Modern Epidemiology
Modern epidemiology accomodates multiple exposures contributing to increasing risk for one disease (many-to-one) and situations where one risk factor contributes to multiple diseases (one-to-many). Individual risk factors can be characterized as one of the following:Necessary and sufficient (e.g., Koch's postulates) Necessary but not sufficient (e.g., adenoma as risk for colon cancer; high-risk HPV for cervical cancer) Sufficient but not necessary (e.g., childhood sun burns as risk for melanoma) Neither sufficient nor necessary (e.g., age as risk for breast cancer)
Figure 4 below is an example of a multifactorial model for disease.
From Warnecke RB, et al.. Approaching health disparities from a population perspective: the National Institutes of Health Centers for Population Health and Health Disparities. Am J Public Health 2008; 98: 1608– 1615
Considerations When Assessing Possible Causal Role of a Risk Factor for Specific Disease
Obviously, there are many factors to assess when considering whether a potential risk factor causes a disease or condition:
- How strong is the association? (odds ratio, relative risk)
- Is there a dose-response relationship?
- If exposure ceases, what happens? Does the condition change?
- Can the findings be replicated?
- Is there biologic plausibility?
- Are there alternative explanations?
- How specific is the association?
- Is this consistent with other knowledge?
- Is there statistical association? If so, is the association
- Spurious, due to chance or bias
- Non-causal OR
- Causal ?
- Is a temporal relationshipobserved?
- Wss the study design adequate?