BIOL 437 Week Ten p.1 (Causality) Flashcards
when a causal association is established
-a protection and control attitude can occur vs. a reaction to the health crisis
cause
-a specific event, condition, or characteristic that precedes the health outcome and is necessary for its occurence
“necessary” cause
- if an environmental exposure is required for the outcome to occur
- without it, effect will not occur
“sufficient” cause
- if the health related event always occurs becuase of the exposure
- with it the effect will result regardless of the presence or absence of other factors
related terms to cause
- risk factor
- at-risk behaviour
- predisposing factor
historical perspecitve of causation
- central topic in philosophy of science
- concepts of disease causation change over time
1950s
-a common belief that we had “conquered” infectious diseases and knew there all there was to know
19th century decline in death rates due to infectious disease and malnutrition
- few decades before medical discoveries of scientific revolution became policies
- not realized until later on
- changes in sanitiation and living standards had a lot to do with the decline
18th century (1776)
-small pox inoculation stopped epidemics
Edward Jenner (1796)
-‘father of vaccination’
>demonstrated immunity to smallpox in people who had previously had cowpox
18th century yellow fever
- contagious
- imported
- quarantine of sick people and ships suspected of having it on board
Miasma theory (end of 18th century)
-all disease due to bad air
miasma theory dominated thinkin (early 19th century)
-prevention was aimed at eliminating miasma in slums and poor sanitation
>sanitation measures were very effective in decreasing death rates
Louis Pasteur
-existence of microorganisms (germ theory)
-inference to direct observation
-prevention: containing spread of microbes
-*germ theory replaced miasma theory= paradigm shift
>explained and predicted immunization and chemotherapy which miasma theory could not
germ theory lead to
- illnessa cuased by different agents
- ‘one disease one-cause model’
- ‘theory of specific etiology of disease’
- specific causation
concept of specificity
- still applied and led to many medical achievements (ex. worldwide eradication of smallpox)
- even when not due to micro-organisms is still valid (ex. James Lind: scurvy and vitamin C)
complexity of relationship between cause and effect
-not addresed to real world until late 1950s
events that put one-cause disease model into question
- opportunitistic infections
- pathogens can be present for a long period of time before causing disease
- shift from infectious to chronic disease (ex. cancer)
epidemiology triangle
-characterizes infectious disease causation by interaction and interdependance between agent, host, environment and time
agent
- a causative factor
ex. pathogen or chemical
host
- an organism
- usually a human
host factors
- intrinsic factors that influence exposure, susceptibility or response to causative agent
ex. race, age
environmental factors
- extrinsic factors that affect the agent and opportunity for exposure
ex. climate, geology, biological factors, psychosocial factors
epidemiological research now
- often examines single risk factors
- greatest impact during early investigation steps before causal info is known
- suspected factors increase the predictive power of the investigation
- one factor at a time method is key to risk factor epidemiology
unplanned or natural experiments
- take advantage of groups of people exposed for non-study purposes
ex. occupational cohorts in specific industries
approach steps to etiology
- Clinical observations
- Available data
- Case-control studies
- Cohort studies
- Randomized trials
determine whether there is an assoication or correlation
- studies of group characteristics (ecological studies)
- studies of individuals characteristics (cohort, case-control and other types of studies)
If association is determined
-determine whether the observed assocition is likely to be a causal one
causal relation attributes
- Association
- Time order
- Direction
association
-cause is assosicated with an effect
time order
-cause is present before or at same time as its effect
direction
-cause acts on its effect
direct causation
-a factor directly causes a disease without an intermediate step
indirect causation
-a factor causes a disease, but only throught an intermediate step or stpes
>intermediate steps are generally always present in causal process involving biology
Causal pies (Rothman)
-accounts for multifactorial nature of causation for many non-infectious diseases
pieces of the pie
-individual factors
*component causes
>intrinsic host factors
>agent factors
>environmental factors
>time
complete pie
- caussal path/mechanism that is “sufficent cause”
- a disease can have more than one suffient cause
a component in every pie or pathway
- is a ‘necessary cause’
* without it the disease cannot occur
disease prevention can be
-accomplished by blocking any single component of a sufficent cause (at least through that pathway)
4 types of causal relationships
- Necessary and sufficent
- Necessary but not sufficient
- Sufficeint but not necessary
- Neither sufficent or necessary
necessary and sufficient
- without the factor, the disease never develops (necessary)
- disease always develops in the presence of the factor (sufficent)
- this situation rarely occurs (one-to-one relationship)
ex. in most infectious diseases, a number of people are exposed, some of whom will manifest the disease and others who will not
necessary but not sufficent
- multiple factors are required, often in a specific temporal sequence
ex. carcinogenesis is a multistage process involving initation and promotion
suffiecent but not neccessary
-factor alone can produce the disease ,but other factors that are acting alone can too
ex. radiation exposure or benezene exposure can each produce leukaemia without the presence of the other
>but cancer does not develop in everyone who has experiened either one
neither sufficent or necessary
- more complex
- probably most accurately represents the causal relationships that operate in most chronic diseases
