Unit 1 Flashcards
Epi
about or upon
Demos
populace or people
Logos
discoursing, writing, or talking about
Epidemiology
the study of disease in populations, with intention to institute control
List the 4 aims of epidemiology:
describe, explain, predict, control
We are not administering a fixed treatment, and we don’t control the exposure factor (temperature). We are not randomizing the animals.
Observational study
Principle type of epidemiologic study:
observational
What are the main types of observational studies?
cohort, case-control, and prevalence
What is action of primary care:
intended to prevent disease
What is the action of secondary care?
detect disease early with intention to reduce impact
What is the action of tertiary care?
extend/improve life after diagnosis
List the steps of the generalized disease pathway?
induction –> incubation period –> signs
Subjects are randomized to treatment, and receive specific treatments (randomization and control)
true experiments
Like a true experiment except no randomization (control without randomization)
quasi-experiments
Neither randomization nor control; subjects self select their treatment
observational studies
Measures of disease frequency:
rate, risk
What is the simplest measure of epidemiology?
count of cases
What type of frequency are we typically interested in?
relative frequency
Mathematically, what do we commonly use for measuring frequencies?
proportions, ratios, rates
What are the 2 ways to express incidence?
- incidence rate (incidence density)
- cumulative incidence (risk, incidence proportion, attack rate)
A person can be an incident case only:
once
Indicates the movement from Well to Diseased:
Incidence
An expression to describe a change in one quantity with respect to another quantity with the denominator featuring a time component:
rate
Denominators for rates are in:
person-time/animal-time
What is the equation for incidence rate?
IR = (# new cases over a pd of time)/ (length of time at risk of developing disease)
When counting animal-time (time at risk), only count time of:
non-diseased animals
Count time for animals until:
- animal gets disease
- death from another cause
- removed from herd
- study terminates
- intervention to render it non-susceptible
Proportion of non-disease individuals at the beginning of a period of study that become disease during the period:
simple cumulative incidence
means they can get the condition:
“at risk”
What’s the equation for cumulative incidence?
CI = (# of new cases over a period of time)/(number of healthy animals at beginning)
A cumulative incidence rate for an outbreak:
attack rate
When is it appropriate to measure attack rate?
when the exposure occurs in a very short and defined period of time
Attack rate =
(# of new cases)/(number of individuals exposed at the START)
Why is IR used less often than CI?
harder to interpret
IR is applicable to a group, while CI is applicable to:
a group or an individual
What is incidence?
When an animal goes from healthy to sick
How is incidence measured?
- can be measures as a rate with time component in the denominator
- risk (fraction from 0-1)
What does incidence represent?
force of disease
Prevalence at a given moment in time, generally the default understanding of prevalence
point prevalence
number of cases identified over a period of time (1 year = annual prevalence):
period prevalence
Prevalence can change based on:
the time that we choose
Why is the other name for “prevalence”, a cross sectional study?
measure can be thought of as a “cross section” of a population
Diseases with long duration can have high prevalence if:
they are not fatal
Incidence can change dramatically, but why may prevalence not change?
if disease has long duration
Diseases that have very short durations, or are highly fatal, will have:
prevalence near 0
What does prevalence convey?
the relative need for follow-up care for those affected with a specific disease
Useful when assessing primary control measures:
indicence
Useful to assess the amount of effort needed delivering secondary and tertiary control:
prevalence
Incidence or Prevalence: Which is more useful in evaluating biosecurity/infection control?
incidence
- Have time in the denominator
- Apply to a group
- Are scientifically precise
rates (incidence density)
- expressed as a proportion
- applies to a group or individal
- commonly used, may not account for withdrawals
cumulative incidence (risk)
Weighted averages of the strata specific rates and strata specific populations
crude risks (be they death or other)
Important strata to consider in epidemiology:
age, breed, sex, production cycle
List the steps of the infectious disease events:
- encounter
- entry
- spread
- multiplication
- damage
- outcome (restart)
the ease with which a disease agent in spread within a population
communicability
Transmission requires:
a susceptible host, an agent, and the “right” environment to establish an effective contact
Effective contact is:
- contact between infected and susceptible animals
- contact the results in transmission of disease agent
Predicts the number of new cases in the next incubation period interval:
reed frost model
While a predominately conceptual model, is it very helpful with envisioning the primary elements in an infectious disease outbreak as well as a good estimate in endemic diseases:
Reed Frost Model
Cases in the time t+1, where 1 represents an interval of time equal to the incubation period of the agent (reed frost)
C(t+1)
Number of susceptible at time t (reed frost)
St
the probability of no contact (reed frost)
q
Cases at time t
Ct
Each individual in the reed frost model is in one of three categories:
S, I, R
Average number of secondary cases that develop from one primary case during its entire communicable period in a population of susceptible hosts
R0 (R naught)
R0 =
(# of contacts per unit time) X (probability of transmission per contact) X (duration of infectiousness)
If R0 is > 1:
the infection is maintained
If R0 is < 1:
the infection cannot be maintained
The ability to protect susceptible individuals within a group due to the high proportion of immune individuals in the same population
herd immunity
Reduces the number of susceptible (St) and alters the probability of transmission per contact/effective contact:
resistance
an increased or unexpected occurrence of cases
epidemic
a normal or expected number of cases in time and space
endemic
epidemic over a large area - country or worldwide
pandemic
new cases, force of disease, probability of becoming a case
incidence
cases at a point in time, “commonness” of disease, probability of being a case
Prevalence
Visual display of disease occurrence in time
epidemic curve
What are the two basic types of epidemics:
common source, propagated
Common-Source Epidemic:
results from exposure to a common source (such as contaminated food or water)
Point (or point-source) epidemic:
exposure is sudden and brief (potato salad at a picnic)
Extended Common Source Epidemic:
exposure period is prolonged
The initial slope of the epidemic curve is stepper when the animal density is:
higher
The initial slope of the epidemic curve is steeper when the infectious period is:
longer
What does the slope of the curve suggest?
efficiency of transmission within the population
What’s an example of a short term trend?
epidemic
What’s an example of a cyclical trend?
- seasonal
- periodic changes in susceptible host population
