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Flashcards in Unit 1 Deck (85):
1

Epi

about or upon

2

Demos

populace or people

3

Logos

discoursing, writing, or talking about

4

Epidemiology

the study of disease in populations, with intention to institute control

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List the 4 aims of epidemiology:

describe, explain, predict, control

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We are not administering a fixed treatment, and we don't control the exposure factor (temperature). We are not randomizing the animals.

Observational study

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Principle type of epidemiologic study:

observational

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What are the main types of observational studies?

cohort, case-control, and prevalence

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What is action of primary care:

intended to prevent disease

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What is the action of secondary care?

detect disease early with intention to reduce impact

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What is the action of tertiary care?

extend/improve life after diagnosis

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List the steps of the generalized disease pathway?

induction --> incubation period --> signs

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Subjects are randomized to treatment, and receive specific treatments (randomization and control)

true experiments

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Like a true experiment except no randomization (control without randomization)

quasi-experiments

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Neither randomization nor control; subjects self select their treatment

observational studies

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Measures of disease frequency:

rate, risk

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What is the simplest measure of epidemiology?

count of cases

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What type of frequency are we typically interested in?

relative frequency

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Mathematically, what do we commonly use for measuring frequencies?

proportions, ratios, rates

20

What are the 2 ways to express incidence?

- incidence rate (incidence density)
- cumulative incidence (risk, incidence proportion, attack rate)

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A person can be an incident case only:

once

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Indicates the movement from Well to Diseased:

Incidence

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An expression to describe a change in one quantity with respect to another quantity with the denominator featuring a time component:

rate

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Denominators for rates are in:

person-time/animal-time

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What is the equation for incidence rate?

IR = (# new cases over a pd of time)/ (length of time at risk of developing disease)

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When counting animal-time (time at risk), only count time of:

non-diseased animals

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Count time for animals until:

- animal gets disease
- death from another cause
- removed from herd
- study terminates
- intervention to render it non-susceptible

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Proportion of non-disease individuals at the beginning of a period of study that become disease during the period:

simple cumulative incidence

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means they can get the condition:

"at risk"

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What's the equation for cumulative incidence?

CI = (# of new cases over a period of time)/(number of healthy animals at beginning)

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A cumulative incidence rate for an outbreak:

attack rate

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When is it appropriate to measure attack rate?

when the exposure occurs in a very short and defined period of time

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Attack rate =

(# of new cases)/(number of individuals exposed at the START)

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Why is IR used less often than CI?

harder to interpret

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IR is applicable to a group, while CI is applicable to:

a group or an individual

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What is incidence?

When an animal goes from healthy to sick

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How is incidence measured?

- can be measures as a rate with time component in the denominator
- risk (fraction from 0-1)

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What does incidence represent?

force of disease

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Prevalence at a given moment in time, generally the default understanding of prevalence

point prevalence

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number of cases identified over a period of time (1 year = annual prevalence):

period prevalence

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Prevalence can change based on:

the time that we choose

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Why is the other name for "prevalence", a cross sectional study?

measure can be thought of as a "cross section" of a population

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Diseases with long duration can have high prevalence if:

they are not fatal

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Incidence can change dramatically, but why may prevalence not change?

if disease has long duration

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Diseases that have very short durations, or are highly fatal, will have:

prevalence near 0

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What does prevalence convey?

the relative need for follow-up care for those affected with a specific disease

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Useful when assessing primary control measures:

indicence

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Useful to assess the amount of effort needed delivering secondary and tertiary control:

prevalence

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Incidence or Prevalence: Which is more useful in evaluating biosecurity/infection control?

incidence

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- Have time in the denominator
- Apply to a group
- Are scientifically precise

rates (incidence density)

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- expressed as a proportion
- applies to a group or individal
- commonly used, may not account for withdrawals

cumulative incidence (risk)

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Weighted averages of the strata specific rates and strata specific populations

crude risks (be they death or other)

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Important strata to consider in epidemiology:

age, breed, sex, production cycle

54

List the steps of the infectious disease events:

1. encounter
2. entry
3. spread
4. multiplication
5. damage
6. outcome (restart)

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the ease with which a disease agent in spread within a population

communicability

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Transmission requires:

a susceptible host, an agent, and the "right" environment to establish an effective contact

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Effective contact is:

- contact between infected and susceptible animals
- contact the results in transmission of disease agent

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Predicts the number of new cases in the next incubation period interval:

reed frost model

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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

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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)

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Number of susceptible at time t (reed frost)

St

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the probability of no contact (reed frost)

q

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Cases at time t

Ct

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Each individual in the reed frost model is in one of three categories:

S, I, R

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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)

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R0 =

(# of contacts per unit time) X (probability of transmission per contact) X (duration of infectiousness)

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If R0 is > 1:

the infection is maintained

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If R0 is < 1:

the infection cannot be maintained

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The ability to protect susceptible individuals within a group due to the high proportion of immune individuals in the same population

herd immunity

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Reduces the number of susceptible (St) and alters the probability of transmission per contact/effective contact:

resistance

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an increased or unexpected occurrence of cases

epidemic

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a normal or expected number of cases in time and space

endemic

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epidemic over a large area - country or worldwide

pandemic

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new cases, force of disease, probability of becoming a case

incidence

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cases at a point in time, "commonness" of disease, probability of being a case

Prevalence

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Visual display of disease occurrence in time

epidemic curve

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What are the two basic types of epidemics:

common source, propagated

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Common-Source Epidemic:

results from exposure to a common source (such as contaminated food or water)

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Point (or point-source) epidemic:

exposure is sudden and brief (potato salad at a picnic)

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Extended Common Source Epidemic:

exposure period is prolonged

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The initial slope of the epidemic curve is stepper when the animal density is:

higher

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The initial slope of the epidemic curve is steeper when the infectious period is:

longer

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What does the slope of the curve suggest?

efficiency of transmission within the population

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What's an example of a short term trend?

epidemic

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What's an example of a cyclical trend?

- seasonal
- periodic changes in susceptible host population