VETS2006 Epidemiology 3

Epidemiology of Infectious Disease 3
How does Infectious Disease Persist in a Population?
via patterns...
What is an Endemic?
consistently present in defined population
Hyperendemic is a disease at continuously high levels
What is an Epidemic
frequency higher than expected/background/endemic level
What is a Pandemic?
widespread occurrence which affects large proportion of the population
What is Sporadic?
irregular/haphazard occurrences
each occurrence can be a single case or in a cluster
What is the epidemic curve?
graphical representation of new cases of disease
vertical axis - new cases
horizontal axis - time
What is common source epidemic?
all cases exposed to a single common source (brief)
not specific to infectious disease
What is propagating epidemic?
animals initially infect other animals which in turn infect further animals
What 3 cases can be identified with propagating epidemic?
Index case(s)
first case known to investigators
prompts investigation

Primary case(s)
initial case(s)
may be determined after trace-back

Secondary cases
infected from primary case(s)
occur after one incubation period
How can a Propagating Epidemic Occur?
there must be enough susceptible animals in the population

infectious individuals must, on average, infect at least one susceptible individual each during the infectious period
What is the Reed-Frost Model
population is divided into 3 groups
infected - rise and fall
susceptible - fall over time
immune - increase over time
What are the Reed-Frost Model Assumptions?
only direct transmission or infection occurs
it's a closed population - no animals can be added
all parameters constant throughout simulation period
What is Herd Immunity?
protection of susceptible members of population by presence of immune animals

susceptible animals less likely to become infected
How does Herd Immunity and the Reed-Frost Model link?
Effect of reducing proportion of susceptible animals in population

shape of curve
total number of cases