Patterns of Disease Flashcards
Epidemic
An increased or unexpected occurrence of cases
- epizootic
Endemic
A normal or expected number of cases in time and space
- enzootic
Pandemic
Epidemic over a large area
- country or worldwide
Incidence
New cases, force of disease, probability of becoming a case
Prevalence
Cases at a point in time, “commonness” of disease, probability of being a case
Epidemic curve
Visual display of disease occurrence in time
- vertical axis: incidence count
- horizontal axis: time
What are the 2 basic types of epidemics?
- common source
- propagated
Common-source epidemic
Results from exposure to a common source
- contaminated food or water
- all cases are infected from a source common to all individuals at risk
Point-source epidemic
If the exposure is sudden and brief
- ex: food poisoning at a picnic
- median incubation time
Extended common source epidemic
If the exposure period is prolonged
Propagating epidemic
Results from animal-to-animal transmission
The initial slope of the epidemic curve is steeper when the animal density is ______
Higher
- remember R0 formula!*
- are changing probability of transmission per contact
The initial slope of the epidemic curve is steeper when the infectious period is _______
Longer
- will drive it faster, and you will get more cases
- changing probability of transmission per contact, and duration of infectiousness
The initial slope of the epidemic curve is steeper when animal numbers ______
Increase
Slope of curve suggests efficiency of ________ within the population
Transmission
- as the slope gets steeper, new cases are being produced faster and transmission is more efficient
Temporal distribution of disease
Short term trends
- epidemics
Cyclical trends
- periodic changes in susceptible host population and in effective contact
- seasonal trends
Secular or long term trends
- long term interaction of host and agent
Potential usefulness of epidemic curve
- characterize the source of an epidemic (general shape)
- assess relative transmission
- determine mode of transmission (common source/propagated)
- determine a seasonal pattern
- possibly assess control measures where effective
- prospectively predict what effect interventions will have
Dissemination rate
Propensity of infection to spread to other herds
- average number of uninfected herds to which an infectious agent is delivered by each infected herd
What does DR depend on?
- host
- environment
- animal movements, behavior of owner, disease control strategies (biosecurity)
- agent
