Measuring Disease 2 Flashcards
What does attack rate tables aim to do?
Calculate the incidence or prevelence of disease in each group and compare them
when do we us attack rates?
When the period of risk is brief such as outbreaks of food poisoning, we use attack rate to describe the proportion of animals that develop the disease
What does prevalence depend upon?
Incidence and duration of disease
P = I x D
How to remember difference between prevalence and incidence?
Prevalence is “AT”
Incidence is “DURING”
What gives the anatomy of the epidemic?
Epidemic curves
What are examples of point source epidemics?
Food poisoning outbreaks
Toxic events
What is a point source epidemic
Pronounced clustering of disease
Common source and event
Often a brief exposure period
What is a propogating epidemic?
Epidemic caused by an infectious agent
Primary cases infect susceptible individual which become secondary cases
What is the inex case?
First primary case that comes to the attention of the investigators
What is a progating epidemic characterized by ?
Build up or amplification
Explain the exponential stage of an epidemic/”incidence” curve
Highly infectious agent with short incubation period produces a steep curve on a relatively small time scale
Rapid spread of infection among the pop
Has constant doubling time
Explain the saturation and peak stage of an epidemic/”incidence” curve
After intial phase not all contacts are with susceptible animals
Rate of epidemic is slowed
What is Rt
Effective reproduction number
- expected number of infections produced by each infectious individual
- Takes into account the production of susceptible individuals in population
What is R0
Basic Reproduction Number
- Refers to the number of secondary cases expected from one primary case in a completely susceptible population
R0 equation
R0 = pcD
p = probabilty of infection on contact
c = rate of contact (i.e. number of contacts / unit time)
D = duration of infectiousness
What is c, rate of contact determined by?
Behaviour
Transmission
Route
Density
Environment
all with in a specific pop
what is cD?
Number of contacts that occur during the infectious period
what is Rt
The effective reproduction number
What is saturation and peak determined by?
Proportion of susceptibles
- Size of epidemic is determined by the proportion of susceptible animals in the population
Epidemics are like fire and susceptibles are?
the fuel
What are 3 potential patterns after decline in the epidemic curve?
Extinction
Stable prevalence and incidence
Endemic cycles
What causes for extinction to occur?
No new susceptibles provided
What are two sources of susceptibles?
Births
Loss of immunity
Stable Prevalence: Equilibrium
Rt = 1
no change in levels of infection
4 factors affecting shape of epidemic curve
- Incubation period of the disease
- Infectivity of the agent
- Proportion of susceptible animals in pop
- Distance between animals (i.e. animal density)
