Measuring Disease 2

Question 1

What does attack rate tables aim to do?

Answer 1

Calculate the incidence or prevelence of disease in each group and compare them

Question 2

when do we us attack rates?

Answer 2

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

Question 3

What does prevalence depend upon?

Answer 3

Incidence and duration of disease

P = I x D

Question 4

How to remember difference between prevalence and incidence?

Answer 4

Prevalence is “AT”

Incidence is “DURING”

Question 5

What gives the anatomy of the epidemic?

Answer 5

Epidemic curves

Question 6

What are examples of point source epidemics?

Answer 6

Food poisoning outbreaks

Toxic events

Question 7

What is a point source epidemic

Answer 7

Pronounced clustering of disease

Common source and event

Often a brief exposure period

Question 8

What is a propogating epidemic?

Answer 8

Epidemic caused by an infectious agent

Primary cases infect susceptible individual which become secondary cases

Question 9

What is the inex case?

Answer 9

First primary case that comes to the attention of the investigators

Question 10

What is a progating epidemic characterized by ?

Answer 10

Build up or amplification

Question 11

Explain the exponential stage of an epidemic/”incidence” curve

Answer 11

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

Question 12

Explain the saturation and peak stage of an epidemic/”incidence” curve

Answer 12

After intial phase not all contacts are with susceptible animals

Rate of epidemic is slowed

Question 13

What is R_t

Answer 13

Effective reproduction number

• expected number of infections produced by each infectious individual
• Takes into account the production of susceptible individuals in population

Question 14

What is R₀

Answer 14

Basic Reproduction Number

• Refers to the number of secondary cases expected from one primary case in a completely susceptible population

Question 15

R₀ equation

Answer 15

R0 = pcD

p = probabilty of infection on contact

c = rate of contact (i.e. number of contacts / unit time)

D = duration of infectiousness

Question 16

What is c, rate of contact determined by?

Answer 16

Behaviour

Transmission

Route

Density

Environment

all with in a specific pop

Question 17

what is cD?

Answer 17

Number of contacts that occur during the infectious period

Question 18

what is R_t

Answer 18

The effective reproduction number

Question 19

What is saturation and peak determined by?

Answer 19

Proportion of susceptibles

• Size of epidemic is determined by the proportion of susceptible animals in the population

Question 20

Epidemics are like fire and susceptibles are?

Answer 20

the fuel

Question 21

What are 3 potential patterns after decline in the epidemic curve?

Answer 21

Extinction

Stable prevalence and incidence

Endemic cycles

Question 22

What causes for extinction to occur?

Answer 22

No new susceptibles provided

Question 23

What are two sources of susceptibles?

Answer 23

Births

Loss of immunity

Question 24

Stable Prevalence: Equilibrium

Answer 24

R_t = 1

no change in levels of infection

Question 25

4 factors affecting shape of epidemic curve

Answer 25

1. Incubation period of the disease
2. Infectivity of the agent
3. Proportion of susceptible animals in pop
4. Distance between animals (i.e. animal density)