Transmission Models

Question 1

Infectious disease events

Answer 1

—> encounter –> entry –> spread –> multiplication –> damage –> outcome

Question 2

Communicability

Answer 2

Ease with which a disease agent is spread within a population

Question 3

What does transmission require?

Answer 3

• susceptible host
• an agent
• the right environment to establish an effective contact

Question 4

Effective contact

Answer 4

• contact between infected and susceptible hosts

- contact that results in transmission of disease agent

Question 5

Mode of transmission

Answer 5

Reservoir –> portals of exit –> portals of entry –> new host

Question 6

Reed Frost model

Answer 6

Used to predict the number of new cases in the next incubation period interval
- envisions primary elements in an outbreak

Question 7

The RF model is primarily ________

Answer 7

Conceptual

• can describe the behavior of a highly communicable disease, or introduction of a new agent
• rarely a good estimate in endemic diseases

Question 8

C(t+1) = St(1-q^Ct)

Answer 8

C(t+1): cases in time (t+1) where 1 represents an interval of time equal to the incubation period of the agent
St: # of susceptible at time t
q: probability of no contact
Ct: cases at time t

Question 9

What are 3 factors that affect the RF equation?

Answer 9

• number of susceptible
• probability of contact
• number of cases at the start

Question 10

_______ drives the equation faster

Answer 10

Low probabilities of contact

Question 11

________ equals a faster epidemic rate

Answer 11

Higher number of susceptibles

Question 12

What does the RF model assume?

Answer 12

• infection is spread solely by adequate contact between individuals (no indirect, no fomite or vector)
• all individuals are susceptible (no prior immunity)
• individual develops infection after contact
• infected individuals will recover and be non-susceptible
• fixed probability of adequate contact
• population is closed

Question 13

The RF model is a ______ model

Answer 13

Deterministic

- based on fixed probability of adequate contact, which does not vary over time or between individuals

Question 14

Stochastic model

Answer 14

Allows probability of contact to be defined by a distribution of possible values

Question 15

What are the 3 classes an individual could be in?

Answer 15

• susceptible
• infected
• recovered
• used by both RF and R0 models*

Question 16

R0 model

Answer 16

Intrinsic (basic) reproductive rate
- average number of secondary cases that develop from one primary case during its entire communicable period in a population of susceptible hosts

Question 17

R0 formula

Answer 17

= # of contacts per unit time x probability of transmission per contact x duration of infectiousness
*first 2 inputs relate to host/environment, last one relates to agent/host

Question 18

R0 = 1

Answer 18

If during the period of communicability, one infected animal can infect one susceptible

Question 19

R0 = 2

Answer 19

If during the period of communicability, one infected animal can infect 2 susceptible

Question 20

R0 = 0.5

Answer 20

If during the period of communicability, 2 infected animals can infect 1 susceptible

Question 21

R0 > 1

Answer 21

Infection is maintained

Question 22

R0 < 1

Answer 22

Infection can not be maintained

- number of new cases are greater than number of old cases (?)

Question 23

How to reduce R0 with number of cases per unit time

Answer 23

• reduce number of animals
• add nonsusceptible animals
• change susceptible to nonsusceptible
• reduce animal density
• restrict animal-animal contact

Question 24

How to reduce R0 with probability of transmission per contact

Answer 24

• restrict contact to be non-effective

- alter mucosal disease

Question 25

Duration of infectiousness

Answer 25

Property of agent - antimicrobial/antiviral therapy - immune stimulation - environmental survival of agent (sanitation, decontamination) - dust and aerosol exposure - vector control/preventing exposure to vector

Question 26

Agent pressure

Answer 26

- number of cases of active disease | - duration of disease/infectiousness (within host/in environment)

Question 27

Herd immunity

Answer 27

Ability to protect susceptible individuals within a group due to the high proportion of immune individuals in the same population - below a critical number of susceptible animals, outbreaks are not supported - innate or acquired

Question 28

Herd immunity reduces the number of ________

Answer 28

Susceptible (St) | - alters probability of transmission per contact/effective contact

Question 29

Herd immunity of _____ is usually sufficient

Answer 29

70-80% - rabies vax is 70%, epidemic will not propagate if 39-57% are vaccinated - human measles requires 94%

Question 30

What does herd immunity depend on?

Answer 30

Properties of agent - infectious dose - duration of communicability - methods of contact, routes of entry