Transmission Models Flashcards
Infectious disease events
—> encounter –> entry –> spread –> multiplication –> damage –> outcome
Communicability
Ease with which a disease agent is spread within a population
What does transmission require?
- susceptible host
- an agent
- the right environment to establish an effective contact
Effective contact
- contact between infected and susceptible hosts
- contact that results in transmission of disease agent
Mode of transmission
Reservoir –> portals of exit –> portals of entry –> new host
Reed Frost model
Used to predict the number of new cases in the next incubation period interval
- envisions primary elements in an outbreak
The RF model is primarily ________
Conceptual
- can describe the behavior of a highly communicable disease, or introduction of a new agent
- rarely a good estimate in endemic diseases
C(t+1) = St(1-q^Ct)
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
What are 3 factors that affect the RF equation?
- number of susceptible
- probability of contact
- number of cases at the start
_______ drives the equation faster
Low probabilities of contact
________ equals a faster epidemic rate
Higher number of susceptibles
What does the RF model assume?
- 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
The RF model is a ______ model
Deterministic
- based on fixed probability of adequate contact, which does not vary over time or between individuals
Stochastic model
Allows probability of contact to be defined by a distribution of possible values
What are the 3 classes an individual could be in?
- susceptible
- infected
- recovered
- used by both RF and R0 models*
R0 model
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
R0 formula
= # 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
R0 = 1
If during the period of communicability, one infected animal can infect one susceptible
R0 = 2
If during the period of communicability, one infected animal can infect 2 susceptible
R0 = 0.5
If during the period of communicability, 2 infected animals can infect 1 susceptible
R0 > 1
Infection is maintained
R0 < 1
Infection can not be maintained
- number of new cases are greater than number of old cases (?)
How to reduce R0 with number of cases per unit time
- reduce number of animals
- add nonsusceptible animals
- change susceptible to nonsusceptible
- reduce animal density
- restrict animal-animal contact
How to reduce R0 with probability of transmission per contact
- restrict contact to be non-effective
- alter mucosal disease
Duration of infectiousness
Property of agent
- antimicrobial/antiviral therapy
- immune stimulation
- environmental survival of agent (sanitation, decontamination)
- dust and aerosol exposure
- vector control/preventing exposure to vector
Agent pressure
- number of cases of active disease
- duration of disease/infectiousness (within host/in environment)
Herd immunity
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
Herd immunity reduces the number of ________
Susceptible (St)
- alters probability of transmission per contact/effective contact
Herd immunity of _____ is usually sufficient
70-80%
- rabies vax is 70%, epidemic will not propagate if 39-57% are vaccinated
- human measles requires 94%
What does herd immunity depend on?
Properties of agent
- infectious dose
- duration of communicability
- methods of contact, routes of entry
