STIs Flashcards
Are STIs density or frequency dependant and what does that mean?
STIs are frequency dependant! - transmission events determined by intimate contact; directy transmitted pathogens are often density dependant and therefore they depend on for example how many people are in a room which is not the case for STIs
Why does frequency dependence matter?
- not expected to have threshold densities
- may cause extinctions - the rate of infection doesn’t slow down as you deplete the susceptibles
- thought that they cannot regulate their host’s populations size
What is special about devil facial tumour disease?
- DFTD prevalence shoots up over time particularly in established sites
- at the same time population seems to decline
- hallmark of frequency dependant disease
What is special about devil facial tumour disease?
- DFTD prevalence shoots up over time particularly in established sites
- at the same time population seems to decline
- hallmark of frequency dependant disease
What is force of infection defined as in frequency dependant diseases? Write the equation
force of infection is defined as partner change rate (c), transmission probability (Bp) and the prevelance of the infection rate at time t
What is Bp?
transmission probability per sexual act
What needs to happen in order for the STI to spread?
In order to spread this rate of change of number of infectious individuals must be greater than 0
What is the formula for R0 in STIs?
cBpD
What are the assumptions we make in the R0 calculation for STIs?
- random mixing - such at each individual changes sexual partners at a constant rate per year (c)
- the population at risk of an STI is also a subset of sexually active individuals
What is the distribution of human sexual behaviours?
W don’t have that, however we have a heterogenous distribution in human sexual behaviours
How do we solve the problem of heterogenous populations?
This problem is solved by dividing the population into two groups, high and low activity groups
How do you account for the different partner change rates? Write the equations
see the notes
What does the assumption of proportionate mixing in STIs mean for the models we can create? Write the equations for lambda
we are assuming proportionate mixing so: high and low activity group members have the same probability that they select an infected partner each time
What is p(t) and what does it need to account for? Write the equations
p(t) needs to account for the fact that a partner can be from the high or low group and that prevelance of infection will difffer between these groups
What is i(t) in STIs?
i (t) are the prevelance of infectious individuals in the high and low activity-groups. We can calculate the probability that partner will be from the high or low group using:
