Evolutionary Epidemiology Flashcards
(18 cards)
Describe the small-pox epidemic
1972
Largest smallpox outbreak in Europe post WW2
R0 for smallpox is between 3-6
In the country of approx 21M people: 18M vaccinated and 15K quarantined in 2 weeks
175 infection cases and 35 deaths
When were antibiotics created?
1941
What does communicable mean?
Infectious
What are the key questions around evolutionary epidemiology
Can we predict where epidemics would arise in the future?
How natural selection acts on pathogenic microbes?
What is the model of infectious disease?
Population of susceptible individuals -> infected -> incubation -> infectious -> spread -> recovered (susceptible, immune or dead)
What does the SIR model track?
Susceptible individuals (s), infectious individuals (I), recovered individuals (R).
- Healthy individuals can die of natural causes. Infectious individuals can die of disease AND natural causes
What are the assumptions of our SIR model?
Recovered individuals are immune and do not gain sensitivity
There is no spatial structure in the population (no quarantine)
Infection transmission follows the Mass Action Law: the rate at which infection spreads in a population is proportional to (Susceptible individuals) x (Infectious individuals)
Disease does not evolve
Draw out the SIR model
Keeps track of population’s disease state, not pathogens
What are the model parameters?
r = reproductive rate
d = (natural death rate)
b = infection rate
T = infection duration
y = recovery rate 1/T
v = death rate from disease (virulence)
How can we flatten the curve?
Reducing the number of S that become I. Reducing the number of contacts or improving hygeine
What are the effects of reducing B (infection rate)
When viruses try to spread under poor conditions, they stop spreading as soon as some of the population gains immunity
What is herd immunity?
Over half of the population is susceptible but the virus is lost from the population
Once the recovered immune population reaches a certain threshold, the disease will stop spreading to the susceptible population
What is R0?
the expected number of infectious contacts made by a single infected individual introduced into a susceptible population
How is R0 calculated?
R0 = the rate at which a single infected individual makes an infectious contact x the expected length of time that such infective individual remains infectious
the expected length of time = 1/the rate of death (or recovery)
R0 for SIR model?
R0 = b / d + v + g
the greaer the infection rate, lower virulence and slower rate of recovery, the more the pathogen will spread
if v decreases, R0 increases?
You may expect that pathogen will always evolve to reduce virulence
Evidence?
Newly emergent diseases tend to be more virulent than diseases that have been infecting us for many years, so this isn’t true.
Describe the virulence-transmission trade-off
The characteristics that allow viruses to spread ( b ), often cause mortality ( v), therefore the viruses must find optimality between maximising b and reducing v
Positive relationship between good for virus and bad for host, thus bad for virus?
if a virus causes a host cell to create more viral particles, then it will be likely to transmit faster, but will also kill more host cells and cause increased virulence
