Lecture 25: Endemic patterns Flashcards
Thursday 28th November 2024
When is an epidemic not an epidemic?
- The successive epidemic waves await replenishment of susceptibles.
- Host-parasite relationship may eventually dampen down to a stable equilibrium (endemic) state.
Describe endemic equilibrium
Stability in the incidence of infection (constant)
Persistence of the parasite in the host population
Each infection produces 1 secondary (new) infection on average, i.e. Effective R (Re) = 1
(NB. Re > 1 means epidemic)
Are endemics overly common in the developed world?
No, they are not overly common in the developing world. Except for chicken pox and influenza.
In the less developed world, are more severe diseases endemic?
Yes.
What type of diseases are endemic in the less developed world?
Neglected Tropical Diseases (NTDs).
What type of animal populations are endemics common in and why?
Wild – no one is treating
Managed (farmed) – it can be advantageous
Is it true that endemic diseases are common in animal populations?
Yes
Does an endemic equilibrium equate to a stable poulation?
Yes
What is the equation for the fraction of susceptibles (S*) in the population at equilibrium?
S = 1 / R0
At Endemicity, 20% of the population is susceptible
As such this is the threshold condition necessary for the parasite to persist.
Should there be less than 20% of the population susceptible, the infection will ‘fade-out’
At Endemicity, 20% of the population is susceptible
As such this is the threshold condition necessary for the parasite to persist.
Should there be less than 20% of the population susceptible, the infection will ‘fade-out’
What does persistence depend on?
Critical community size (CCS)
Rate of contact (mixing) for transmission
Duration of infectious period
Survival of host
What does CCS stand for?
Critical Community Size
What is the critical community size?
‘The minimum host population size required for the pathogen to persist’
What is the critical community size of particular concern for?
microparasites
Macroparasites:
May survive outside of the host
Can/often aggregate – high burdens in a small number of hosts
is the birth rate high enough for measles persistance in england?
Yes
What happens if you increase the infectious period, but mantain R0?
Likely low host mortality
Eliminates cycles – improves persistence
Increases prevalence
A ‘slower moving’ infection
Do longer infectious periods reduce host mortality, improve persistance, and improve prevalence?
Yes
Ebola
1976 Sudan
The first known outbreak killing 151.
Ebola
1976 Zaire
280 deaths.
Ebola
1995 – 2018
Multiple small outbreaks
Ebola
2013 – 2016 West Africa
28616 cases, 11310 deaths.
Ebola
Case Incidence in Guinea, Liberia, and Sierra Leone.
- 40% death rate
- R0 estimates vary between 1.5 – 2.5
- All three countries declared Ebola free by June 2016
Describe the small ebola outbreak in 2014 in the Democratic Republic of Congo
Also in 2014 there was a small outbreak in the Democratic Republic of Congo - 66 cases, and 49 deaths.
