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.
Describe the ebola outbreak August 2018- June 2020
3481 cases, 2299 deaths (66% mortality)
Do the many isolated outbreaks of ebola suggest the presence of a reservoir host?
Yes
What makes ebola a zoonotic disease?
It can be transmitted from animal to human.
What is the primary reservoir host of ebola?
Fruit bats
Do fruit bats get infected with ebola?
No
Describe human-ebola cases
- There are almost certainly constant human Ebola cases. But these are in isolated communities
- Human to human transmission will drive a large outbreak.
What does a reservoir host refer to?
a population or species
What does a carrier refer to?
referring to an individual
Is it true that reservoir hosts and carriers are clinically normal (not diseased) but infectious (all the time or intermittently) ?
Y
Is it true that macroparasites are highly persistant?
Yes
Name 3 types of macroparasites
Hookworm, Ancylostoma, duodenale
What is the Basic Reproduction Number R0 for helminths?
The average number of female offspring produced through the lifespan of one female worm that survive to maturity in the absence of density-dependent constraints on population growth
Measurements of macroparasite infections
- Infection intensity/mean burden per host is directly related to R0 (e.g. accumulation of environmental infective Ascaris larvae).
- Prevalence determined by the mean worm burden and degree of parasite aggregation
- Low prevalences of infection may occur with high worm burdens if parasites are highly aggregated within the host population
Is it true that for macroparasites, Accumulative exposure determines infection burden?
Yes
Immunity is transient (dependent on parasite burden): Continual re-infection.
Immunity is transient (dependent on parasite burden): Continual re-infection.
Does Trichuris trichiura stunt growth?
Yes
Summary characteristics of gastrointestinal helminth worms for endemic persistence
- Host density (CCS) is not limiting factor to transmission
- External “reservoir” of transmission stages
- Long generation time and period of infectiousness
- Immunity is transient
- Continual re-infection
- Mode of transmission: often contaminative – not requiring host to host transmission
