Lecture 25. Endemic Patterns Flashcards
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
What is endemic equilibrium?
Stability in the incidence of infection (constant)
Persistence of the parasite in the host population
Where is endemicity not common?
In the developed world (except chickenpox and influenzas)
What are NTDs?
Neglected Tropical Diseases
What is the equation to calculate the fraction of susceptibles (S*) in the population at equilibrium?
S = 1/R0
What does the determinants of persistence depend on?
Critical community size (CCS)
Rate of contact (mixing) for transmission
Duration of infectious period
Survival of host
What is the critical community size (CCS)?
The minimum host population size required for the pathogen to persist
How does ebola virus persist?
Ebola is a Zoonotic pathogen
It can be transmitted from animal to human
Fruit bats being the primary reservoir host
Fruit bats are not diseased by the Ebola Virus
There are almost certainly constant human Ebola cases But these are in isolated communities
Human to human transmission will drive a large outbreak
How does Phocine distemper (PDV) persist in NW Europe?
Seals congregate in discrete patches where direct close contact transmission occurs
In populations far below the critical community size (CCS) for PDV
R0= 2.8 requires birth cohort to make up 1/2.8 = 35% of the population size (i.e. the condition where R0 S (Re) >1)
But seals pup once/yr, max. replacement rate of 20%
What is an innocuous host of PDV?
Harp seals
What is a reservoir host?
Referring to a population or species
What is a carrier?
Referring to an individual
