epidemiology Flashcards
DALYs definition:
the number of healthy years of life lost due to premature death and disability
Yersinia pestis
define R0
R0= “the average number of new cases arising from one infectious case introduced into a population of wholly susceptible individuals” aka basic case reproduction number
define S
susceptible individuals in a population
how do you calculate the true reproductive rate of a disease? or the amount of secondary infections due to case #1.
Effective” R (Re) = R0 x S
how does an epidemic end?
when Re decreases to <1. only when new susceptible are introduced can the Re number go back to >1, causing and epidemic.
How do epidemics continue?
Susceptibles (S–increases):
born
migrate into a population
No immunity (SI model)
Pathogen mutates (e.g. antigenic drift) and can re-infect/or continually infect individuals
Immunity wanes
what causes Epidemic fade-out?
“The elimination of the infectious agent due to chance”
In small populations rather than large populations:
generation (birth) of threshold susceptibles is slow
numbers of infecteds is low
Patterns in epidemic data- what can they tell us?
Infecteds through time: prevalence & incidence
(Retrospective analysis of previous infections)
Origin of the outbreak
Index case – the first case in an outbreak of disease
Mode of spread through the population
Potential incubation period and time of exposure
Clues to identify the infectious agent
(R0 value comparisons)
define incubation period -
the period between infection and clinical onset of the disease
define latent period
the time from infection to infectiousness.
Point Epidemic
Single common exposure and incubation period
Does not spread by host-to-host transmission
E.g. food-borne disease outbreaks
Continuous Common Source Epidemic
Prolonged exposure to source over time
Cases do not all occur within the span of a single incubation period
Curve decay may be sharp or gradual
E.g. water-borne cholera: 1-3 days incubation
Propagated Progressive Source Epidemic
E.g. measles: 10 days incubation
Spread between hosts.
Larger curves until susceptibles are depleted, or intervention is made.
This pattern most likely in a small population.
In a larger population, it would all ‘merge’ together.
endemic vs epidemic
The successive epidemic waves await replenishment of susceptibles.
Host-parasite relationship may eventually dampen down to a stable equilibrium (endemic) state.
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
Re = S x R0
endemicy
Not (overly) common in the developed world.
Exceptions – Chicken pox, influenzas
In the less developed world, more severe diseases are endemic.
Neglected Tropical Diseases (NTDs).
The WHOs 20 priority NTDs, are so because they are endemic in large populations
Endemic diseases are common in animal populations.
Wild – no one is treating
Managed (farmed) – it can be advantageous
what are the determinants of persistence for an endemic?
Critical community size (CCS)
Rate of contact (mixing) for transmission
Duration of infectious period
Survival of host
what is CCS – critical community size
The minimum host population size required for the pathogen to persist’
Of particular concern for microparasites
Macroparasites:
May survive outside of the host
Can/often aggregate – high burdens in a small number of hosts
How does Ebola Virus persist?
1976 Sudan: The first known outbreak killing 151.
1976 Zaire: 280 deaths.
1995 – 2018: Multiple small outbreaks.
2013 – 2016 West Africa: 28616 cases, 11310 deaths.
40% death rate
R0 estimates vary between 1.5 – 2.5
All three countries declared Ebola free by June 2016
what was the death rate and R0 of ebola?
40% death rate
R0 estimates vary between 1.5 – 2.5
All three countries declared Ebola free by June 2016
2014 there was a small outbreak in the Democratic Republic of Congo , explain the cases and why it was seemingly isolated
- 66 cases, and 49 deaths.
Outbreak August 2018 - June 2020, 3481 cases, 2299 deaths (66% mortality)
As well as many other seemingly isolated outbreaks.
How?
It suggests a reservoir host.
what is ebola? and how is it transmitted?
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?
1988 & 2002 PDV epidemics in NW European harbour seals harbour and some grey seals.
90% infected
60% rapid mortality
R0 = 2.8
Solid immunity
PDV is not sustained by NW European population
*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%
*
*Suggests there is a reservoir host
Definition innocuous:
not harmful
