epidemiology 23-28 Flashcards
what are microparasites?
parasites which are small and multiply inside their host
what are macroparasites?
large parasites that multiply externally of the host
what are DALYs?
- disability adjusted life years
- the number of healthy years of life lost due to premature death and disability
what is an epidemic?
an increase in incidence of disease in excess of that expected
how is R0 calculated?
R0 = p x c x d
p - the probability that a contact results in transmission
c - the frequency of host contacts between infectious and susceptible individuals
d - the average amount of time the host is infectious
what is the definition of R0?
the average number of new cases arising from 1 infectious case introduced into a population of wholly susceptible individuals
how is the effective R calculated?
Re = R0 * fraction of susceptible individuals
how do epidemics continue?
- susceptible population increases:
-> more people are born
-> migration into a population
-> no immunity - the pathogen mutates and can reinfect or continually infect individuals
- immunity wanes
what is the index case?
the first case in an outbreak of disease
what is meant by the incubation period?
the period between infection and clinical onset of disease
what is the latent period?
the time from infection to infectiousness
what is a point epidemic?
- a single common exposure and no incubation period
- does not spread by host-to-host transmission
- usually food contamination
what is a continuous common source epidemic?
- prolonged exposure to source over time
- cases do not occur within the span of a single incubation period
- e.g. water borne cholera
what is a propagated progressive source epidemic?
- spread between hosts
- larger curves are seen until susceptibles are depleted or intervention is made
how is an endemic equilibrium maintained
- stability in the incidence of infection
- persistence of the parasite in the host population
- each infection produces 1 secondary infection on average
how is S calculated at endemicity
S = 1/R0
what do determinants of persistence depend on?
- critical community size
- rate of contact for transmission
- duration of infectious period
- survival of host
what is the critical community size?
the minimum host population size required for the pathogen to persist
what is the definition of R0 for macroparasites?
the average number of female offspring produced through the lifespan of one female worm that survives to maturity in the absence of density dependent constraints on population growth
how is herd immunity reached? can it be reached without intervention?
- achieved by vaccination and intervention
- a pathogen that generated total herd immunity would go extinct, which goes against evolution
what is vectorial capacity?
the average number of potentially infective bites that will be delivered by all the vector’s feeding upon a single host in one day
how is R0 calculated for vectors
R0 = c * d
c = vectorial capacity
d = duration of host infectiousness in days
how can vector transmission be controlled?
- human bait traps
- non human bait traps
- urban breeding site source reduction
- rural drainage of breeding sites
how do GM mosquitoes prevent transmission?
the males are sterile so offspring cannot be produced
why are vector borne diseases most affected by climate change?
- environmental parasite stages are particularly sensitive to the climate
- parasite development is often climate dependent
how long does the extrinsic incubation period need to be for a vector to be infectious?
it needs to be shorter than the average adult life expectancy of the vector
how can ENSO be responsible for periodic epidemic cycles?
- flooding increases water breeding grounds for mosquitoes
- higher temps increases the proportions of vectors surviving the feeding cycle
- expanding into other places due to warmer climate
what is the R0 of SARS-CoV-2 estimated to be?
between 2 and 4
what is the definition for vaccine efficacy?
the percentage of those vaccinated who subsequently become immunised
