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IDHE Short course | London, 30 March-3 April 2009
Disease Control in Humanitarian Emergencies (DCE)1
Francesco Checchi
Disease Control in Humanitarian Emergencies (DCE)
Department of Epidemic & Pandemic Alert and Response (EPR)
Principles of infectious disease transmission
Short course on Infectious Diseases in Humanitarian Emergencies
London, 30 March 2009
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Infectious diseasesInfectious diseases
� Most crisis-attributable indirect morbidity
and mortality
� Prions
� Viruses
� Bacteria
� Fungi
� Protozoa
� Multicellular parasites
� Infectious or communicable?
smallestsmallest
largestlargest
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How does transmission take place?How does transmission take place?
� Route of transmission
� Much more important than whether pathogen is a virus, bacterium,
or anything else
� Several routes possible for one pathogen
� Transmission cycle� Most complex for vector-borne infections
� Reservoir� Organism or habitat in which pathogen does not go extinct
� Primary vs. secondary reservoir
� Is disease…Anthroponosis? Zoonosis? Neither?
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Disease Control in Humanitarian Emergencies (DCE)4
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Transmission cycle and reservoir: examplesTransmission cycle and reservoir: examples
� Most airborne-droplet diseases (e.g. ARI, measles, meningococcal meningitis):
� Humans are reservoir
� Cycle is direct human to human transmission
� For ARI specifically: colonisation of upper airways
� Faecal-oral diseases:
� Cholera vs. polio: what can vaccination achieve?
� Tetanus (e.g. post-tsunami “epidemic” in Aceh)
� Not homogeneous processes
� Chronic carriers and “super-shedders”
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Lake KivuLake Kivu
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Transmission cycle of Rift Valley Fever virusTransmission cycle of Rift Valley Fever virus
� Zoonosis
� Each sub-cycle is one “epidemiological system”
� Where is the reservoir?
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How much transmission is occurring?How much transmission is occurring?
� Natural progression parameters
� Incubation period, duration of infectiousness, serial interval
� Reproductive ratio
� Determinants
� Implications
� Transmission level and transmission rate
� Endemic versus epidemic
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Reproductive ratioReproductive ratio
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Reproductive ratio (continued)Reproductive ratio (continued)
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What determines the level of transmission?What determines the level of transmission?
� Reproductive ratio (R)� average number of infections arising from one infection
� ever-changing quantity across time and space (i.e. context-specific)
� Basic reproductive ratio (R0): everyone is susceptible and there is no control (= maximum value that R can take = transmission potential)
� One unique R value…� …for each route of transmission: e.g. R for HIV = R(vaginal) + R(anal) +
R(transfusion)…
� …for each “epidemiological system”
� Implications:� R>1 : transmission is increasing
� R<1 : transmission is decreasing, and the disease is on the way to extinction
� R≈1 : transmission is stable, humans are a reservoir
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What determines the reproductive ratio?What determines the reproductive ratio?
�“c” represents contact, very context-specific:
� Faecal-oral: contacts with faecalmatter
� Vector-borne: bites of susceptible vectors on infectious humans x bites of infectious vectors on susceptible humans
� Crucially dependent on % susceptible
�“p” greatly dependent on pathogen virulence
�“d” affected by host-pathogen interactions
� Immune response, CFR
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““Stop TBStop TB”” targets: >70% detected, >80% cured targets: >70% detected, >80% cured –– why?why?
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What determines the rate of transmission?What determines the rate of transmission?
� We know R determines the level of transmission, but how fast will new infections occur?
� Depends on
� R
� Serial interval
� Imagine two diseases of equal R but different serial interval, in a population of 1000 people:
� R = 5, serial interval = 10 d
� Start with 1 infection: after 10d, 5 new infections; 10d later, 5x5=25; 10 d later, 25x5 =125; total (1+5+25+125)/30d = 156/(1000 people x 30d) = 5.2/1000 person-d
� R = 5, serial interval = 30 d
� Start with 1 infection: after 30d, 5 new infections; total (1+5)/(1000 people x 30d) = 6/30d = 0.2/1000 person-d
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The basic parameters for some diseasesThe basic parameters for some diseases
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From transmission to disease and deathFrom transmission to disease and death
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Endemic versus epidemic diseasesEndemic versus epidemic diseases
� Somewhat outdated, rarely clear-cut
� Depends on context: e.g. malaria
� Epidemic (“epidemic-prone” disease):
� Disease “usually absent” (low exposure = high susceptibility = if a case is introduced, R≈ R0), or
� Incidence in excess of expected range (R>>1)
� Endemic:
� Disease “always present”
� Stable incidence over relatively long time interval (excluding seasonality)
� Two possibilities:
� R≈1
� R<1, but constant spillover to humans from reservoir
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Transmission cycle of Rift Valley Fever virusTransmission cycle of Rift Valley Fever virus
� R>1 always (endemic)
RR≈≈≈≈≈≈≈≈1 always (endemic)1 always (endemic) RR>1 sometimes >1 sometimes
(epizootic)(epizootic)
RR<1 always (zoonosis <1 always (zoonosis
due to epizootic)due to epizootic)
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How do we know an epidemic is happening?How do we know an epidemic is happening?
� Arbitrary definitions
� Epidemic thresholds:
(will talk more about epidemic/outbreak thresholds later)