Presentation on theme: "Infectious Disease Epidemiology EPIET Introductory Course, 2006 Lazareto, Menorca Prepared by: Mike Catchpole, Johan Giesecke, John Edmunds, Bernadette."— Presentation transcript:
1 Infectious Disease Epidemiology EPIET Introductory Course, 2006 Lazareto, Menorca Prepared by: Mike Catchpole, Johan Giesecke, John Edmunds, Bernadette Gergonne
2 Epidemiology: Why Bother? Human disease does not occur at randomEpidemiology leads to the identification of causal and preventive factors in human disease
3 Burden of disease in adult men and women, Established Market Economies: 1990 *Pertussis, polio, measles and tetanusSource: World Bank
4 Epidemiology: basic concepts The study of the distribution and determinants of disease frequency in (human) populationsFrequencyDistributionDeterminants
5 What is special about infectious disease epidemiology?
6 Specific conceptsAttack rate, immunity, vector, transmission, carrier, subclinical disease, serial interval, index case, source, exposure, reservoir, incubation period, colonization, generations, susceptible, non-specific immunity, clone, resistance, repeat episodes …But why do we need these concepts?
7 Infectious disease: the unique factor Infectious diseases can be spread from human to human (or animal to human)
8 Chain of Transmission Reservoir Susceptible Host Agent Person to person transmissionReservoirSusceptible HostPortal of exitPortal of entryAgentMode of transmission
9 Chain of transmission Reservoir Human Person with symptomatic illness Carriers:AsymptomaticIncubatingConvalescentChronicAnimal: zoonosisEnvironmental: soil, plant, water
10 Chain of transmission Mode of Transmission Direct Direct contact Secretions, Blood, Faeces/urineDroplet spreadIndirectFood/waterAerosolAnimal vectorsFomitesMedical devices and treatments
11 Chain of transmission Portal of exit Human/animal Respiratory tract Genito-Urinary tractFaecesSalivaSkin (exanthema, cuts, needles, blood-sucking arthropods)Conjunctival secretionsPlacentaEnvironmentalCooling towers
12 Chain of transmission Portal of entry Respiratory tract Mouth (faecal-oral transmission)SkinMucous membranesBlood
13 Level of disease occurence Sporadic level: occasional cases occurring at irregular intervalsEndemic level: persistent occurrence with a low to moderate levelHyperendemic level: persistently high level of occurrenceEpidemic or outbreak: occurrence clearly in excess of the expected level for a given time periodPandemic: epidemic spread over several countries or continents, affecting a large number of people
14 What causes incidence to increase? Portal of exitPortal of entryAgentSusceptible HostMode of transmissionReservoir
15 Why does an epidemic occur ? Agent and host in adequate numberRecent increase in amount of the agentRecent increase in infectivity / virulence of the agentRecent introduction of the agentEnhanced mode of transmissionIncrease of host exposureChange in the susceptibility of the host response to the agentIntroduction through new portals of entry
18 Infectious Disease Epidemiology: five major differences A case can also be an exposureSub-clinical infections influence epidemiologyContact patterns play major roleImmunityThere is sometimes a need for urgency
19 1. Case = exposure Unique to infectious disease epidemiology. Usually, the sets of exposures and outcomes are completely apart e.g. smoking and cancer.
20 The average number of cases an infectious individual will generate Dependent on 4 factors:1) The number of contacts made (c)2) The probability of infection given contact (p)3) The duration of infectiousness (D)4) The proportion of contacts who are susceptible (S)
21 The basic reproduction number, R0 Useful summary statisticDefinition:the average number of secondary cases a typical infectious individual will cause in a completely susceptible populationMeasure of the intrinsic potential for an infectious agent to spread
22 The basic reproduction number, R0 If everyone is susceptible then the average number of secondary infections generated by a single infectious individual is given by:R0 = p x c x DCan be estimated if we know p, c, & D, or from proportion susceptible, outbreaks in susceptible populations, the average age at infection (and many other ways)
23 R0, threshold for invasion If R0 < 1 then infection cannot invade a populationimplications: infection control mechanisms unnecessary (therefore not cost-effective)If R0 > 1 then (on average) the pathogen will invade that populationimplications: control measure necessary to prevent (delay) an epidemic
24 After invasion: the effective reproduction number, R(t) Initial invasion, R(t) = R0As pathogen invades, the number of susceptibles declines through recovery (or death)Eventually, insufficient susceptibles to maintain chains of transmissionOn average each infectious person infects < 1 other, epidemic dies outPeak of epidemic R(t) = 1
26 Determinants of STI incidence =p c Dp Risk of transmissionc Rate of sexual partner changeD Duration of infectivity7
27 STI Control Strategies =p c Dp condoms, acyclovir, zidovudinec health education, negotiating skillsD case ascertainment (screening,partner notification), treatment,compliance, health seeking behaviour, accessibility of services8
28 Cases of Gonorrhoea and Genital Herpes seen in STI clinics in England, 1971-1999
29 Determinants of STD incidence cT = 1pDcT Critical threshold for maintenancep Risk of transmissionD Duration of infectivity9
30 Sexual partners in last 12 months* * UK National Study of Sexual Attitudes and Lifestyles
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