Infectious Disease Epidemiology Principles of Epidemiology Lecture 7 Dona Schneider, PhD, MPH, FACE

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  • Infectious Disease EpidemiologyPrinciples of EpidemiologyLecture 7Dona Schneider, PhD, MPH, FACE

    Epidemiology (Schneider)

    Concepts in Infectious EpidemiologyAgent, host, environmentClassification of human infections by modes of transportationIncubation periodSpectrum of diseaseHerd immunity

    Epidemiology (Schneider)

    Major Factors Contributing to the Emergence of Infectious DiseasesHuman demographics and behaviorTechnology and industryEconomic development and land useInternational travel and commerceMicrobial adaptation and changeBreakdown of public health measures

    Epidemiology (Schneider)

    Epidemiologic Triad ConceptsInfectivity ability to invade a host (# infected / # susceptible) X 100Pathogenicity ability to cause disease(# with clinical disease / # of infected) X 100Virulence ability to cause death(# of deaths / # with disease (cases)) X 100All are dependent upon the condition of the hostImmunity (active, passive)NutritionSleepHygiene

    Epidemiology (Schneider)

    Mode of TransmissionPerson-to-person (respiratory, orogenital, skin)Examples: HIV, measlesVector (animals, insects)Examples: rabies, yellow feverCommon vehicle (food, water)Examples: salmonellosisMechanical vectors (personal effects) such as doorknobs, or toothbrushes are called FOMITES

    Epidemiology (Schneider)

    Classification by Mode of TransmissionDynamics of Spread through Human PopulationsSpread by a common vehicleIngestionSalmonellosisInhalationLegionellosisInoculationHepatitisPropagation by serial transfer from host to hostRespiratoryMeaslesAnal-oralShigellosisGenitalSyphilis

    Epidemiology (Schneider)

    Principle Reservoir of InfectionMan Infectious hepatitisOther vertebrates (zoonoses) TularemiaAgent free-living HistoplasmosisPortal of Entry/Exit in Human HostUpper respiratory tract DiphtheriaLower respiratory tract TuberculosisGastrointestinal tract Typhoid feverGenitourinary tract GonorrheaConjunctiva TrachomaPercutaneous LeptospirosisPercutaneous (bite of arthropod) Yellow fever

    Epidemiology (Schneider)

    Cycles of Infectious Agent in NatureMan-man InfluenzaMan-arthropod-man MalariaVertebrate-vertebrate-manPsittacosisVertebrate-arthropod-vertebrate-man Viral encephalitisComplex CyclesHelminth infections River blindness

    Epidemiology (Schneider)

    Incubation PeriodThe interval between the time of contact and/or entry of the agent and onset of illness (latency period)The time required for the multiplication of microorganisms within the host up to a threshold where the parasitic population is large enough to produce symptoms

    Epidemiology (Schneider)

    Each infectious disease has a characteristic incubation period, dependent upon the rate of growth of the organism in the host andDosage of the infectious agentPortal of entryImmune response of the hostBecause of the interplay of these factors, incubation period will vary among individualsFor groups of cases, the distribution will be a curve with cases with longer incubation periods creating a right skew

    Epidemiology (Schneider)

    Spectrum of DiseaseExposureSubclinical manifestationsPathological changesSymptomsClinical illnessTime of diagnosisDeathWhether a person passes through all these stages will depend upon infection and prevention, detection and therapeutic measures

    Epidemiology (Schneider)

    Iceberg Concept of InfectionLysis of cellCELL RESPONSECell transformationorCell dysfunctionIncomplete viral maturationSubclinicalDiseaseExposurewithout cell entryClinical DiseaseFatalClinical andsevere diseaseModerate severityMild IllnessInfection withoutclinical illnessExposure without infectionHOST RESPONSEBelow visualchangeDiscernableeffect

    Epidemiology (Schneider)

    Spectrum of Disease (cont.)Example90% of measles cases exhibit clinical symptoms66% of mumps cases exhibit clinical symptoms>>>99%99%1:995 to 20 yearsRash, feverMeasles60%1.5:1Young adultFever, coughInfluenza50%2:15 to 20 yearsRashRubella80% to 95%1.5:1Adult14%7:110 to 15 years10%11:15 to 9 years5%20:1< 5 yearsIcterusHepatitis A50% to 75%2:1 to 3:116 to 25 years1% to 10%10:1 to 100:16 to 15 years1%> 100:11 to 5 yearsMononucleosisEpstein-Barr0.1% to 1.0%+ 1000:1ChildParalysisPolioClinical casesEstimated ratioAge at infectionClinical featureVirus

    Epidemiology (Schneider)

    Herd ImmunityThe decreased probability that a group will develop an epidemic because the proportion of immune individuals reduces the chance of contact between infected and susceptible personsThe entire population does not have to be immunized to prevent the occurrence of an epidemic Example: smallpox, measles

    Epidemiology (Schneider)

    Epidemiology (Schneider)

    Epidemiology (Schneider)

    Investigating an EpidemicDetermine whether there is an outbreak an excess number of cases from what would be expectedThere must be clarity in case definition and diagnostic verification for each case

    Epidemiology (Schneider)

    Investigating an Epidemic (cont.)Plot an epidemic curve (cases against time)Calculate attack ratesIf there is no obvious commonality for the outbreak, calculate attack rates based on demographic variables (hepatitis in a community)If there is an obvious commonality for the outbreak, calculate attack rates based on exposure status (a church supper)

    Epidemiology (Schneider)

    Investigating an Epidemic (cont.)Determine the source of the epidemic If there is no obvious commonality for the outbreak, plot the geographic distribution of cases by residence/work/school/location to reduce common exposures If there is an obvious commonality for the outbreak, identify the most likely cause and investigate the source to prevent future outbreaks

    Epidemiology (Schneider)

    Index CasePerson that comes to the attention of public health authoritiesPrimary CasePerson who acquires the disease from an exposureAttack rateSecondary CasePerson who acquires the disease from an exposure to the primary caseSecondary attack rate

    Epidemiology (Schneider)

    Calculation of Attack Rate for Food X64%114776%13310Attack RateTotalWellIllAttack RateTotalWellIllDid not eat the food (not exposed)Ate the food (exposed)Attack Rate = Ill / (Ill + Well) x 100 during a time periodAttack rate = (10/13) x 100 = 76% ( 7/11) x 100 = 64%RR = 75/64 = 1.2

    Epidemiology (Schneider)

    Secondary Attack RateUsed to estimate to the spread of disease in a family, household, dorm or other group environment.Measures the infectivity of the agent and the effects of prophylactic agents (e.g. vaccine)Secondary attack rate (%)Total number of cases initial case(s)Number of susceptible persons in the group initial case(s)=x 100

    Epidemiology (Schneider)

    Mumps experience of 390 families exposed to a primary case within the family15258415210-19872044204505-9501002503002-4SecondaryPrimaryNo. susceptible before primary cases occurredTotalAge in yearsCasesPopulationSecondary attack rate 2-4 years old = (150-100)/(250-100) x 100 = 33%

    Epidemiology (Schneider)

    Case Fatality RateReflects the fatal outcome (deadliness) of a disease, which is affected by efficacy of treatment

    Case fatality rate (%)Number of deaths due to disease XNumber of cases of disease X=x 100

    Epidemiology (Schneider)

    Assume a population of 1000 people. In one year, 20 are sick with cholera and 6 die from the disease.

    The cause-specific mortality rate in that year from cholera =

    The case-fatality rate from cholera =620=0.3=30%61000=0.006=0.6%

    Source: Timmreck, 1998

    Source: Timmreck, 1998

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