EPI 5240: Introduction to Epidemiology Overview of Communicable Disease Epidemiology October 26, 2009 - PowerPoint PPT Presentation

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EPI 5240: Introduction to Epidemiology Overview of Communicable Disease Epidemiology October 26, 2009

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EPI 5240: Introduction to Epidemiology Overview of Communicable Disease Epidemiology October 26, 2009 Dr. N. Birkett, Department of Epidemiology & Community Medicine, – PowerPoint PPT presentation

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Title: EPI 5240: Introduction to Epidemiology Overview of Communicable Disease Epidemiology October 26, 2009


1
EPI 5240Introduction to EpidemiologyOverview
of Communicable Disease EpidemiologyOctober 26,
2009
  • Dr. N. Birkett,
  • Department of Epidemiology Community Medicine,
  • University of Ottawa

2
Session Overview
  • Basic concepts of infectious disease epidemiology
  • Spectrum of disease
  • Transmission methods
  • Epidemics and outbreaks
  • Key concepts
  • Methods of investigation/control
  • Approaches to disease surveillance
  • Sample outbreak investigation exercise

3
Agents of Infectious Diseases
  • Bacteria
  • Viruses
  • Rickettsia
  • Fungi (mycoses)
  • Protozoa
  • Helminths
  • Prions

4

Infections Sources and agents (1)
  • Foodborne
  • Botulism
  • Clostridium Perfringens
  • Salmonellosis
  • Shigellosis
  • Staphylococcal disease
  • Travelers disease
  • Trichinosis
  • Person-to-person spread
  • Aseptic Meningitis
  • Viral hepatitis
  • Respiratory Infections (influenza)
  • Herpes Simplex
  • Streptococcal disease (rheumatic fever)
  • Tuberculosis
  • Leprosy
  • Water Foodborne
  • Amebiasis
  • Cholera
  • Giardiasis
  • Legionellosis
  • E Coli

5

Infections Sources and agents (1)
  • Arthropod Borne
  • Encephalitis (West Nile)
  • Lyme Disease
  • Malaria
  • Plague
  • Rocky Mountain Spotted Fever
  • Vaccine preventable
  • Chickenpox
  • Diphtheria
  • Measles
  • Mumps
  • Poliomyelitis
  • Tetanus
  • Sexually Transmitted
  • HIV/AIDS
  • Gonorrhea
  • Syphilis
  • Chlamydia Trachomatis
  • Zoonotic
  • Psittacosis
  • Q fever
  • Rabies
  • Hantavirus
  • Prions
  • Kuru
  • vCJD
  • Opportunistic fungal/fungal
  • Coccidioidomycosis
  • Candidiasis

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7
Variation in incubation period
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10
Where would Lassa Fever go? Ebola virus?
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12
Some Definitions
  • INFECTION
  • An infectious agent has entered and established
    itself in (or on) a host.
  • COLONIZATION
  • Organism is present on the surface of the body
    and grows at a rate sufficient to maintain its
    numbers
  • There is no host reaction to the organism.
  • CONTAMINATION
  • The presence of an organism on the surface of a
    body or inanimate object of an infectious agent
    which can serve as a source of infection. The
    organism need not be actively growing.

13
Terminology (1)
  • Infectivity
  • The ability of an agent to invade and multiply in
    a host (an infection).
  • Dose of organism required to establish infection
    in 50 of animals.
  • Pathogenicity
  • The ability of an agent to produce clinically
    apparent illness.

14
Terminology (2)
  • Virulence
  • The proportion of clinical cases which produce
    severe disease and/or permanent sequelae.
  • Immunogenicity
  • The ability of an agent to produce specific
    immunity against the agent
  • Can be produced in general body or within
    specific sites such as the GI tract.
  • Determines the ability of an agent to re-infect a
    host
  • Measles vs. gonorrhea
  • Seroepidemiology
  • The use of the immune status of people to study
    disease severity, distribution, etc.
  • Lassa fever

15
Terminology (3)
  • Reservoir
  • Living organisms or inanimate matter in which
    infectious agent normally lives and multiplies
  • Fomites (Vehicle)
  • Inanimate objects contaminated with infectious
    agent (not the reservoir). Example would be toys
    in a daycare centre.
  • Vector
  • An animate source of an infectious agent. The
    vector may be infected with the organism (e.g.
    mosquitoes and malaria) or just be a mechanical
    carrier (e.g. flies). There is disagreement about
    whether vectors are restricted to insects or can
    also include small mammals.

16
Terminology (4)
  • Zoonoses
  • Diseases transmitted to humans from animals (e.g.
    anthrax)
  • Carriers
  • An infected person without apparent clinical
    disease who remains infectious (e.g. Typhoid
    Mary)
  • Index Case
  • The person (case) who brings the infection to the
    attention of the medical community or the public
    agency. Sometimes used to refer to the person
    who brings the infection into a community. This
    will often (but not always) be the same person.

17
Terminology (5)
  • Attack Rate
  • The probability that people will get ill from the
    disease. Usually applied in an outbreak
    situation. It is a cumulative incidence
    (incidence proportion) type measure.
  • new cases in group
  • ------------------------------------
  • people in group

18
Terminology (6)
  • Secondary Attack Rate
  • Probability of infection in a closed group who
    are at risk but excluding the index case(s).
    Formula is
  • new cases in group index case(s)
  • -----------------------------------------------
    ------
  • people in group index case(s)
  • Risk of H. Influenza infection in family members
    within 30 days of disease in an index case
    (compared to general public)
  • 600

19
Terminology (7)
  • Case Fatality Rate (CFR)
  • The probability of death in people with an
    infection.

20
Pathogenic Mechanisms (1)
  • Direct Tissue Invasion
  • Production of Toxins
  • Allergic Host Reaction
  • Resistant/latent infection (carriers)
  • Enhancement of host susceptibility to drugs (e.g.
    Reyes syndrome and ASA).
  • Immune Suppression

21
Reservoirs (examples of transmission patterns)
  • Human Human Human
  • Animal Animal Animal
  • Human

Animal
Vector
Vector
Human
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Mechanisms of Spread (1)
Indirect transmission
Direct transmission
  • kissing
  • sexual intercourse
  • hand shaking
  • droplets
  • spores in soil

Vehicle borne fomites (e.g. toys) food IV
fluid Vector borne mechanical (e.g. soiled
feet of insect) biological (e.g.
malaria) Airborne dust droplet nuclei
24
Factors Influencing Spread of Disease in a
Population (1)
  • Period of infectivity in relation to symptoms
  • Mumps
  • Carrier state
  • Herd immunity
  • Attack Rate
  • Secondary attack rate
  • Type of spread
  • Person to person
  • common vehicle
  • vector-borne
  • zoonoses
  • Transmission mechanisms
  • sexually transmitted vs. droplet spread

25
Develop disease
Immune
26
Epidemics (1)
  • Epidemic (now often called outbreak)
  • the occurrence in a community or region of cases
    of a disease/condition/behaviour clearly in
    excess of normal expectancy
  • Endemic
  • the occurrence of a disease/condition at a
    relatively constant level in a given setting
    (often required to be at a high rate)
  • Pandemic
  • an epidemic covering a very wide area and
    affecting a large proportion of the population
  • Pathogen
  • Infectious and non-infectious substance capable
    of producing tissue damage or initiating a
    process which can lead to a disease.

27
Epidemics (2)
  • Common conditions increasing likelihood of an
    epidemic
  • The introduction of a new pathogen or an
    increased amount of, or a change in the virulence
    (infectivity) of, a pathogen.
  • An adequate number of exposed and susceptible
    persons.
  • An effective means of transmission between the
    source of the pathogen and the susceptible person.

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Epidemics (4)
  • Types of epidemics
  • Common source
  • Point source
  • Ongoing exposure
  • Need not be geographically localized
  • Propagated/progressive
  • Mixed
  • Note that epidemics can arise from behaviour as
    well as from traditional infectious sources.
  • Intravenous drug users and HIV/HPC
  • Mass hysteria
  • Epidemic curve
  • Spot maps

30
Epidemic Curves point source (1)
31
Epidemic Curves propagated (2)
10 days
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34
Epidemics (3)
  • Incubation period and causal agent

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38
Epidemic of hepatitis in institution
39
Epidemic of hepatitis in institution
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41
SARS
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44
Epidemic Control (1)
  • Twin goals
  • Understand the cause, etc.
  • Minimize the impact to the affected community
  • Goals can conflict
  • need to collect full information base
  • need to take action in absence of full
    information
  • Effective and clear communication with general
    public is essential
  • SARS outbreak
  • designate one spokesperson
  • regular press briefings

45
Epidemic Control (2)
  • Step 1 Identify potential investigation team
    members and resources/prepare for fieldwork.
  • Identify a lead investigator
  • Step 2 Establish the existence of an outbreak
  • Step 3 Verify the diagnosis
  • contact patients, verify history, symptoms, lab
    tests, etc.

46
Epidemic Control (3)
  • Step 4 Construct a working case definition
  • Uses a few simple clinical criteria
  • May be restricted by person, place and/or time
  • do not include an exposure or risk factor you
    want to test.
  • Is loose or sensitive if used for
    case-finding
  • Is specific or tight if used for hypothesis
    testing.
  • Your case definition may (should) change as you
    acquire more information.
  • Step 5 Find cases systematically and develop a
    case listing.

47
Epidemic Control (4)
  • Step 6 Perform descriptive epidemiology
  • time epidemic curve
  • place spot map
  • person look for commonalities. Tends to follow
    case series methods at this point but may involve
    comparison to known community information.
  • Can use cohort or case-control methods is
    appropriate
  • Step 7 Develop hypotheses.
  • Step 8 Evaluate hypotheses.

48
Epidemic Control (5)
  • Step 9 As necessary, reconsider and refine
    hypotheses and conduct additional studies
  • Step 10 Implement control and prevention
    measures.
  • your PRIMARY goal. Implement as soon as feasible
    even if information base is incomplete.
  • Step 11 Communicate your findings.
  • Step 12 Maintain surveillance to monitor trends
    and evaluate control/prevention measures.

49
Actions to take in first 24 hours
  • Determine extent of outbreak
  • Establish etiological agent (if possible).
  • Identify all persons at risk
  • Identify key clinical epidemiological features
  • age, sex, race
  • candidate exposures of interest
  • data of onset, place of onset, etc.
  • Obtain relevant environmental samples
  • when in doubt, take a sample! Think CSI.
  • Designate a public spokesperson and set-up media
    communications system
  • Organize investigative team
  • Call for help (if needed)

50
Actions for acute outbreak control
  • Isolation
  • separation of infected persons or animals from
    others during the period of communicability
  • usually isolate for at least two incubation
    periods.
  • Quarantine
  • restrictions on the activities of well people who
    (may) have been exposed to a communicable disease
    during its period of communicability.
  • active surveillance is an alternative
  • usually quarantine for at least two incubation
    periods.
  • More controversial than isolation since it
    affects people who are not currently ill (and may
    never get ill).
  • Immunization
  • passive or active. Passive (IGG) is more useful
    for acute outbreaks.
  • Chemoprophylaxis

51
General approaches to outbreak control (1)
  • Reduce host susceptibility
  • Immunization (active and passive)
  • nutrition
  • improved income, etc.
  • Interrupt transmission of the agent
  • Quarantine/isolation
  • Case treatment
  • contract tracing
  • inspections
  • environmental clean-up
  • animal population control
  • rabies vaccination of wild animals
  • insect spraying
  • monitor for animal infections

52
General approaches to outbreak control (2)
  • Inactivate agent
  • water purification chlorination
  • Personal hygiene measures
  • hand washing (1 strategy)
  • protective clothing (masks, gowns)
  • avoid at risk situations
  • Family/community measures
  • preventing sexual abuse of children leads to
    reduction in STDs
  • Needle exchange and related programmes.

53
Surveillance
  • the continuing scrutiny of all aspects of
    occurrence and spread of disease that we
    pertinent to effective control
  • Reportable diseases.
  • Sentinel practices
  • Animal/water surveys
  • Environmental monitoring
  • Mortality (vital statistics)
  • Provincial laboratory tests
  • Epidemic investigations
  • Disease registries
  • CIHI and related data.

54
Reportable Diseases
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1

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59
Summary
  • Infectious diseases present unique
    epidemiological challenge due to transmission
    patterns
  • Infectious diseases are not dead in 2009
  • H1N1
  • Outbreaks involve
  • Control
  • Investigation
  • Key control methods include
  • Immunization (which is under threat)
  • Isolation/quarantine
  • chemoprophylaxis
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