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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,
University of Ottawa
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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
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Agents of Infectious Diseases
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Bacteria
Viruses
Rickettsia
Fungi (mycoses)
Protozoa
Helminths
Prions
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Infections: Sources and agents (1)
Foodborne
 Botulism
 Clostridium Perfringens
 Salmonellosis
 Shigellosis
 Staphylococcal disease
 Traveler’s disease
 Trichinosis
Water & Foodborne
 Amebiasis
 Cholera
 Giardiasis
 Legionellosis
 E Coli
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Person-to-person spread
• Aseptic Meningitis
• Viral hepatitis
• Respiratory Infections
(influenza)
• Herpes Simplex
• Streptococcal disease
(rheumatic fever)
• Tuberculosis
• Leprosy
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Infections: Sources and agents (1)
Vaccine preventable
• Chickenpox
• Diphtheria
• Measles
• Mumps
• Poliomyelitis
• Tetanus
Sexually Transmitted
• HIV/AIDS
• Gonorrhea
• Syphilis
• Chlamydia Trachomatis
Arthropod Borne
• Encephalitis (West Nile)
• Lyme Disease
• Malaria
• Plague
• Rocky Mountain Spotted
Fever
Zoonotic
• Psittacosis
• Q fever
• Rabies
• Hantavirus
Prions
• Kuru
• vCJD
Opportunistic fungal/fungal
• Coccidioidomycosis
• Candidiasis
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Variation in incubation period
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Where would Lassa Fever go? Ebola virus?
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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.
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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.
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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
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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.
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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.
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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
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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
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Terminology (7)
• Case Fatality Rate (CFR)
– The probability of death in people with an
infection.
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Pathogenic Mechanisms (1)
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Direct Tissue Invasion
Production of Toxins
Allergic Host Reaction
Resistant/latent infection (carriers)
Enhancement of host susceptibility to
drugs (e.g. Reye’s syndrome and ASA).
• Immune Suppression
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Reservoirs (examples of
transmission patterns)
• Human
Human
Human
• Animal
Animal
Human
Animal
Animal
Vector
Vector
Human
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Mechanisms of Spread (1)
Direct transmission
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kissing
sexual intercourse
hand shaking
droplets
spores in soil
Indirect transmission
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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
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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
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Person to person
common vehicle
vector-borne
zoonoses
• Transmission mechanisms
– sexually transmitted vs. droplet spread
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Develop disease
Immune
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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.
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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
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Epidemic Curves: point source (1)
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Epidemic Curves: propagated (2)
10 days
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Epidemics (3)
• Incubation period and causal agent
Time frame
Examples
Hours
Food toxins
Heavy metals
Days
Bacterial infections
Salmonella / cholera
Weeks
Measles / mumps / Hep A
Months
Hep B / Rabies
Years
Kuru / cancer
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Epidemic of hepatitis in institution
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Epidemic of hepatitis in institution
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SARS
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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
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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.
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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.
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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.
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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.
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Actions to take in first 24 hours
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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)
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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
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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
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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.
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Surveillance
‘the continuing scrutiny of all aspects of occurrence
and spread of disease that we pertinent to effective
control’
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Reportable diseases.
Sentinel practices
Animal/water surveys
Environmental monitoring
Mortality (vital statistics)
Provincial laboratory tests
Epidemic investigations
Disease registries
CIHI and related data.
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Reportable Diseases
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#1
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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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