Download B2B Pop Health, April 6_2009, part 2

Back to Basics, 2009
POPULATION HEALTH (1):
Infectious Diseases and
Outbreak Investigation
N Birkett, MD
Epidemiology & Community Medicine
Other resources available on Individual & Population Health
web site
April 7, 2009
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Infectious Disease Summary
N. Birkett, Back-to-Basics, 2009
• Material relates to MCC objective 78.5
• Terminology
• Nature of diseases
• Outbreaks/epidemics
– Identification
– Methods of control
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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 (2)
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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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
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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.
• Secondary Attack Rate
– Probability of infection in a closed group who are at
risk but excluding the index case(s). Formula is:
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Terminology (6)
• 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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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
• 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 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 (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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Epidemics (4)
• Types of epidemics
– Common source
• Point source
• Ongoing exposure
• Need not be geographically localized
– Propagated/progressive
– Mixed
• Epidemic curve
• Spot maps
• Note that epidemics can arise from behaviour as
well as from traditional infectious sources.
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Epidemic Curves: point source (1)
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Epidemic Curves: propagated (2)
10 days
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Factors Influencing Spread of
Disease in a Population (1)
• Period of infectivity in relationship to symptoms
– Includes consideration of carrier states
• Herd immunity
• Type of spread
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Person to person
Common vehicle
Vector-borne
Zoonoses
• Transmission mechanics
– Consider sexual vs. droplet spread
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Develop disease
Immune
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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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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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