Back to Basics, 2013
POPULATION HEALTH:
Infectious Diseases and
Outbreak Investigation
N. Birkett, MD
Epidemiology & Community Medicine
Other resources available on Individual & Population Health
web site
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78-5 Outbreak Management (1)
• Physicians are crucial participants in the control of
outbreaks of disease.
• They must be able to
• diagnose cases,
• recognize outbreaks,
• report these to public health authorities and
• work with authorities to limit the spread of the outbreak.
• A common example includes physicians working in nursing
homes and being asked to assist in the control of an outbreak of
influenza or diarrhoea.
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78-5 Outbreak Management (2)
Key Objectives
• Know the defining characteristics of an outbreak and
how to recognize one when it occurs.
• Demonstrate essential skills involved in controlling an
outbreak and its impact on the public, in collaboration
with public health authorities as appropriate.
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78-5 Outbreak Management (3)
Enabling Objectives
• Define an outbreak in terms of an excessive number of cases beyond that
usually expected.
• Describe and understand the main steps in outbreak management and
prevention.
• Demonstrate skills in effective outbreak management including infection
control when the outbreak is due to an infectious agent.
• Describe the different types of infection control practices and justify which
type is most appropriately implemented for different outbreak conditions.
• Demonstrate effective communication skills with patients and the
community as a whole.
• Describe appropriate approaches to prevent or reduce the risk of the
outbreak recurring.
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Infectious Disease Summary
• Terminology
• Nature of diseases
• Outbreaks/epidemics
– Identification
– Methods of control
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Infections: Sources and agents (1)
Foodborne
• Botulism
• Clostridium perfringens
• Hepatitis A
• Norovirus
• Salmonellosis
• Shigellosis
• Staphylococcal disease
• Trichinosis
Water & Foodborne
• Amebiasis
• Cholera
• Giardiasis
• Legionellosis
• E coli
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Person-to-person spread
• Aseptic meningitis
• Hepatitis B and C
• Respiratory Infections
(e.g., influenza)
• Herpes simplex
• Streptococcal disease
• Tuberculosis
• Leprosy
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Infections: Sources and agents (2)
Vaccine preventable
• Chickenpox
• Diphtheria, pertussis,
tetanus
• Hepatitis A and B
• HPV
• Influenza
• Measles, mumps, rubella
• Meningococcal
• Pneumococcal
• Poliomyelitis
Sexually Transmitted
• HIV/AIDS
• Gonorrhea
• Syphilis
• Chlamydia trachomatis
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Arthropod Borne
• Encephalitis (West Nile)
• Lyme Disease
• Malaria
• Plague
• Rocky Mountain Spotted
Fever
Zoonotic
• Psittacosis
• Q fever
• Rabies
• Hantavirus
• West Nile
Prions
• Kuru
• vCJD
Fungal
• Candidiasis
• Coccidioidomycosis
• Histoplasmosis
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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.
• Pathogen
– Infectious and non-infectious substances capable of producing
tissue damage or initiating a process which can lead to a
disease.
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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 the
same host
• e.g., 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).
• E.g. 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).
• Usually insects or small mammals such as rodents.
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Terminology (4)
•
Zoonoses
– Diseases transmitted to humans from animals (e.g. anthrax)
• Carrier
– An infected person without apparent clinical disease who
remains infectious (e.g. Typhoid Mary)
• Index Case
– The first case to be diagnosed in an outbreak
– Sometimes defined as the first case noticed in the outbreak
• Contacts
– People who have possibly been infected due to relevant contact
with an infectious case
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Terminology (5)
• Attack Rate
– The probability that people will get ill from the
disease. Usually applied in an outbreak situation.
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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:
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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)
•
•
•
•
•
Direct Tissue Invasion (Group A Strep)
Production of Toxins (Diphtheria)
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
• Touch (many URI’s,
scabies)
• Bite (rabies)
• Kiss (mononucleosis)
• Sexual contact (Chlamydia)
• Droplet, over 5μ, spread 1-2
meters (resp. virus)
• Soil (tetanus)
• Transplacental (hepatitis B)
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Indirect transmission
• Vehicle borne (inanimate)
• fomites (e.g. toys)
• Food
• IV fluid
• organism may or may not
multiply
• E. coli in hamburger
• Vector borne (animate)
• mechanical (e.g. soiled feet
of insect)
• biological (e.g. malaria)
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Mechanisms of Spread (2)
Airborne transmission
• Droplet nuclei
–
–
–
–
–
tuberculosis
measles
varicella
smallpox
? Influenza
• Dust
– anthrax
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Epidemics (1)
• Epidemic
– now often called an ‘outbreak’, especially if localized
– the occurrence (in a community or region) of a
disease/condition/behaviour clearly in excess of normal
expectancy
• Pandemic
– an epidemic covering a very wide area and affecting a large
proportion of the population (SIM web link)
– Controversial definition
• Consider H1N1 outbreak
• Endemic
– the occurrence of a disease/condition at a relatively constant
level in a given setting, usually with on-going transmission
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Epidemics (2)
• Conditions increasing likelihood of an epidemic
– Agent conditions
• Introduction of a new pathogen
• Increased amount of a pathogen
• Change in the virulence of a pathogen.
– Population conditions
• 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 possible causal agents
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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Distribution of cases by onset of symptoms:
point source type
Number of cases
20
15
10
Maximum incubation period
Average
incubation period
Minimum
incubation period
5
0
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Exposure
Units of Time
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Distribution of cases by onset of symptoms
Continuous Source type
Number of cases
20
15
10
5
0
Exposure
begins &
continues
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Units of Time
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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
– Direct
– Indirect
– Airborne
• Transmission mechanics
– Consider sexual vs. droplet spread
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Herd Immunity
Develop disease
Immune
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Epidemic Control (1)
• Twin goals:
– Minimize the impact to the affected community
– Understand the cause and mode of transmission
• Goals can conflict:
– Need to take action in absence of full information
– Need to collect full information base
• Effective and clear communication with general public is
essential
– Designate one spokesperson
– Regular press briefings
– Lessons from the SARS outbreak
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Outbreak control measures (1)
Isolation
•
separation of infectious 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
– Quarantine for the longest usual incubation period
•
Often at least two incubation periods.
– More controversial than isolation since it affects people who are not
currently ill (and may never get ill).
– we expect some who are quarantined to get sick
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Outbreak control measures (2)
Immunization
• passive or active
• Passive can be more useful for acute outbreak control
Chemoprophylaxis
• Treat people with disease
• Preventive use of antibiotics for people exposed
– tuberculosis
– meningococcal infection
Harm Reduction (for long term outbreaks)
• Focus on prevent adverse effects from harmful exposures
– Needle exchange
– Safe Injection sites
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Outbreak control measures (3)
Reduce host susceptibility
•
Immunization (active and passive)
•
nutrition
•
improve living conditions and other social determinants of health
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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Outbreak control measures (4)
Inactivate agent
• water purification; chlorination
Personal hygiene measures
• hand hygiene (#1 strategy)
• cough etiquette
• protective clothing (masks, gowns)
• avoid at risk situations
Family/community measures
• preventing sexual abuse of children leads to reduction in STIs
• Needle exchange and related programmes.
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Surveillance (1)
The continuing scrutiny of all aspects of occurrence and
spread of disease that are pertinent to effective control
• Reportable diseases.
– legal obligation to report designated diseases to local public health
department
– list includes
• AIDS
• food poisoning
• encephalitis
• STIs
• influenza
• Lassa fever
• SARS
• plus many more
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Surveillance (2)
The continuing scrutiny of all aspects of occurrence and
spread of disease that are 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.
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