March 30, 20101 Back to Basics, 2010 POPULATION HEALTH (3B): Infectious Diseases and Outbreak...
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Transcript of March 30, 20101 Back to Basics, 2010 POPULATION HEALTH (3B): Infectious Diseases and Outbreak...
March 30, 2010 1
Back to Basics, 2010POPULATION HEALTH (3B):
Infectious Diseases and Outbreak Investigation
N Birkett, MDEpidemiology & Community Medicine
Other resources available on Individual & Population Health web site
March 30, 2010 2
Infectious Disease SummaryN. Birkett, Back-to-Basics, 2010
• Material relates to MCC objective 78.5
• Terminology
• Nature of diseases
• Outbreaks/epidemics– Identification– Methods of control
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Foodborne Botulism Clostridium Perfringens Salmonellosis Shigellosis Staphylococcal disease Traveler’s disease Trichinosis
Water & Foodborne Amebiasis Cholera Giardiasis Legionellosis E Coli
Person-to-person spread• Aseptic Meningitis• Viral hepatitis• Respiratory Infections
(influenza)• Herpes Simplex• Streptococcal disease
(rheumatic fever)• Tuberculosis• Leprosy
Infections: Sources and agents (1)
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Arthropod Borne• Encephalitis (West Nile)• Lyme Disease• Malaria• Plague• Rocky Mountain Spotted
Fever
Sexually Transmitted• HIV/AIDS• Gonorrhea• Syphilis• Chlamydia Trachomatis
Vaccine preventable• Chickenpox• Diphtheria• Measles• Mumps• Poliomyelitis• Tetanus
Infections: Sources and agents (2)
Zoonotic• Psittacosis• Q fever• Rabies• Hantavirus
Opportunistic fungal/fungal• Coccidioidomycosis• Candidiasis
Prions• Kuru• vCJD
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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)
• 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 Animal
Human
Vector
Animal
Vector
Human
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Mechanisms of Spread (1)
• 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
Direct transmission Indirect transmission
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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 ExamplesHours 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
– 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 communityGoals can conflict:• need to collect full information base• need to take action in absence of full informationEffective 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’
• Reportable diseases.• Sentinel practices• Animal/water surveys• Environmental monitoring• Mortality (vital statistics)• Provincial laboratory tests• Epidemic investigations• Disease registries• CIHI and related data.