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Infectious Disease

VIEWS: 8 PAGES: 31

									          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                                                                 1
          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

April 7, 2009                                      2
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          Infections: Sources and agents (1)

      Foodborne
       Botulism
       Clostridium Perfringens
       Salmonellosis             Person-to-person spread
       Shigellosis               • Aseptic Meningitis
       Staphylococcal disease    • Viral hepatitis
       Traveler’s disease        • Respiratory Infections
       Trichinosis                 (influenza)
                                  • Herpes Simplex
                                  • Streptococcal disease
         Water & Foodborne          (rheumatic fever)
          Amebiasis              • Tuberculosis
          Cholera                • Leprosy
          Giardiasis
          Legionellosis
          E Coli

April 7, 2009                                                4
          Infections: Sources and agents (2)
Vaccine preventable                 Arthropod Borne
• Chickenpox                        • Encephalitis (West Nile)
• Diphtheria                        • Lyme Disease
• Measles                           • Malaria
• Mumps                             • Plague
• Poliomyelitis                     • Rocky Mountain Spotted
• Tetanus                             Fever


Sexually Transmitted           Zoonotic
• HIV/AIDS                     • Psittacosis
• Gonorrhea                    • Q fever               Prions
• Syphilis                     • Rabies                • Kuru
• Chlamydia Trachomatis        • Hantavirus            • vCJD


                Opportunistic fungal/fungal
                • Coccidioidomycosis
                • Candidiasis
April 7, 2009                                                    5
                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.

April 7, 2009                                           6
                    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

April 7, 2009                                                   7
                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.
April 7, 2009                                                 8
                 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.

April 7, 2009                                                   9
                 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:



April 7, 2009                                                  10
                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

April 7, 2009                             15
                Reservoirs (examples of
                 transmission patterns)
• Human                    Human       Human

• Animal                   Animal      Animal
                               Human
                  Animal

                              Vector
    Vector

                 Human

April 7, 2009                                   16
         Mechanisms of Spread (1)
Direct transmission       Indirect transmission
•    kissing              •   Vehicle borne
•    sexual intercourse   •   fomites (e.g. toys)
•    hand shaking         •   food
                          •   IV fluid
•    droplets             •   Vector borne
•    spores in soil       •   mechanical (e.g. soiled
                              feet of insect)
                          •   biological (e.g. malaria)
                          •   Airborne
                          •   dust
                          •   droplet nuclei
April 7, 2009                                             17
                   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.
April 7, 2009                                                  18
                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.
April 7, 2009                                       19
                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

April 7, 2009                                   20
                       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.
April 7, 2009                                       21
  Epidemic Curves: point source (1)




April 7, 2009                     22
   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

April 7, 2009                                         25
                Develop disease
                Immune
April 7, 2009                     26
                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
April 7, 2009                                       27
 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

April 7, 2009                                                                  28
    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
April 7, 2009                               29
    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.
April 7, 2009                                             31

								
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