Transmission of Infection Pamela S. Falk, MPH Director

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Transmission of Infection
Pamela S. Falk, MPH Director Healthcare
Epidemiology University of Texas Medical
Branch Galveston, Texas
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The chain of infection
Causative Agent
Susceptible host
Reservoir
Portal of exit
Portal of entry
Mode of transmission
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Causative Agents

• Bacteria
• Viruses
• Fungi
• Protozoa
• Helminths
• Prions

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Disease producing characteristics of infectious
agents

• Invasiveness
• Pathogenicity
• Virulence
• Infectious dose
• Viability in the free state
• Host specificity
• Antigenic variation
• Ability to develop resistance to antimicrobial
  agents

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Reservoir

• Definition
• place in which an infectious agent can survive
  but may or may not multiply
• Common reservoirs
• humans
• animals
• equipment
• medication/intravenous fluid

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Human Reservoirs

• Persons with acute or subclinical illness
• Carriers
• during incubation
• convalescent carriers
• chronic carriers
• intermittent carriers

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Portal of exit

• The path by which an infectious agent leaves the
  reservoir
• respiratory tract
• GU tract
• GI tract
• skin/mucous membrane
• blood
• transplacental

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Mode of transmission

• The mechanism for transfer of an infectious
  agent from a reservoir to a susceptible host
• Major modes of transmission are
• contact
• droplet
• airborne
• common vehicle
• vector-borne

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Contact transmission

• direct contact person-to-person spread,
  actual physical contact
• indirect contact contact with contaminated
  intermediate object

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Contact transmission

• most frequent mode of transmission within the
  healthcare environment
• Examples
• touching open and draining wounds
• touching blood
• touching rashes or vesicles
• touching equipment soiled with body fluids

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Contact transmission

• prevention strategies include
• hand hygiene
• cleaning and disinfection of medical devices and
  equipment

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Droplet transmission

• droplets are generated by an infected or
  colonized person during coughing, sneezing,
  talking, suctioning, etc
• droplets propelled a short distance (lt 1 meter)
  and deposited on a susceptible hosts nasal
  mucosa or mouth

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Droplet transmission

• examples
• meningococcus
• influenza
• pertussis
• prevention strategy
• wear a mask when close to patient (lt1 meter)
• hand hygiene

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Airborne transmission

• Droplet nuclei (tiny particles), dust particles
  or skin squames containing microorganisms are
  transmitted to a susceptible host by air currents

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Airborne transmission

• Examples
• chickenpox
• measles
• tuberculosis
• Prevention strategies
• place patient in separate room with door closed
• patient should wear mask when leaving room or
  facility
• staff should be immune to measles and varicella
• staff should wear a mask (respirator) if
  tuberculosis is a possibility

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Common vehicle transmission

• Microorganisms are transmitted to susceptible
  hosts from common items, e.g.
• food
• water
• medications
• devices/equipment

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Multidose vials

• Label with date opened and discard at 30 days or
  manufacturers recommendation
• Discard immediately if suspected or visible
  contamination occurs
• cloudy, precipitate, or discoloration present

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Vector-borne transmission

• Transfer of microorganisms by insects, flies,
  rats, or other vermin
• uncommon mode of transmission in healthcare
  facilities

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Portal of entry

• The path by which an infectious agent enters the
  susceptible host
• respiratory tract
• GU tract
• GI tract
• skin/mucous membrane
• parenteral
• transplacental

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Susceptible Host

• A person lacking effective resistance to a
  particular pathogenic organism

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Host characteristics that influence
susceptibility to and severity of disease

• Age
• Socioeconomic status
• Disease history
• Lifestyle
• Heredity
• Nutritional status

• Immunization status
• Diagnostic/therapeutic procedures
• Medications
• Pregnancy
• Trauma

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Nonspecific Host Defense Mechanisms

• Normal (endogenous) flora
• Natural antibodies
• Natural barriers to entry of microorganisms
• skin and mucous membranes
• respiratory tract cilia, cough mechanism
• intestinal tract gastric acid
• GU tract mechanical flushing
• eye tears
• Nutritional status

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Examples of steps in the chain of
infectionSalmonellosis

• Causative agent a bacteria, Salmonella
• infective dose 1 million organisms if host has
  normal gastric acidity
• viable in free state
• Reservoirs
• human gt active case, carriers
• animals gt poultry, cattle, reptiles
• environment gt contaminated food products,
  untreated sewage, biologic waste products

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Examples of steps in the chain of
infectionSalmonellosis

• Portal of exit GI tract, GU tract
• Modes of transmission
• Contact (contaminated hands)
• Common vehicle (food, water)
• Portal of entry GI tract
• Susceptible host anyone

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The chain of infection
Causative Agent
Susceptible host
Reservoir
Portal of exit
Portal of entry
Mode of transmission
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