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The Chain of Transmission is a visual tool that helps illustrate all the components at play during the acquisition of infectious agents. It is applicable to all environments (e.g. hospital, home and community), but the examples given below will be focused on healthcare settings. Each element in the chain is a topic in and of itself, and the subject of further analysis.

Susceptible Host

A person who does not have immunity to the infectious agent or is susceptible as result of being immune-compromised.

Infectious Agents

Infectious agents include bacteria, viruses, yeasts, fungi, parasites and prions. Any of these can act as pathogens and cause infection.

Portal of Entry

A pathway by which an infectious agent can enter a host.

Reservoir

A reservoir is a place where an infectious agent can live and reproduce.

Mode of Transmission

A reservoir is a place where an infectious agent can live and reproduce.

Portal of Exit

A pathway by which an infectious agent can escape its reservoir.
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Infectious Agents

Infectious agents include bacteria, viruses, yeasts, fungi, parasites and prions. Any of these can act as pathogens and cause infection.
  • Patients harbouring infectious agents are largely responsible for their introduction into healthcare settings. Once there, they can contaminate medical equipment and the environment, and are difficult to eradicate.
References:

Peel Public Health. (2011). The Chain of Infection. Retrieved from: http://www.peelregion.ca/health/discon/htmfiles/take-control/2011/sec2/2-1-The-Chain-of-Infection.pdf
Centres for Disease Control. (2017). Transmission-Based Precautions. Retrieved from: https://www.cdc.gov/infectioncontrol/basics/transmission-based-precautions.html

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Portal of Entry

A pathway by which an infectious agent can enter a host. Examples include:
  • Broken skin (cracks, wounds, bed sores)
  • Mucous membranes (eyes, and tissues inside the nose, mouth and vagina)
  • Respiratory track (inhalation)
  • Gastrointestinal tract (ingesting contaminated food or water)
References:

Peel Public Health. (2011). The Chain of Infection. Retrieved from: http://www.peelregion.ca/health/discon/htmfiles/take-control/2011/sec2/2-1-The-Chain-of-Infection.pdf
Centres for Disease Control. (2017). Transmission-Based Precautions. Retrieved from: https://www.cdc.gov/infectioncontrol/basics/transmission-based-precautions.html

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Reservoir

A reservoir is a place where an infectious agent can live and reproduce.
  • Patients are most often the reservoirs for infectious agents within healthcare settings. However, other reservoirs exist, and include food (foodborne pathogens), water and HVAC systems (Legionella).
References:

Peel Public Health. (2011). The Chain of Infection. Retrieved from: http://www.peelregion.ca/health/discon/htmfiles/take-control/2011/sec2/2-1-The-Chain-of-Infection.pdf
Centres for Disease Control. (2017). Transmission-Based Precautions. Retrieved from: https://www.cdc.gov/infectioncontrol/basics/transmission-based-precautions.html

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Mode of Transmission

Refers to the way(s) in which infectious agents travel from reservoirs to hosts.
  • Modes of Transmission includes Contact, Droplet, Airborne, and various combinations of these routes.
  • Contact: The most common form of transmission, whereby persons become exposed to pathogens through contact with a person who is infected, or coming into contact with an object that has been contaminated. Also see Direct Contact and Indirect Contact
  • Droplet: Transmission that occurs as result of large, respiratory droplets containing infectious agents that come into contact with the mucous membranes (e.g. eyes, nose, mouth/throat membranes) of a susceptible host. Droplets can be expelled during coughing, sneezing, laughing and some types of medical procedures (e.g. suctioning), but, as result of their size, typically only travel up to two meters before falling to the ground. Droplets have the potential to land on medical equipment and environmental surfaces, which creates risk for Indirect Contact transmission. Influenza is an example of an infection that spreads through Droplet and Contact transmission.
  • Airborne: Transmission that occurs as result of tiny, respiratory droplet nuclei particles (1-5 microns in diameter), which enter a susceptible host through inhalation. Droplet nuclei can be expelled during all activities that pass air out of the nose/mouth (e.g. talking, laughing, coughing, etc.) as well as during aerosol-generating medical procedures (e.g. intubation, suctioning, sputum induction, etc.). Once in the air, particles can remain suspended for long periods of time. Tuberculosis is an example of an infection that spreads through Airborne transmission.
References:

Peel Public Health. (2011). The Chain of Infection. Retrieved from: http://www.peelregion.ca/health/discon/htmfiles/take-control/2011/sec2/2-1-The-Chain-of-Infection.pdf
Centres for Disease Control. (2017). Transmission-Based Precautions. Retrieved from: https://www.cdc.gov/infectioncontrol/basics/transmission-based-precautions.html

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Portal of Exit

  • A portal of exit is any way that exists for an infectious agent to leave its reservoir. Examples include:
    • When patients cough and sneeze droplets are expelled into the air
    • When patients have vomiting and diarrhea they can easily contaminate their bedspace and bathroom
    • When patients have weeping wounds blood and body fluids can contaminate patient equipment and the environment
References:

Peel Public Health. (2011). The Chain of Infection. Retrieved from: http://www.peelregion.ca/health/discon/htmfiles/take-control/2011/sec2/2-1-The-Chain-of-Infection.pdf
Centres for Disease Control. (2017). Transmission-Based Precautions. Retrieved from: https://www.cdc.gov/infectioncontrol/basics/transmission-based-precautions.html


Content created by Claudia Crussell-Balogh, CIC, MHSc.