A. Chain of Infection

Topic Progress:

A chain is an interconnected series of links. A chain of infection, then, is the group of links that must be present in order for infection to occur. There are five links necessary for an infection to exist:

  1. Pathogen
  2. Source or Reservoir
  3. Transmission Mode
  4. Portal of Entry
  5. A Susceptible Host

Link #1: Pathogen

A pathogen is any organism capable of producing disease. A pathogen’s ability to produce disease is based on both virulence and numbers.

  1. Virulence is the strength of an organism’s ability to produce disease.
  2. Numbers means that in order to produce disease, there must be a high enough number of pathogenic microorganisms present in a susceptible host (see Link #5).

Link #2: Source or Reservoir

  1. Reservoirs are locations in the body where germs live and grow and are the source of pathogens.
  2. The primary source of pathogens in the Dental Healthcare Setting (DHCS) is the patient’s mouth; specifically, pathogens can be found in saliva, blood, and respiratory secretions.

Link #3: Mode of Transmission

This is the link that we have control over since there are precautions that can be taken against transmission (see Section 4). There are several ways disease is transmitted:

  1. Direct: Person-to-person contact; Infected agents are found in droplets spread directly through sneezing & coughing or contact with infected body fluids (blood) or lesions. SARS-CoV-2, Hepatitis, HIV, and TB are examples of diseases spread through direct contact. Examples include placing an ungloved hand into someone’s mouth, saliva splattering into the DHCP’s eyes or mouth, breathing in the aerosol from an infected person’s cough, etc.
  2. Indirect: Infected agents are first transmitted to an object, surface, or another person, and then transferred to an individual. Examples include touching infected/contaminated countertops, charts or other items then touching your mouth, nose or eyes, or a poke from a contaminated instrument or a contaminated needle (needlestick injury).
  3. Airborne (Droplet infection): Disease is spread through droplets of moisture in the air that contain bacteria or viruses. Most contagious respiratory diseases are transmitted through droplets in the air (generally caused by coughing &/or sneezing). Some of the diseases that are airborne are SARS-CoV-2, TB, flus, and colds. These are called aerosol transmittable disease.
    1. Aerosol, Spray, or Spatter (Airborne): Terms used to describe droplets of potentially infectious microorganisms. 
    2. Aerosols are the finest form of droplets and disease is generally spread through inhaling infectious droplets. Aerosols are created by using high-speed-handpieces, ultrasonic scalers and air-water syringes. Aerosols can remain suspended in the air for a while.
    3. Sprays & Spatter are larger than aerosols and generally spread infection through contact with mucous membranes or non-intact skin (skin in which a cut, scrape, or needle prick has occurred). Spays & Spatter are created in the same ways as aerosols and are often associated with using the air-water syringe.
      • Examples of direct airborne transmission include:
        • Contaminated splatter, aerosol, or mist from the patient’s saliva (or even prophy paste) goes directly into the DHCP’s nose, mouth, eyes, or open cut.
      • Examples of indirect airborne transmission include:
        • Contaminated splatter, aerosol, or mist from the patient’s saliva (or even prophy paste) lands on the countertop. The DCHP touches the countertop with bare hands and then touches his/her nose, mouth, eyes, or open cut.
    4. Blood-Borne (bloodborne) Transmission: Disease is spread through blood carrying a pathogen.
      Some Blood-Borne (bloodborne) pathogens included HIV, HBV, HCV. Blood-Borne diseases can be spread by direct contact or indirect contact.
      • Examples of indirect transmission include:
        • Splatter containing contaminated blood from a patient lands on a counter top. An unprotected person touches the contaminated surface with a bare hand and then touches their mouth.
        • A needlestick injury or poke with a contaminated instrument.
      • Examples of direct transmission include:
        • Splatter containing contaminated blood lands in the eye, nose, or mouth of another person. Unprotected sex is an example of direct transmission.

Link #4: Portal of Entry

A Portal of Entry is the means through which an infectious microorganism enters the body.

  1. The primary portals of entry for airborne pathogens are the mouth, eyes and nose.
  2. Bloodborne pathogens typically enter the body by a poke from contaminated needle or instrument or via non-intact skin (e.g. open cut).
  3. Mucous membranes in the eyes and mouth are a portal of entry.

Link #5: Susceptible Host

A susceptible host is someone who is unable to resist infection by a specific infectious agent.

  1. A compromised immune system or comorbidities (unhealthy conditions such as heart disease, cancer, diabetes) can make one a susceptible host.
  2. Factors such as fatigue, stress, poor eating habits can make one a susceptible host.
  3. It is important to note that at times even healthy individuals are susceptible hosts for pathogens.