Infection Control: New York State Mandatory Training

Element II: The modes and mechanisms of transmission of pathogenic organisms in the healthcare setting and strategies for prevention and control.


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Bacillus anthracis

The "Chain of Infection" is a basic component of understanding the prevention and control of infection that most healthcare workers recall from their early days of training. It is a critical concept in infection control that is worth reviewing:

Chain of Infection

Chain of Infection

The pathogen is the microorganism that causes infection. They include bacteria, viruses, fungi and parasites. There must be an adequate number of pathogens to cause disease. Infectious agents transmitted during healthcare derive primarily from human sources but inanimate environmental sources have also been implicated in transmission (Siegel, et al., 2007).

The reservoir is the place where microorganisms live, such as in humans and animals, in soils, food, plants, air or water. The reservoir must meet the needs of the pathogen in order for the pathogen to survive and multiply. Human reservoirs include patients, healthcare personnel, and household members and other visitors. Such source individuals may have active infections, may be in the asymptomatic and/or incubation period of an infectious disease, or may be transiently or chronically colonized with pathogenic microorganisms, particularly in the respiratory and gastrointestinal tracts. The endogenous flora of patients (e.g., bacteria residing in the respiratory or gastrointestinal tract) also are the source of HAIs (Siegel, et al., 2007).

The means of escape are how the microorganism leaves the reservoir. These portals can be:

  • Respiratory- for example, viruses that cause the common cold, Mycobacterium. tuberculosis, and Haemophilus influenza utilizes this means of exit from the reservoir.
  • Genitourinary- for example, sexually transmitted diseases such as syphillus or HIV.
  • Alimentary - for example, salmonella, rotavirus, C. difficile, Giardia.
  • Skin - for example, scabies, impetigo.
  • Blood and body fluids - HIV, Hepatitis B and C.
  • Transplancental - for example, Rubella and HIV.

Some microorganisms have more than one means of escape - for example, chickenpox can be spread via respiratory source or the patient's skin. Bloodborne pathogens such as HIV, Hepatitis B and C can be spread through blood and from fluid from the genitourinary system.

The mode of transmission is how the pathogen moves from place to place. This can occur through three principle routes:

  • Contact transmission, which is further divided into (Siegel, et al., 2007):

    • Direct transmission occurs when microorganisms are transferred from one infected person to another person without a contaminated intermediate object or person. Opportunities for direct contact transmission between patients and healthcare personnel include:
      • blood or other blood-containing body fluids from a patient directlyenters a caregiver's body through contact with a mucous membrane or breaks (i.e., cuts, abrasions) in the skin.
      • mites from a scabies-infested patient are transferred to the skin of a caregiver while he/she is having direct ungloved contact with the patient's skin.
      • a healthcare provider develops herpetic whitlow on a finger after contact with HSV when providing oral care to a patient without using gloves or HSV is transmitted to a patient from a herpetic whitlow on an ungloved hand of a healthcare worker.

    • Indirect transmission involves the transfer of an infectious agent through a contaminated intermediate object or person. In the absence of a point-source outbreak, it is difficult to determine how indirect transmission occurs. However, extensive evidence suggests that the contaminated hands of healthcare personnel are important contributors to indirect contact transmission. Examples of opportunities for indirect contact transmission include (Siegel, et al., 2007):
      • Hands of healthcare personnel may transmit pathogens after touching an infected or colonized body site on one patient or a contaminated inanimate object, if hand hygiene is not performed before touching another patient.
      • Patient-care devices (e.g., electronic thermometers, glucose monitoring devices) may transmit pathogens if devices contaminated with blood or body fluids are shared between patients without cleaning and disinfecting between patients.
      • Shared toys may become a vehicle for transmitting respiratory viruses or pathogenic bacteria among pediatric patients.
      • Instruments that are inadequately cleaned between patients before disinfection or sterilization (e.g., endoscopes or surgical instruments) or that have manufacturing defects that interfere with the effectiveness of reprocessing may transmit bacterial and viral pathogens.
      • Clothing, uniforms, laboratory coats, or isolation gowns used as personal protective equipment (PPE), may become contaminated with potential pathogens after care of a patient colonized or infected with an infectious agent. Although contaminated clothing has not been implicated directly in transmission, the potential exists for soiled garments to transfer infectious agents to successive patients.

  • Droplet transmission is, technically, a form of contact transmission, and some infectious agents transmitted by the droplet route also may be transmitted by the direct and indirect contact routes. However, in contrast to contact transmission, respiratory droplets carrying infectious pathogens transmit infection when they travel directly from the respiratory tract of the infectious individual to susceptible mucosal surfaces of the recipient, generally over short distances, necessitating facial protection. Respiratory droplets are generated when an infected person coughs, sneezes, or talks or during procedures such as suctioning, endotracheal intubation, cough induction by chest physiotherapy and cardiopulmonary resuscitation (Siegel, et al., 2007).

  • Studies have shown that the nasal mucosa, conjunctivae and less frequently the mouth, are susceptible portals of entry for respiratory viruses. The maximum distance for droplet transmission is currently unresolved; historically, the area of defined risk has been a distance of less than 3 feet around the patient. Using this distance for donning masks has been effective in preventing transmission of infectious agents via the droplet route. There is some evidence to suggest that some droplets (SARS and smallpox) could reach persons located 6 feet or more from their source. It is likely that the distance droplets travel depends on the velocity and mechanism by which respiratory droplets are propelled from the source, the density of respiratory secretions, environmental factors such as temperature and humidity, and the ability of the pathogen to maintain infectivity over that distance. Based on these considerations, it may be prudent to don a mask when within 6 to 10 feet of the patient or upon entry into the patient's room, especially when exposure to emerging or highly virulent pathogens is likely. More studies are needed to improve understanding of droplet transmission under various circumstances (Siegel, et al., 2007).

    Droplet size is another variable under discussion. Droplets traditionally have been defined as being >5 Ám in size. Droplet nuclei, particles arising from desiccation of suspended droplets, have been associated with airborne transmission and defined as <5 Ám in size. Observations of particle dynamics have demonstrated that a range of droplet sizes, including those with diameters of 30 microns or greater, can remain suspended in the air. The behavior of droplets and droplet nuclei affect recommendations for preventing transmission. Whereas fine airborne particles containing infectious pathogens can remain a lot in the air, requiring an airborne infection isolation room (AIIR) to prevent its dissemination within a facility; organisms transmitted by the droplet route cannot remain aloft in the air and therefore do not require special air handling and ventilation. Examples of infectious agents that are transmitted via the droplet route include Bordetella pertussis, influenza virus 23, adenovirus 111 , rhinovirus, Mycoplasma pneumoniae, SARS-associated coronavirus (SARS-CoV), group A streptococcus, and Neisseria meningitides (Siegel, et al., 2007)."

  • Airborne transmission occurs by dissemination of either airborne droplet nuclei or small particles in the respirable size range containing infectious agents that remain a lot in the air over time and distance (e.g., spores of Aspergillus spp, and Mycobacterium tuberculosis) (Siegel, et al., 2007).

    Microorganisms carried in this manner may be dispersed over long distances by air currents and may be inhaled by susceptible individuals who have not had face-to-face contact with (or been in the same room with) the infectious individual. Preventing the spread of pathogens that are transmitted by the airborne route requires the use of special air handling and ventilation systems (e.g., AIIRs) to contain and then safely remove the infectious agent. Infectious agents to which this applies include Mycobacterium tuberculosis, rubeola virus (measles), and varicella-zoster virus (chickenpox). In addition, published data suggest the possibility that variola virus (smallpox) may be transmitted over long distances through the air under unusual circumstances and AIIRs are recommended for this agent as well; however, droplet and contact routes are the more frequent routes of transmission for smallpox. In addition to AIIRs, respiratory protection with NIOSH certified N95 or higher level respirator is recommended for healthcare personnel entering the AIIR to prevent acquisition of airborne infectious agents (Siegel, et al., 2007).

The means of entry is how the microorganism enters the host. Often this is the same means from which the organism left the reservoir.

The susceptible host is the person who may become infected. Infection is the result of a complex interrelationship between a potential host and an infectious agent. Most of the factors that influence infection and the occurrence and severity of disease are related to the host. However, characteristics of the host-agent interaction as it relates to pathogenicity, virulence and antigenicity are also important, as are the infectious dose, mechanisms of disease production and route of exposure. There is a spectrum of possible outcomes following exposure to an infectious agent.

Some persons exposed to pathogenic microorganisms never develop symptomatic disease while others become severely ill and even die (Siegel, et al., 2007). Some individuals are prone to becoming transiently or permanently colonized but remain asymptomatic. Still others progress from colonization to symptomatic disease either immediately following exposure, or after a period of asymptomatic colonization. The immune state at the time of exposure to an infectious agent, interaction between pathogens, and virulence factors intrinsic to the agent are important predictors of an individuals' outcome (Siegel, et al., 2007).

Host factors such as extremes of age and underlying disease (e.g. diabetes), human immunodeficiency virus/acquired immune deficiency syndrome [HIV/AIDS], malignancy, and transplants can increase susceptibility to infection as do a variety of medications that alter the normal flora (e.g., antimicrobial agents, gastric acid suppressants, corticosteroids, antirejection drugs, antineoplastic agents, and immunosuppressive drugs). Surgical procedures and radiation therapy impair defenses of the skin and other involved organ systems. Indwelling devices such as urinary catheters, endotracheal tubes, central venous and arterial catheters and synthetic implants facilitate development of HAIs by allowing potential pathogens to bypass local defenses that would ordinarily impede their invasion and by providing surfaces for development of biofilms that may facilitate adherence of microorganisms and protect from antimicrobial activity. Some infections associated with invasive procedures result from transmission within the healthcare facility, others arise from the patient's endogenous flora.

The host may also have acquired immunity to the pathogen such as may occur through previous infection with the pathogen or through immunization (Siegel, et al., 2007).

The occurrence and presence of all these factors and events is considered the "chain of infection". In the healthcare setting, all of these factors come into play in the spread and the control of infection. Effective infection control strategies prevent disease transmission by interrupting one or more links in the chain of infection (CDC, 2003).

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