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
Chain of Infection
The pathogen is the microorganism that causes infection. Pathogens include bacteria, viruses, fungi and parasites. There must be an adequate number of pathogens to cause disease. Infectious agents transmitted during healthcare derived primarily from human sources but inanimate environmental sources have also been implicated in transmission.
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.
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,
- 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
The mode of transmission is how the pathogen
moves from place to place. This can occur through three principle
- Contact transmission, which is further divided
- 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 directly enters 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 herpes simplex virus (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:
- 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
- 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
- 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
- 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.
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).
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.
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. 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.
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.
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.
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