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Chapter: Medical Microbiology: An Introduction to Infectious Diseases: Nosocomial Infections and Infection Control

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Nosocomial Infections and Their Sources

Infections occurring during any hospitalization are either community acquired or nosoco-mial.

NOSOCOMIAL INFECTIONS AND THEIR SOURCES

Infections occurring during any hospitalization are either community acquired or nosoco-mial. Community infections are those present or incubating at the time of hospital admis-sion. All others are considered nosocomial. For example, a hospital case of chickenpox could be community acquired if it erupted on the fifth hospital day (incubating) or noso-comial if hospitalization was beyond the limits of the known incubation period (20 days). Infections appearing shortly after discharge (2 weeks) are considered nosocomial, although some could have been acquired at home. Infectious hazards are inherent to the hospital environment; it is there that the most seriously infected and most susceptible patients are housed and often cared for by the same staff.

The infectious agents responsible for nosocomial infections arise from various sources, including patients’ own normal flora. In addition to any immunocompromising disease or therapy, the hospital may impose additional risks by treatments that breach the normal defense barriers. Surgery, urinary or intravenous catheters, and invasive diagnostic procedures all may provide normal flora with access to usually sterile sites. Infections in which the source of organisms is the hospital rather than the patient include those derived from hospital personnel, the environment, and medical equipment.

Hospital Personnel

Physicians, nurses, students, therapists, and any others who come in contact with the pa-tient may transmit infection. Transmission from one patient to another is calledcross-infection. The vehicle of transmission is most often the inadequately washed hands of amedical attendant. Another source is the infected medical attendant. Many hospital out-breaks have been traced to hospital personnel, particularly physicians, who continue to care for patients despite an overt infection. Transmission is usually by direct contact, al-though airborne transmission is also possible. A third source is the person who is not ill but is carrying a virulent strain. For Staphylococcus aureus and group A streptococci, nasal carriage is most important, but sites such as the perineum and anus have also been involved in outbreaks. An occult carrier is less often the source of nosocomial infection than a physician covering up a boil or a nurse minimizing “the flu.” The carrier is difficult to detect unless the epidemic strain has distinctive characteristics or the epidemiologic circumstances point to a single person.

Environment

The hospital air, walls, floors, linens, and the like are not sterile and thus could serve as asource of organisms causing nosocomial infections, but the importance of this route hasgenerally been exaggerated. With the exception of the immediate vicinity of an infectedindividual or a carrier, transmission through the air or on fomites is much less important than that caused by personnel or equipment. Notable exceptions are when the environ-ment becomes contaminated with Mycobacterium tuberculosis from a patient or Legionella pneumophila in the water supply. These events are most likely to result in dis-ease when the organisms are numerous or the patient is particularly vulnerable (eg, after heart surgery or bone marrow transplant).

Medical Devices

Much of the success of modern medicine is related to medical devices that support or mon-itor basic body functions. By their very nature, devices such as catheters and respirators carry a risk of nosocomial infection, because they bypass normal defense barriers, provid-ing microorganisms access to normally sterile fluids and tissues. Most of the recognized causes are bacterial or fungal. The risk of infection is related to the degree of debilitation of the patient and various factors concerning the design and management of the device. Any device that crosses the skin or a mucosal barrier allows flora in the patient or environ-ment to gain access to deeper sites around the outside surface. Possible access inside the device (eg, in the lumen) adds another and sometimes greater risk. In some devices, such as urinary catheters, contamination is avoidable; in others, such as respirators, complete sterility is either impossible or impractical to achieve.

The risk of contamination leading to infection is increased if organisms that gain ac-cess can multiply within the system. The availability of water, nutrients, and a suitable temperature largely determine which organism will survive and multiply. Many of the Gram-negative rods such as Pseudomonas, Acinetobacter, and members of the Enterobac-teriaceae can multiply in an environment containing water and little else. Gram-positive bacteria generally require more physiologic conditions.

Even with proper growth conditions, many hours are required before contaminating organisms become numerous. Detailed studies of catheters and similar devices show the risk of infection begins to increase after 24 to 48 hours and is cumulative even if the de-vice is changed or disinfected at intervals. It is thus important to discontinue transcuta-neous procedures as soon as medically indicated. The medical devices most frequently associated with nosocomial infections are listed below. The infectious risk of others can be estimated from the principles discussed previously. New devices are constantly being introduced into medical care, occasionally without adequate consideration of their poten-tial to cause nosocomial infection.

Urinary Catheters

Urinary tract infection (UTI) accounts for 40 to 50% of all nosocomial infections, and at least 80% of these are associated with catheterization. The infectious risk of a single uri-nary catheterization has been estimated at 1%, and indwelling catheters carry a risk that may be as high as 10%. The major preventive measure is maintenance of a completely closed system through the use of valves and aspiration ports designed to prevent bacterial access to the inside of the catheter or collecting bag. Unfortunately, breaks in closed sys-tems eventually occur if the system is in place for more than 30 days. The urine itself serves as an excellent culture medium once bacteria gain access. Although Escherichiacoli is still a leading cause of nosocomial UTIs, other Enterobacteriaceae and Pseudomonas are more likely than in the community setting.

Vascular Catheters

Needles and plastic catheters placed in veins (or, less often, in arteries) for fluid adminis-tration, monitoring vital functions, or diagnostic procedures are a leading cause of noso-comial bacteremia. These sites should always be suspected as a source of organisms whenever blood cultures are positive with no apparent primary site for the bacteremia. Contamination at the insertion site is generally staphylococcal, with continued growth in the catheter tip. Organisms may gain access somewhere in the lines, valves, bags, orbottles of intravenous solutions proximal to the insertion site. The latter circumstance usually involves Gram-negative rods. Preventive measures include aseptic insertion tech-nique and appropriate care of the lines, including changes at regular intervals.

Respirators

Machines that assist or control respiration by pumping air directly into the trachea have a great potential for nosocomial pneumonia if the aerosol they deliver becomes contam-inated. Bacterial growth is significant only in the parts of the system that contain water; in systems using nebulizers, bacteria can be suspended in water droplets small enough to reach the alveoli. The organisms involved include Pseudomonas, Enterobacteriaceae, and a wide variety of environmental bacteria such as Acinetobacter.The primary con-trol measure is periodic changing and disinfection of the tubing, reservoirs, and nebu-lizer jets.

Blood and Blood Products

Infections related to contact with blood and blood products are generally a risk for health care workers rather than patients. Manipulations ranging from phlebotomy and hemodial-ysis to surgery carry varying risk of blood containing an infectious agent reaching mu-cous membranes or skin of the health care worker. The major agents transmitted in this manner are hepatitis B, hepatitis C, and human immunodeficiency virus (HIV). Control requires meticulous attention to procedures that prevent direct contact with blood, such as the use of gloves, eyewear, and gowns. Cuts and needle sticks among health care workers carry a risk approaching 2%. Identification of hepatitis virus and HIV carriers is a part of a protective process that must be balanced by patient privacy considerations. Health care facilities all have established policies concerning serologic surveillance of patients and the procedures to follow (eg, testing, prophylaxis) when blood-related accidents occur. Similarly, products for transfusion undergo extensive screening in order to protect the re-cipient.

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