Infections occurring during any hospitalization could be either community acquired or nosocomial. Community-acquired infections are defined as those present or incubating at the time of hospital admission. 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 nosocomial 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.
Community-acquired infections are acquired before admission
Nosocomial infections are acquired in hospital
The infectious agents responsible for nosocomial infections arise from various sources, including patients’ own microbiota. 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 opportunistic microbes 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.
Endogenous infections are part of hospital risk
Physicians, nurses, students, therapists, and any others who come in contact with the patient may transmit infection. Transmission from one patient to another is called cross-infection. The vehicle of transmission is most often the inadequately washed hands of a medical attendant. Another source is the infected medical attendant. Many hospital outbreaks have been traced to hospital personnel, particularly physicians, who continue to care for patients despite an overt infection. Transmission is usually by direct contact, although 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 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.”
✺ Cross-infection is usually by direct contact
✺ Infected medical attendants are particularly dangerous
Infection from carriers can transmit to patients
The hospital air, walls, floors, linens, and the like are not sterile and, thus, could serve as a source of organisms causing nosocomial infections, but the importance of this route has generally been exaggerated. With the exception of the immediate vicinity of an infected individual 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 environment becomes contaminated with Mycobacterium tuberculosis from a patient or Legionella pneumophila in the water supply. These events are most likely to result in disease when the organisms are numerous or the patient is particularly vulnerable (eg, after heart surgery or bone marrow transplantation).
Environmental contamination is relatively unimportant
M tuberculosis and Legionella are risks
Much of the success of modern medicine is related to medical devices that support or monitor basic body functions. By their very nature, devices such as catheters, implants, and respirators carry a risk of nosocomial infection because they bypass normal defense barriers, providing 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 microbes in the patient or environment 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.
✺ Equipment that crosses epithelial barriers provides microbial access
The risk of contamination leading to infection is increased if organisms that gain access 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 Enterobacteriaceae can multiply in an environment containing water and little else. Gram-positive bacteria generally require more physiologic conditions.
Conditions for bacterial growth increase risk
Even with proper growth conditions, many hours are required before contaminating organisms multiply to numbers sufficient to cause disease. Detailed studies of catheters and similar devices show that the risk of infection begins to increase after 24 to 48 hours and is cumulative even if the device is changed or disinfected at intervals. It is thus important to discontinue transcutaneous procedures as soon as medically indicated. The medical devices most frequently associated with nosocomial infections are listed in the following text. 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 potential to cause nosocomial infection.
Transcutaneous and indwelling devices should be changed as soon as possible
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 urinary 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 systems eventually occur when the system is in place for more than 30 days. The urine itself serves as an excellent culture medium once bacteria gain access.
Closed urinary drainage systems are still violated
Needles and plastic catheters placed in veins for fluid administration, monitoring vital functions, or diagnostic procedures are a leading cause of nosocomial 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, or bottles of intravenous solutions proximal to the insertion site. The latter circumstance usually involves gram-negative rods. Preventive measures include aseptic insertion technique and appropriate care of the lines, including changes at regular intervals.
✺ Skin is primary source for intravenous contamination
Machines that assist or control respiration by pumping air directly into the trachea have a great potential for causing nosocomial pneumonia if the aerosol they deliver becomes contaminated. 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 control measure is periodic changing and disinfection of the tubing, reservoirs, and nebulizer jets.
Changing controls nebulizer contamination
Infections related to contact with blood and blood products are generally a risk for healthcare workers rather than patients. Manipulations ranging from phlebotomy and hemodialysis to surgery carry varying risk of blood containing an infectious agent reaching mucous membranes or skin of the healthcare 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 healthcare 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. Healthcare 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 to protect the recipient.
Risk of hepatitis B, hepatitis C, and HIV is related to blood manipulation
Screen is determined by institutional policy