ACP - ACP Hospitalist - April 2008 - MKSAP primer: Health care-associated infections

Clinical Information

MKSAP primer: Health care-associated infections

Adapted from ACP's latest Medical Knowledge Self-Assessment Program

Approximately 2% to 10% of all patients hospitalized in the U.S. develop a health care-associated infection. It is estimated that 1.6 million patients acquire 4 million infections each year and that as many as 90,000 of these patients subsequently die. Costs are estimated to exceed $32 million to $825 million annually.

An infection is considered to be health care associated if it develops in a patient who has been hospitalized for at least 48 hours and was not incubating the infection at the time of admission. An infection present on admission, if acquired during a previous admission or outpatient medical or surgical procedure, may also be considered health care associated.

Common pathogens causing health care-associated infections

Clostridium difficile

Clostridium difficile is an anaerobic gram-positive rod that produces two toxins, both capable of damaging the mucosa of the colon and causing pseudomembranous colitis. Hospitalized patients are at risk for C. difficile infection because antibiotics, enemas, intestinal stimulants and antineoplastic chemotherapeutic agents may alter the normal colonic flora and allow colonization by increased numbers of toxin-producing C. difficile. The rate of acquisition is proportionate to the length of hospital stay. Approximately 13% of patients hospitalized for one to two weeks and 50% of those hospitalized for more than four weeks may become colonized. C. difficile spores can remain viable on surfaces of inanimate objects for months and are transmitted directly from patient to patient via the hands of health care workers or indirectly by contaminated equipment such as bedpans, urinals and call bells and contaminated environmental surfaces such as bed rails, floors and toilet seats.

Prevention includes assiduous hand washing with soap and water, aggressive cleaning with appropriate disinfectants, use of barrier precautions (gloves and gowns), restriction of antimicrobial agents that predispose to colonization (e.g., clindamycin) and isolation of infected patients.

Legionella species

Legionella is a cause of nosocomial pneumonia, primarily in immunosuppressed or infirm patients. Although 40 Legionella species are recognized, Legionella pneumophila is the most pathogenic and causes approximately 90% of cases of Legionnaires’ disease. Of the 14 or more identified serogroups of L. pneumophila, serogroup 1 accounts for more than 80% of the reported cases.

Legionnaires’ disease is acquired by inhalation of water aerosols or aspiration of water contaminated by Legionella. Aerosols have been generated from cooling towers, air conditioners, humidifiers, condensers, respiratory therapy equipment, and whirlpool baths. Nasogastric tubes increase the risk of Legionella infection, presumably by facilitating aspiration.

Prevention strategies should focus on surveillance for nosocomial pneumonia potentially due to Legionella, testing all patients with nosocomial pneumonia for Legionella, and removal or treatment of suspected or proven sources of Legionella to eradicate the pathogen.

Methicillin-resistant Staphylococcus aureus

MRSA is now a major pathogen in hospitals and is increasingly becoming a major pathogen in the community. It accounts for 25% to 60% of isolates in hospitals and approaches 70% for isolates in intensive care units. Almost one-third of patients with MRSA colonization or prior infection develop a subsequent MRSA infection. Risk factors for infections include the duration of administration of antibiotics, presence of a severe underlying illness, exposure to a health care system and exposure to an intensive care unit.

Important factors in the prevention and control of MRSA are aggressive hand hygiene, identification of colonized and infected patients, isolation of patients, use of dedicated equipment, appropriate cleaning of the environment, and, for some patients, decolonization of the skin or the anterior nasal cavity.

Mycobacterium tuberculosis

Mycobacterium tuberculosis is occasionally transmitted nosocomially. Most infections are transmitted from person to person. Perhaps the greatest risk to patients or to persons caring for patients is delay in diagnosis. Whereas patients should wear standard surgical masks to minimize droplet spread, health care workers should wear special masks with increased filtration capacity in order to filter out the smaller droplet nuclei. Patients should be isolated in rooms with negative pressure and appropriate ventilation apparatus (airborne isolation rooms).

Vancomycin-resistant enterococci

Vancomycin-resistant enterococci (VRE) have become a major problem in almost every health care facility. Experimental and epidemiologic data suggest that these resistant strains result from an interplay between exposure to antibiotics and poorly defined host factors. Gastrointestinal colonization appears to be a prerequisite to infection. Risk factors for VRE colonization and infection include use of vancomycin, third-generation cephalosporins, and antianaerobic agents. VRE can be transmitted from person to person or from the environment. Most hospital transmission probably occurs via the hands of health care workers. VRE may survive up to 60 minutes on the hands of persons caring for VRE-colonized patients. Enterococci are recovered in 7% to 30% of environmental surfaces cultured during outbreaks of VRE. VRE outbreaks have also been linked to contaminated electronic thermometers and ear oximeters.

Neisseria meningitidis

Neisseria meningitidis is a gram-negative diplococcus that is the leading cause of epidemic meningitis worldwide. Approximately 2,600 patients in the U.S. develop meningococcal meningitis each year, and 12% of these patients die. Transmission in the community is person to person (usually by the respiratory route), and close contact with an infected person is required. Transmission in a health care setting requires contact with respiratory secretions of an infected patient and being in close proximity (within three feet) of that patient. Most experts provide chemoprophylaxis for exposed health care workers after close contact with the respiratory secretions of an infected patient. Persons who were wearing masks while in contact with a patient are not considered exposed. The use of the tetravalent meningococcal vaccine in health care workers is restricted to immunization during outbreaks of N. meningitidis.

Other microorganisms

Other microorganisms such as Aspergillus species, influenza virus, respiratory syncytial virus, and varicella-zoster virus have been described as causing nosocomial infections.

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Common sites of health care-associated infections

Approximately 80% of all health care-associated infections involve the bloodstream, surgical sites, urinary tract and lower respiratory tract. About 20% of infections involve the gastrointestinal tract, eyes, skin, heart valves and other sites.

Bloodstream infections

Bloodstream infections account for approximately 13% of all health care-associated infections. However, this percentage may be too low, as many patients with vascular lines are treated in outpatient settings. The mortality rate ranges from 12% to 25%, and added costs to the hospital exceed $40,000 for each patient with a bloodstream infection.

Bloodstream infections may be transient, intermittent (e.g., a deep-seated abscess) or continuous (e.g., endocarditis). Infections are categorized as primary when no source is identified or secondary when a source is documented. Primary infections are commonly associated with catheters and are therefore preventable. Catheter-related bloodstream infections can be uncomplicated localized, primarily exit-site infection, infection of the tunneled portion of a catheter, or combinations of the above. In less-developed countries, contaminated intravenous fluids remain an important cause of bloodstream infections. Most catheter-related bloodstream infections are due to coagulase-negative staphylococci, enterococci, and Staphylococcus aureus. Less common pathogens are Candida species and various gram-negative rods including Escherichia coli, Klebsiella species, and Pseudomonas species.

Prevention of catheter-associated bloodstream infections involves preventing transmission of microorganisms (i.e., careful hand hygiene, maximal barrier precautions, use of chlorhexidine to disinfect the patient’s skin), selecting an insertion site to maximize the ability to clean and maintain dressings, and removing the catheter as soon as possible.

Surgical site infections

Surgical site infections cause 29% of all nosocomial infections and 14% of all nosocomial adverse events and are therefore associated with significant morbidity and mortality and excess costs to the health care system. Infections that occur within 30 days of a surgical procedure are considered nosocomial. When prosthetic materials are used, infections that occur up to one year after insertion are considered nosocomial. Most surgical site infections are caused by S. aureus, followed by enterococci, coagulase-negative staphylococci, various gram-negative microorganisms and yeast.

Host characteristics that increase the risk of developing an infection include morbid obesity, old age, diabetes mellitus, severe underlying illness, a prolonged preoperative hospital stay and a preoperative infection. These factors are difficult to modify. However, awareness of their role in enhancing susceptibility to infection should lead to precautionary measures such as tight glucose control in patients with diabetes mellitus or postponing surgery, when possible, in patients with a severe underlying illness.

The most important way to prevent surgical site infections is appropriate use and timing of perioperative antibiotic prophylaxis. Antibiotics significantly reduce the risk of infection in many patients undergoing clean and clean-contaminated surgical procedures that involve the gastrointestinal, respiratory, genital and urinary tracts, especially when perioperative antibiotics are begun before the incision is made. Operating rooms must meet certain ventilation standards to assure appropriate air filtration and pressure relationships. A patient’s hair should only be removed when necessary and only by using clippers immediately before the procedure. Core intraoperative body temperature should be maintained above 36.5°C (97.7°F). One study reported that using higher rates of supplemental oxygen also decreased infection. Reporting of surgical site infections may serve to reduce their incidence, presumably because of heightened awareness of the factors that lead to infection.

Urinary tract infections

More than 900,000 nosocomial urinary tract infections (UTIs) occur in the U.S. each year, and UTIs are the most common health care-associated infections in both acute and long-term health care facilities. Ninety percent to 95% of UTIs are due to indwelling bladder catheters, and 5% to 10% result from manipulation of the genitourinary tract. Catheters increase the risk for developing a UTI because they avert the normal defenses of the urologic system in various ways:

1. Bacteria can be inoculated directly into the bladder during insertion of the catheter;
2. Both the inside and outside walls of the catheter serve as conduits from the external environment to the bladder;
3. The glycocalyx that forms on the catheter surface protects bacteria from antibiotics and host defenses;
4. The catheter damages the glycosaminoglycan layer of the bladder and blunts the leukocyte immune response to infection; and
5. Residual urine that does not completely drain from the bladder serves as a reservoir for bacterial growth.

E. coli, P. aeruginosa, Klebsiella species, and Enterobacter species are the most common causes of urinary tract infections. However, other bacteria that are acquired in the health care environment and become part of the colonic flora may also cause UTIs. Prophylactic antibiotics have not been shown to prevent UTIs or to decrease febrile episodes due to these infections. These agents do, however, increase the frequency of antibiotic-resistant bacteria isolated from urine culture specimens.

Lower respiratory tract infections and ventilator-associated pneumonia

In the U.S., 18% of nosocomial infections involve the lower respiratory tract. Risk factors for nosocomial pneumonia include severe underlying illness, extremes of age, chronic lung disease, immunosuppression, depressed sensorium, intra-abdominal or thoracic surgery, use of H₂-receptor blockers, large-volume aspiration and mechanical ventilation. Nosocomial pneumonia is also more common during the autumn and winter months. The mortality rate from nosocomial pneumonia is the highest of all nosocomial infections, with an attributable rate of approximately 30%.

Patients requiring mechanical ventilation have the highest risk of developing infection. Ventilator-associated pneumonia is associated with increased mortality, mean length of hospital stay, and health care-associated costs. Infections are most often caused by P. aeruginosa, S. aureus, Enterobacter species, K. pneumoniae, and E. coli. Many infections in immunosuppressed patients are due to inhalation of aerosols or droplets contaminated with Legionella species, Aspergillus species, respiratory syncytial virus or influenza virus.

The pathogenesis of ventilator-associated pneumonia is particularly relevant to prevention strategies. Oropharyngeal colonization by either endogenously or exogenously acquired flora and gastric colonization provide reservoirs of organisms that can also be sustained by the biofilm that develops on endotracheal tubes. These organisms are then aspirated into the normally sterile lower respiratory tract.

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Strategies for preventing health care-associated infections

Prevention strategies are effective in reducing the incidence of nosocomial infections in both patients and health care workers. These include careful attention to hand hygiene, prevention of occupational exposures, and immunization of health care workers (see table).

Hand hygiene

Hand hygiene or washing is considered the single most important practice to prevent the transmission of infection in the health care setting. Washing hands with soap and water and disinfecting hands with disinfectant soap and water or alcohol-based hand scrubs are effective. Hand washing reduces bacterial counts on the surface of hands from 10⁷ to 10³ colony-forming units and has been shown to reduce the incidence of nosocomial infection in many settings. Alcohol rubs are equally effective in reducing contamination of the hands compared with hand washing. Despite these data, physicians have been shown to be among the least compliant with hand hygiene.

Occupational exposures

Occupationally acquired infections among health care workers include those that are transmitted by blood (e.g., HIV, hepatitis B and C viruses, cytomegalovirus, Ebola virus or other hemorrhagic fevers); by air or droplet contact (e.g, measles, tuberculosis, varicella, rubella, variola, severe acute respiratory syndrome, influenza, respiratory syncytial virus, mumps, parvovirus B19, pertussis); by the oral–fecal route (e.g., hepatitis A, Salmonella, norovirus, rotavirus); by direct contact (e.g., scabies, lice, herpes simplex virus); or by exposure to body secretions other than blood (e.g., meningococcal infection, brucellosis, trypanosomiasis, rabies). Prevention should be based on how these agents are transmitted but should include appropriate hand hygiene, respiratory protection, gowns, gloves, immunizations, and, in some cases, specialized isolation rooms.

As noted, HIV and hepatitis B and C viruses are the primary agents associated with bloodborne occupational exposures. Data from more than 25 studies have shown that the risk of infection is 27% to 47% following exposure to hepatitis B virus, 1.9% (range, 0% to 22%) following exposure to hepatitis C, and 0.32% after exposure to HIV. Primary prevention strategies include appropriate use of standard precautions, education of health care workers, modification of causative procedures and work practices, use of engineered controls and technological advances and immunization of at-risk persons.

The rationale for offering postexposure chemoprophylaxis for HIV is similar to that of hepatitis B virus and is based on in vitro evidence, maternal–fetal transmission studies, retrospective case–control studies of occupational HIV infection and recent clinical experience. Prophylaxis recommendations are updated periodically but are based on the type of exposure, the vehicle (e.g., hollow-bore needles), the depth of penetration, and the type of host.

Immunization of health care workers

Immunization of health care workers is recommended to prevent infection in the workers themselves and to prevent transmission of communicable diseases to patients. In order to prevent morbidity to both health care workers and patients, the hepatitis B series, measles, mumps, rubella, varicella and yearly influenza vaccination are strongly encouraged for all persons in the health care field. Other vaccines are administered when a specific type of work (e.g., laboratory studies using pathogenic vectors) requires providing immunity to these pathogens.

The information included herein should never be used as a substitute for clinical judgment and does not represent an official position of ACP. Click here for more information on MKSAP.

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