Incidence of HAI

The Centers for Disease Control and Prevention (CDC) has established the National Healthcare Safety Network (NHSN) program to monitor the incidence of HAI in the United States. Data collected in NHSN are used to improve patient safety at the local and national levels. In aggregate, the CDC analyzes and publishes surveillance data to estimate and characterize the national burden of health care–associated infections. Regardless of a hospital’s size or medical school affiliation, the rates of infections at each body site are consistent across institutions. The majority of HAIs are urinary tract infections (33%), followed by pneumonia (15%), surgical site infections (15%), and bloodstream infections (13%). The remaining 24% are other miscellaneous infections. Each HIA adds 5 to 10 days to the affected patient’s hospital stay. Of individuals with hospital-acquired bloodstream or lung infections, 40% to 60% die each year. Likewise, patients with indwelling urinary catheters have a threefold increased chance of dying from urosepsis—a bloodstream infection that is a complication of a urinary tract infection—than those who do not have one.

Attack rates vary according to the type of hospital. Large, tertiary-care hospitals that treat the most seriously ill patients often have higher rates of HIA than do small, acute-care community hospitals; large medical school–affiliated (teaching) hospitals have higher infection rates than do small teaching hospitals. This difference in the risk of infection is probably related to several factors, including but not limited to the severity of illness, the frequency of invasive diagnostic and therapeutic procedures, and variation in the effectiveness of infection control programs. Within hospitals, the surgical and medical services have the highest rates of infection; the pediatric and nursery services have the lowest. Moreover, within services, the predominant type of infections varies—that is, surgical site infections are the most common on the surgical service, whereas urinary tract or bloodstream infections are the most common on medical services or in the nursery.

Types of HAI

The majority of HAIs are endogenous in origin—that is, they involve the patient’s own microbial flora. Three principal factors determine the likelihood that a given patient will acquire an infection:

In general, hospitalized individuals have increased susceptibility to infection. Corticosteroids, cancer chemotherapeutic agents, and antimicrobial agents all contribute to the likelihood of HAI by suppressing the immune system or altering the host’s normal flora to that of resistant microbes. Likewise, foreign objects, such as urinary or intravenous catheters, break the body’s natural barriers to infection. Nonetheless, these medications or devices are necessary to cure the patient’s primary medical condition. Finally, exerting influence over the virulence of the pathogens is not possible because it is not possible to immunize patients against HAI. Patients with serious community-acquired infections are frequently admitted to the hospital, and the disease may spread by either direct contact; by contact with contaminated food, water, medications, or medical devices (fomites); or by airborne transmission. Thus, the HAI may never be completely eliminated, only controlled.

Urinary Tract Infections

Gram-negative rods produce the majority of health care–associated urinary tract infections, and Escherichia coli is the number one organism involved. Gram-positive organisms, Candida spp., and other fungi cause the remainder of the infections. The risk factors that predispose patients to acquire a health care–associated urinary tract infection include advanced age, female gender, severe underlying disease, and the placement of indwelling urinary catheters.

Emergence of Antibiotic-Resistant Microorganisms

The organisms that cause HAIs have changed over the years because of selective pressures from the use (and overuse) of antibiotics (see Chapter 11). Risk factors for the acquisition of highly resistant organisms include prolonged hospitalization and prior treatment with antibiotics. In the pre-antibiotic era, most HAIs were caused by S. pneumoniae and group A Streptococcus (Streptococcus pyogenes). In the 1940s and 1950s, with the advent of treatment of patients with penicillin and sulfonamides, resistant strains of S. aureus appeared. Then, in the 1970s, treatment of patients with narrow-spectrum cephalosporins and aminoglycosides led to the emergence of resistant aerobic gram-negative rods, such as Klebsiella, Enterobacter, Serratia, and Pseudomonas. During the late 1970s and early 1980s, the use of more potent cephalosporins played a role in the emergence of antibiotic-resistant, coagulase-negative staphylococci, enterococci, methicillin-resistant S. aureus (MRSA), and Candida spp. The 1990s witnessed the emergence of beta-lactamase–producing, high-level gentamicin-resistant, and vancomycin-resistant enterococci (VRE). The twenty-first century has seen the emergence of vancomycin-resistant Staphylococcus aureus (VRSA).

Patients’ normal flora will change quickly after hospitalization from viridans streptococci, saprophytic Neisseria spp., and diphtheroids to potentially resistant microorganisms found in the hospital environment. The colonized nares, skin, gastrointestinal tract, or genitourinary tract can later serve as reservoirs for endogenously acquired infections. Moreover, if patients colonized with resistant microorganisms return to nursing homes in the community harboring these organisms, they can also transfer them to other patients. This further increases the pool of patients who harbor multidrug-resistant organisms when they, in turn, are hospitalized. These new patients recontaminate the hospital environment and serve as potential reservoirs for spread to additional patients.

Hospital Infection Control Programs

Hospital infection control programs are designed to detect and monitor HAIs and to prevent or control their spread. The infection control committee is multidisciplinary and should include a microbiologist, an infection control practitioner (often a nurse with special training), a hospital epidemiologist (usually an infectious disease physician), and a pharmacist. The infection control practitioner collects and analyzes surveillance data, monitors patient care practices, and participates in epidemiologic investigations. Daily review of charts of patients with fever or positive microbiology cultures allows the infection control practitioner to recognize problems with HAIs and to detect outbreaks as early as possible. The infection control practitioner is also responsible for the education of health care providers in techniques, such as hand washing and isolation precautions, that minimize the acquisition and spread of infections.

It is the infection control practitioner’s job to identify all cases of an outbreak. The investigation of the cluster of cases during a particular outbreak involves its characterization in terms of commonalities, such as location in the hospital (nursery, intensive care unit), same caregiver, or prior respiratory or physical therapy. Risk factors—including underlying diseases, current or prior antimicrobial therapy, and placement of a urinary catheter—are also assessed. This information helps the infection control committee determine the reservoir of the organism in the hospital—that is, the place where it exists and the means by which the organism is transmitted from its reservoir to the patient.

Microorganisms are spread in hospitals through several modes:

Once the reservoir is known, the infection control practitioner can implement control measures, such as reeducation regarding hand washing (in the case of spread by health care workers) or hyperchlorination of cooling towers in the case of legionellosis.

Role of the Microbiology Laboratory

The microbiology laboratory supplies the data on organism identification and antimicrobial susceptibility profile that the infection control practitioner reviews daily for evidence of HAI. Thus, the laboratory personnel must be able to detect potential microbial pathogens and then accurately identify them to species level and perform appropriate susceptibility testing. The microbiology laboratory staff should also monitor multidrug-resistant organisms by tabulating data on antimicrobial susceptibilities of common isolates and studying trends indicating emerging resistance. Significant findings should be immediately reported to the infection control practitioner. If an outbreak is suspected, the laboratory works in tandem with the infection control committee by (1) saving all isolates, (2) culturing possible reservoirs (patients, personnel, or the environment), and (3) performing typing of strains to establish relatedness between isolates of the same species. Microbiology laboratories are also obligated by law to report certain isolates or syndromes to public health authorities. For example, Table 79-1 lists organisms to be reported to state health authorities in Texas. Other states have similar criteria.

Characterizing Strains Involved in an Outbreak

The ideal system for typing microbial strains involved in outbreaks should be standardized, reproducible, sensitive, stable, readily available, inexpensive, applicable to a wide range of microorganisms, and field tested in other epidemiologic investigations. Although no such perfect system is currently available, a number of methods are used to aid in typing epidemic strains. There are two major ways to type strains using either phenotypic traits or molecular typing methods.

Classic phenotypic techniques include biotyping (analyzing unique biologic or biochemical characteristics), the use of antibiograms (analyzing antimicrobial susceptibility patterns), and serotyping (serologic typing of bacterial or viral antigens, such as bacterial cell wall [O] antigens). Bacteriocin typing, which examines an organism’s susceptibility to bacterial peptides (proteins), and bacteriophage typing, which examines the ability of bacteriophages (viruses capable of infecting and lysing bacterial cells) to attack certain strains, have been useful for typing Pseudomonas aeruginosa and S. aureus, respectively; these techniques, however, are not widely available.

Genotypic, or molecular, methods have largely replaced phenotypic methods as a means of confirming the relatedness of strains involved in an outbreak. Plasmid analysis and restriction endonuclease analysis of chromosomal DNA are widely used. Plasmids are extrachromosomal pieces of genetic material (nucleic acids) that self-replicate (reproduce). Plasmids may be transferred from one bacterial cell to another by conjugation or transduction (see Chapter 2). Plasmid analysis has often been used to explain the occurrence of unusual or multiple-antibiotic resistance patterns. It has been shown that plasmids or R factors (resistance genes carried on plasmids) can cause outbreaks when a specific plasmid is transmitted from one genus of bacteria to another. Plasmid profiles, patterns created when plasmids are separated based on molecular weight by agarose gel electrophoresis, can also be used to characterize the similarity of bacterial strains. Relatedness of strains is based on the number and size of plasmids, with strains from identical sources showing identical plasmid profiles. Plasmids themselves or chromosomal DNA may also be typed by means of restriction endonuclease digestion patterns. Restriction enzymes recognize specific nucleotide sequences in DNA and produce double-stranded cleavages that break the DNA into smaller fragments. The fragments of various sizes are separated using gel electrophoresis based on molecular weight. The specific recognition sequence and cleavage site have been defined for a great many of these enzymes.

Modifications of the basic restriction endonuclease technique have been developed to reduce the number of bands generated to fewer than 20 in an attempt to make the gels easier to interpret. These include pulsed-field gel electrophoresis (PFGE) and hybridization of ribosomal RNA with short fragments of DNA. Plasmid restriction digests have been used to type S. aureus and coagulase-negative staphylococci, and PFGE is the preferred method for typing enterococci, enteric gram-negative rods, and other gram-negative rods.

Other molecular methods, such as PCR (polymerase chain reaction), are used in conjunction with these methods for strain typing. Molecular methods are discussed in more detail in Chapter 8.

Preventing HAI

The CDC published guidelines in the 1970s specifying isolation precautions in hospitals. Techniques for isolation precautions included (1) health care workers washing their hands between caring for different patients; (2) segregation of infected patients in private rooms or cohorting of patients (placing patients with the same clinical syndrome in semiprivate rooms) if private rooms are not available; (3) wearing of masks, gowns, and gloves when caring for infected patients; (4) bagging of contaminated articles, such as bed linens, when removed from the room; (5) cleaning of all isolation rooms after the patient is discharged; and (6) placement of cards on the patient’s door specifying the type of isolation and instructions for visitors and health care workers. Categories of isolation were also established and included (1) strict isolation for highly contagious diseases such as chickenpox, pneumonic plague, and Lassa fever; (2) respiratory isolation for diseases such as measles or Haemophilus influenzae or Neisseria meningitidis; (3) enteric precautions for diseases such as amebic dysentery, Salmonella, and Shigella; (4) contact isolation for patients infected with multidrug-resistant bacteria; (5) acid-fast bacilli (AFB)(tuberculosis) isolation for persons with M. tuberculosis; (6) drainage and secretion precautions for persons with conjunctivitis and burns; and (7) blood and body fluid precautions for individuals with acquired immunodeficiency syndrome (AIDS). Over time, a system of disease-specific precautions was added to the category-specific ones, and hospitals were given the option of using one of the two systems. Disease-specific precautions were more cost-effective, in that only those precautions specifically necessary were used to interrupt the transmission of a single disease.

In 1996, the CDC developed a new system of standard precautions synthesizing the features of universal precautions (described in Chapter 4) and body substance isolation. Standard precautions are used in the care of all patients and apply to blood; all body fluids, secretions, and excretions except sweat, regardless of whether they contain visible blood; nonintact skin; and mucous membranes.

In addition, transmission-based precautions are used for patients known (or suspected) to be infected with pathogens spread by airborne or droplet transmission or by contact with dry skin or fomites. Box 79-1 lists infection control measures for standard precautions. Table 79-2 lists the infectious agents or syndromes along with the respective infection control measures for each transmission-based precaution. Many infection control practitioners find these guidelines a lot less cumbersome to implement than the old category- and disease-specific measures. Hospitals, however, may modify these guidelines to fit their individual situations as long as their number of HAIs remains low.

Surveillance Methods

Most routine environmental cultures in the hospital are now considered to be of little use and should not be performed unless there are specific epidemiologic reasons. The decision to perform these cultures should be determined by the microbiologist, infection control practitioner, and hospital epidemiologist. However, certain surveillance cultures are still performed as a method of limiting outbreaks. These include culturing cooling towers or hot water sources for Legionella spp., culturing water and dialysis fluids for hemodialysis as well as endotoxin testing, culturing blood bank products, especially platelets, and surveillance cultures for vancomycin-resistant enterococci (VRE), methicillin (or oxacillin)-resistant S. aureus (MRSA), and vancomycin-resistant S. aureus (VRSA) using rectal and oropharyngeal swabs. Physical rehabilitation centers often culture hydrotherapy equipment (whirlpools) quarterly to verify that cleaning methods are adequate; some centers culture more frequently.

Routine surveillance of air handlers, food utensils, food equipment surfaces, and respiratory therapy equipment is no longer recommended; neither is monitoring infant formulas prepared in-house nor items purchased as sterile. A better approach is for the infection control team to monitor patients for the development of an HAI that might be related to the use of contaminated commercial products. In the event of an outbreak or an incident related to suspected contamination, a microbiologic study would be indicated. However, most often, such infections are actually caused by in-use contamination, rather than contamination during the manufacturing process. Suspect lots of fluid and catheter trays should be saved, and the U.S. Food and Drug Administration should be notified if contamination of an unopened product is suspected.

Although some institutions still require preemployment stool cultures and ova and parasite examinations on food handlers, most now recognize that this is of limited value. It is much more important for food handlers to submit specimens for these tests if they develop diarrhea. Similarly, most hospitals no longer screen personnel routinely for nasal carriage of S. aureus. Although a significant percentage of the general population, including hospital personnel, are known to carry this organism, most individuals rarely shed enough organism to pose a hazard and there is no simple way to predict which nasal carriers will disseminate staphylococci.

All steam and dry-heat sterilizers and ethylene oxide gas sterilizers should be checked at least once each week with a liquid spore suspension.

Hospitals that perform bone marrow transplantation or treat hematologic malignancies may also conduct surveillance cultures of severely immunocompromised patients who occupy laminar flow rooms. In these instances, the isolation of specific organisms may have predictive value for subsequent systemic infection. Air sampling for fungi during construction is also indicated, especially if patients are immunocompromised and are being treated near the construction site.

The U.S. Pharmacopeia published requirements for monitoring of sterile compounding in hospital pharmacies. The laminar flow hoods, biologic safety cabinets, clean rooms, and donning areas must be monitored weekly or monthly so that intravenous or intrathecal products and drugs used in the operating room are made (compounded) under sterile conditions.