Dr. Burbridge is a Professor of Radiology at the University of Saskatchewan College of Medicine, and a Radiologist in the Department of Medical Imaging at the Royal University Hospital in Saskatoon, SK, Canada.

Radiology department personnel regularly encounter patients with infectious diseases via inpatient and outpatient imaging examinations. These contacts include patients who have been placed on infectious disease precautions or those who may be carriers of infectious organisms. Vancomycin-resistant enterococcus (VRE) is a pathogen that may impact a wide range of radiology department personnel, from those in patient reception to those in interventional radiology.

Enterococci are spherical bacteria present in large numbers in the lower gastrointestinal tract of humans. Two common species, Enterococcus faecalis (E faecalis) and Enterococcus faecium (E faecium), account for > 85% of the entercocci present in the intestine. E faecalis is found in concentrations of 105 to 107 colony-forming units per gram of feces in normal humans.¹ Enterococci are hardy organisms, able to survive on environmental surfaces for extended periods. Several studies have found multidrug resistant (MDR) strains of enterococci on various objects in the patient environment, including bed rails, night tables, curtains, bathroom sinks, toilet seat rings, electronic thermometers, and other patient-care equipment.^2-6

Enterococci are intrinsically resistant to many antibiotics. Unlike acquired resistance, intrinsic resistance is based upon chromosomal genes, which typically are nontransferable. Penicillin, ampicillin, piperacillin, imipenem, and vancomycin are among the few antibiotics that show consistent inhibitory, but not bactericidal, activity against E faecalis.

E faecium are less susceptible to β-lactam antibiotics than E faecalis because the penicillin-binding proteins of the former have markedly lower affinities for the antibiotics.^7-9 Enterococci have also shown consistent ability to develop antibiotic resistance to a variety of medications, including ampicillin, gentamicin, and other aminoglycosides. Enterococci may acquire antibiotic resistance through exchange of resistance-encoding genes. The first reports of strains highly resistant to penicillin appeared in the 1980s.⁹ Thereafter, enterococci were classified as infectious organisms that demonstrate multiple drug resistance (MDRO) by the Centers for Disease Control and Prevention (CDC). The past 2 decades have witnessed the rapid emergence of MDR enterococci. High-level gentamicin resistance occurred in 1979 and was quickly followed by numerous reports of nosocomial infection in the 1980s. Simultaneously, sporadic outbreaks of nosocomial E faecalis and E faecium infection appeared with penicillin resistance due to β-lactamase production.

Finally, MDR enterococci that had lost susceptibility to vancomycin were reported in Europe and the United States in the late 1990s.⁹ Among several phenotypes for vancomycin-resistant enterococci, VanA (resistance to vancomycin and teicoplanin) and VanB (resistance to vancomycin alone) are the most common. In the United States, VanA and VanB account for approximately 60% and 40% of vancomycin-resistant enterococci (VRE) isolates, respectively.⁹

Colonization vs. infection

Colonization suggests that the organism is present in or on the body, but is not causing illness. Infection suggests that the organism is present in or on the body and is causing illness.¹⁰ VRE predominantly resides in the gastrointestinal tract. Colonization rates for VRE range between 3% and 47%.^11-13 Traditionally, it was thought that patients developed VRE infection from their own intestinal source of bacteria, but it has been demonstrated that the hands of healthcare workers (HCWs) and surfaces in the healthcare environment can be vectors for spread of the bacteria.^4,14 More importantly for radiology, Wood et al demonstrated the presence of enterococci on radiology department imaging receptors during a random sampling of equipment from 4 healthcare sites in their region.¹⁵ Contamination of x-ray tables, x-ray cassettes, x-ray tubes, portable x-ray machines, portable ultrasound machines, ultrasound probes, and the surfaces of the rooms and other equipment seems highly likely and is probably more common in areas where patients are at increased risk of harboring and spreading bacteria, such as the operating room and intensive care units. These sites are frequently visited by mobile imaging devices and the personnel trained to use them. The risk of exposure or contamination is probably lower for outpatient imaging, but there is no guarantee that the patient being imaged is not infected or colonized. Additionally, the imaging technologist may be colonized or may have just returned to the department contaminated with enterococci after exposure to the pathogen in an active care area outside the imaging department. Thus, the potential for cross-contamination of outpatients, inpatients, and support personnel must be considered and planned for.

Prevention strategies

MDROs are transmitted by the same routes as antimicrobial-susceptible infectious agents. Person-to-person transmission in healthcare settings, usually via the hands of HCWs, is a major factor in the growth of MDRO incidence and its prevalence in acute care facilities. Preventing the emergence and transmission of these pathogens requires a comprehensive approach that includes administrative involvement and measures, such as staffing, communication strategies, assessment processes to ensure adherence to infection control measures, healthcare personnel education and training, judicious antibiotic use, comprehensive surveillance for targeted MDROs, application of infection control precautions during patient care, environmental measures (eg, cleaning and disinfection ), and decolonization therapy when appropriate.¹⁶ For MDROs, (eg, MRSA, VRE), the CDC recommends standard and contact precautions to prevent the spread of these organisms.¹⁶

Standard precautions

Standard precautions are based on the principle that all bodily fluids, secretions, excretions, nonintact skin, and mucous membranes may contain transmissible infectious agents. Standard precautions include practices that apply to all patients, regardless of suspected or confirmed infection status, in any setting where healthcare is delivered. These include: hand hygiene; the use of gloves, gowns, masks, eye protection, or face shields, depending on the anticipated exposure; and safe injection practices. Also, items in the patient environment likely to have been contaminated with infectious body fluids must be handled in a manner to prevent transmission of infectious agents (eg, wear gloves for direct contact, and properly clean and disinfect or sterilize reusable equipment before use on another patient).¹⁶

The application of standard precautions is determined by the nature of the HCW-patient interaction and the anticipated extent of body fluid or pathogen exposure. For some interactions (eg, performing venipuncture), only gloves may be needed; during other interactions (eg, intubation), the use of gloves and a gown, face shield, or mask and goggles, is necessary. Education and training on the principles and rationale for recommended practices are critical elements of standard precautions because they facilitate appropriate decision-making and promote adherence when HCWs are faced with new circumstances. Standard precautions are also intended to protect patients by ensuring that healthcare personnel do not carry infectious agents on their hands or via equipment used in patient care.¹⁶ Table 1 outlines the clinical application of standard precautions in the healthcare setting.

Contact precautions

Contact precautions are intended to prevent transmission of infectious agents, including epidemiologically important microorganisms that are spread by direct or indirect contact with the patient or the patient‘s environment. Contact precautions also apply where the presence of excessive wound drainage, fecal incontinence, or other discharges from the body suggest an increased potential for extensive environmental contamination and risk of transmission. A single-patient room is preferred for patients who require contact precautions. Personnel caring for patients on contact precautions wear a gown and gloves for all interactions that may involve contact with the patient or potentially contaminated areas in the patient‘s room. Donning PPE upon entering the room and discarding before leaving is done to contain pathogens, especially those implicated in transmission through environmental contamination.¹⁶

Patient care equipment and instruments/devices

Medical equipment and instruments/devices must be cleaned and maintained according to the manufacturers‘ instructions to prevent patient-to-patient transmission of infectious agents. Cleaning to remove organic material must always precede high-level disinfection and sterilization of critical and semi-critical instruments and devices because residual proteinaceous material reduces the effectiveness of disinfection and sterilization. Noncritical equipment, such as commodes, intravenous pumps, and ventilators, must be thoroughly cleaned and disinfected before use on another patient. All such equipment and devices should be handled in a manner that will prevent HCW contact with potentially infectious material. It is important to include computers, and keyboards used for imaging equipment, used in patient care, in policies for cleaning and disinfection of noncritical items. The literature on contamination of computers with pathogens has been summarized and 2 reports have linked computer contamination to colonization and infections in patients. Although keyboard covers and washable keyboards are in use, the infection control benefits of those items and optimal management have not been determined.^17-19

Imaging vectors

As noted earlier, Wood et al have demonstrated the presence of VRE on medical imaging equipment.¹⁵ For example, ultrasound probes are optimal vectors for transmission of infectious agents between patients. They are used for a wide variety of applications, including portable imaging in the operating room and intensive care units, neonatal imaging, and localization for interventional radiology procedures. The probes may encounter dozens of patients per day. Fowler et al demonstrated the presence of large bacterial counts on ultrasound probes and the importance of proper probe cleaning to minimize transmission risk.²⁰

Interventional radiology patients and personnel are at significant risk for exposure to MDR pathogens via patients with active infections. The immunocompromised and those with drainage or infusion devices that are associated with wounds that breech the skin surface, or that provide access to body cavities or vessels require special consideration. Nolan, in describing an outbreak of VRE among a group of pediatric oncology patients, found that one patient was infected while undergoing an interventional radiology procedure.²¹ Heightened awareness of infectious disease policies and procedures is warranted in this department. However, adherence to proper infection-control guidelines is a significant issue and has been demonstrated to be inconsistently executed, as described by Reddy et al.²² A wide variety of procedure-specific guidelines are available, and it behooves those involved in the management of patients in this arena to familiarize themselves and others with proper procedures and protocols. An example of procedure-specific guidelines for intravascular catheters is provided by O‘Grady et al.²³

Conclusion

Sir Francis Bacon stated, “Knowledge is power.” Radiology departments are a central focus of patient care, and those who work in this department have the power to minimize the impact of infectious agents on themselves and others and, in particular, to be sensitive to the unique preventive policies and procedures for MDROs, such as VRE. Vigilance and education are both required to slow and halt the spread of these resistant pathogens and to protect patients and healthcare workers from their potentially serious effects.

References

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