Fomites' Role in Disease Transmission is Still Up for Debate
By Kelly M. Pyrek
The role fomites play in the transmission of infectious disease is still being contested in the infection control community, with no conclusive decision emerging from clinical studies. A fomite is defined as an inanimate object that serves to transmit an infectious agent from person to person.
According to the work of David J. Weber, MD, MPH, and William A. Rutala, "The acquisition of nosocomial pathogens depends on a complex interplay of the host, pathogen and environment."1 Breaches in the host's skin integrity allow microbes to invade, while microorganisms must be present in a minimum inoculating dose sufficient to trigger infection, virulence, infectivity and the ability to produce a latent infection. Nosocomial infections can result from endogenous or exogenous flora, with the latter often present on an environmental reservoir or fomite.
In the chain of infection (see "Breaking the Chain of Infection," July 2002 Infection Control Today), fomites can serve as the reservoir, with pathogens being spread from the inanimate environment to an animate environment (the patient) via the hands of healthcare workers (HCWs).
According to Weber and Rutala, "A key concept in considering the hospital environment as an infectious hazard for patients is proof of a causative role of inanimate objects in human disease." They point to several levels of proof offered by researcher Frank Rhame:2
- The organism can survive after inoculation on to the fomite.
- The pathogen can be cultured from fomites in use.
- The pathogen can proliferate on the fomite.
- At least some small part of acquisition of infection cannot be accounted for by other methods of transmission.
- Studies show an association between exposure to the contaminated fomite and infection.
In an editorial in Infection Control and Hospital Epidemiology, Weber and Rutala pondered the following: Do patients colonized or infected with VRE contaminate their environment? What is the role of surface contamination in the transmission of vancomycin-resistant enterocci (VRE)? Is surface contamination linked to the transmission of other nosocomial pathogens?3 They concede that in VRE outbreaks, "It often has been difficult to determine whether cross-transmission occurred due to contaminated common equipment (e.g., stethoscopes), acquisition of transient hand carriage by healthcare personnel due to direct contact with a colonized or infected patient or acquisition of transient hand carriage by healthcare personnel due to contact with a contaminated surface. Cross-transmission of VRE occasionally has been linked to contaminated medical devices, including an electronic thermometer and a fluidized bed. Disinfection or removal of the contaminated equipment terminated the outbreaks."
Survival of the Fittest
Weber and Rutala point to various studies conducted to analyze the survival rates of enterococci inoculated onto environmental surfaces. Noskin, et al.,4 reported that Enterococcus faecalis survived for five days and Enterococcus faecium for seven days on countertops. Both species survived on bed rails for 24 hours without significant die-off, on telephone handpieces for 60 minutes, on the diaphragmatic surface of stethoscopes for 30 minutes and on gloved and ungloved fingers for at least 60 minutes. Other investigators have demonstrated survival of VRE for more than three days on inoculated surfaces5 or equipment contaminated by colonized or infected patients.6 Survival of 18 hours on pieces of sterile cotton sheets also has been demonstrated.7
When trying to prevent the transmission of disease, the most important environmental reservoirs and typical pathogens in a hospital are water (Legionella, Pseudomonas); food (Salmonella); air (Aspergillus); endoscopes (Pseudomonas, Mycobacteria); invasive equipment (Pseudomonas); and surfaces (adenovirus). Other common environmental reservoirs are electronic thermometers (C. difficile, VRE); glass thermometers (salmonella); bandages (zygomycetes); air-fluidized beds (enteroccocus); mattresses (Pseudomonas, Acinetobacter); plaster (Pseudomonas); and urine-measuring devices (Serratia).8
Contamination of the inanimate environment-especially bed rails, bed sheets and patient gowns -- has been most closely associated with methicillin-resistant Staphylococcus aureus (MRSA), C. difficile and antibiotic-resistant Enterococcus. These microorganisms also have been found on blood pressure cuffs, dietary trays, intravenous pumps, stethoscopes, utility room sinks, bathroom doors and a sink drain in a patient room.
VRE has been one of the most studied areas of environmental contamination and its presence has been detected on the following:9
- EKG pressure monitor dials and doorknobs: 12 percent of cultures (Karanfil, 1992)
- Patient gowns, linens, bed rails, IV pumps and blood pressure cuffs: 28 percent of cultures (Boyce 1994)
- Patient gowns, linens, bed rails, blood pressure cuffs: 37 percent of cultures (Boyce 1995)
- Bed linen, side rails, bedside tables: 7 percent of cultures (Slaughter 1996)
In a study that appeared in the Lancet in 1996, 12 of 29 ventilated patients acquired VRE; the resistant strain was isolated from 157 of 1,294 environmental cultures. The study concluded that it is impossible to separate patient-to-patient transmission via HCWs or the contaminated environment, but that the latter was not a major source of infection.10
"It will be extremely difficult to disentangle the contributions of the animate and inanimate reservoirs of VRE in leading to transient hand carriage of VRE by medical personnel," Weber and Rutala write.11 "Clearly, proper handwashing with an antimicrobial agent before and after each contact with patients or their immediate environment is crucial in preventing person-to-person transmission of nosocomial pathogens. Unfortunately, compliance with CDC handwashing guidelines has been noted in less than one half of the instances in which it is indicated.12 For this reason, additional contact precautions have been recommended, including wearing gloves when entering the rooms of patients with VRE. We believe there is sufficient evidence to state that inanimate surfaces likely play a role in the transmission of VRE. Supportive evidence includes environmental cultures demonstrating widespread surface contamination in rooms of many patients colonized or infected with VRE and experimental evidence that VRE can survive on environmental surfaces for hours and that hands can become colonized with VRE via patient or environmental surfaces."
What infection control experts conclude from these various studies is that the inanimate environment may contribute to the spread of some pathogens such as VRE, but that the precise contribution is probably 0 percent to less than 25 percent. It has been found that non-critical surfaces play a minor role in the transmission of most pathogens, although surface contamination may occur with some pathogens and hands may become colonized. Most experts believe that meticulous disinfection and handwashing can prevent most transmission of disease.13
"Even though surface contamination may play a role in disease transmission, changes in routine disinfection only are unlikely to reduce disease transmission because recontamination of the patient environment likely is rapid," Weber and Rutala write.14 "Preliminary studies suggest that current protocols for terminal cleaning may not eliminate VRE from environmental surfaces.15 In conclusion, we believe that widespread environmental contamination with VRE is likely in the rooms of colonized or infected patients. Good handwashing and use of recommended barrier precautions are indicated to prevent cross-transmission of VRE. There is no evidence that changing routine cleaning protocols is likely to alter the level of surface contamination. However, terminal cleaning protocols may need to be altered. Research efforts should focus on improving compliance by healthcare providers with currently recommended handwashing and barrier precautions."
The Role of Surveillance in Environmental Reservoirs
Most infection control experts do not believe routine microbiologic surveillance of the inanimate hospital environment is warranted. "Although many epidemics of nosocomial infections have stemmed from reservoirs of pathogens in the inanimate hospital environment, the contribution of the environment to the acquisition and spread of endemic nosocomial infections has been thought to be insignificant," Rutala says.16
In 1974 the Committee on Infections Within Hospitals of the American Hospital Association concluded, "The occurrence of nosocomial infection has not been related to levels of microbial contamination of air, surfaces and fomites ... meaningful standards for permissible levels of such contamination do not exist ... routine environmental microbiologic sampling programs done with no specific epidemiologic goal in mind are unnecessary and economically unjustifiable."17
Despite this guidance, a nationwide survey of hospital to infection control programs in 1976-1977 revealed 70 percent of respondents were conducting moderate to extensive microbiologic surveillance of hospital environments routinely.18
To test the relationship between environmental contamination and nosocomial infections, healthcare professionals launched a study of the University of Wisconsin Hospital that moved from a 56-year-old building into a new and more spacious facility.19 Cultures of fomites and surfaces (including sink drains, faucets and inner walls of ice machines) water and air were taken in the old and new hospitals immediately before taking occupancy and after 6 to 12 months of occupancy.
The most common nosocomial pathogens, including ancinetobacter species, Enterobacteria and Staphylococcus aureus, were isolated from 17 percent of 276 specimens obtained from the old hospital and in 14 percent of 311 cultures from the new hospital immediately before occupancy. After 6 to 12 months of occupancy, 11.3 percent of the 311 cultures from the new hospital yielded common pathogens. Acinetobacter was recovered from cultures in the old hospital but was found infrequently in the new facility. Pseudomonas also was recovered frequently from cultures in the old hospital, particularly from sink drains. Staphylococcus aureus was recovered from both facilities almost equally. The study closely watched the nosocomial infections among patients before and after the move and concluded that there was no decline in the rate of infection immediately after moving into the new hospital's supposed "less contaminated" environment.
According to the study's researchers, "Despite major differences in environmental contamination between the old and new hospitals, the incidence of nosocomial infection in patients remained unchanged. We conclude that organisms in the inanimate hospital environment contribute negligibly to endemic nosocomial infection and that routine microbiologic surveillance of the inanimate environment is not cost effective."20
The study suggests that the increase in environmental contamination in the new hospital after occupancy can be attributed to organisms originating in the human environment of the hospital-from infected patients and from the hands of HCWs. At any time, at least 40 percent of HCWs carry gram-negative bacilli and 10 percent carry Staphylococcus aureus.21
So, is it a chicken-and-egg scenario? Is bacteria transferred onto the hands from fomites or vice versa? One of the badges of a healthcare professional is a pager, a fomite waiting for an opportunistic bacteria. Curious about the role pagers play in the transmission of infection, researchers Deepjot Singh, MD, Hanspreet Kaur, MD, William Gardner, MD and Lisa B. Treen, BSMT, assessed the bacterial contamination of pagers in a large community teaching hospital and level I trauma center.22
One hundred healthcare workers whose pagers were cultured included house staff, attending physicians, registered nurses, respiratory therapists and several medical students. In the survey, participants were asked about their service assignments, how often they cleaned their pagers, what cleaning agents were used and the time interval since the last cleaning. Pagers were removed from their cases and their fronts and four sides were each pressed into agar plates. Pagers were then cleaned with a 70 percent isopropyl alcohol swab and allowed to dry. Cultures were then repeated.
Queries about HCWs' cleaning practices revealed 88 of 100 participants had never cleaned their pagers, and two HCWs had cleaned their pagers more than once in the previous six months. None of the participants reported cleaning their pagers in the two weeks prior to the study. Those who cleaned their pagers regularly used an alcohol swab.
Culture results revealed that bacterial counts were highest for respiratory therapists and lowest for house staff. Medical students had the lowest bacterial counts, however, they represented just three of the 100 participants. The bacterial carriage rates were highest on pagers carried by HCWs in the emergency department and healthcare personnel from the intensive care unit (ICU) had a significantly higher bacterial load than those in a general medical unit.
All pagers revealed the presence of coagulase-negative staphylococci and Staphylococcus aureus; other organisms found included Bacillus species, Streptococcus species and Candida species. Three pagers belonging to house staff grew methicillin-resistant Staphylococcus aureus (MRSA). Gram-negative bacilli and enterococci were not found on any of the pagers. The study found that cleaning the pagers with alcohol resulted in a 94 percent decrease in the total bacterial colony count and completely eliminated Staphylococcus aureus.
The study conducted by Singh, et al., acknowledges other studies pointing to stethoscopes, electronic thermometers and otoscopes as potential vectors of nosocomial infections. One study demonstrated the transfer of Micrococcus luteus from inoculated stethoscopes to skin and suggested that other bacteria could be transferred in this manner.23-24 Another study revealed that staphylococcal species were isolated from 21 percent of stethoscopes in a hospital setting.25
It has been documented that 22 percent of HCWs cleaned their stethoscopes regularly.26 This study was conducted in a 450-bed general hospital to evaluate the bacterial contamination of stethoscopes, determine bacterial survival on stethoscope membranes and study the efficacy of 70 percent alcohol or liquid soap for membrane disinfection. Among the 355 stethoscopes tested, 234 carried at least two different bacterial species and 31 carried potentially pathogenic bacteria. Although some bacteria deposited onto membranes could survive 6 to 18 hours, none survived after disinfection.
The pager study revealed one common thread in so many studies of the environment as potential vector: gram-negative bacilli and enterococci were not isolated from fomites because they require a warm, moist environment. Devices such as pagers and stethoscopes may not be environments conducive to the growth of gram-negative bacilli. The study did not assess viral contamination, which the researchers acknowledge may be transmitted by fomites.27
The researchers concluded: "Hospital pagers are rarely cleaned and are often touched during or after the examination of patients, without handwashing. Our study suggests that pagers can be contaminated with potential pathogens, including antibiotic-resistant strains, and that simple cleaning with 70 percent isopropyl alcohol significantly decreases bacterial load, including pathogenic bacteria. However, their role as vectors of nosocomial infection remains unproved."