Fomites' Role in Disease Transmission is Still Up for Debate
By Kelly M. Pyrek
Therole fomites play in the transmission of infectious disease is still beingcontested in the infection control community, with no conclusive decisionemerging from clinical studies. A fomite is defined as an inanimate object thatserves 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 ofthe host, pathogen and environment."1 Breaches in the host'sskin integrity allow microbes to invade, while microorganisms must be present ina minimum inoculating dose sufficient to trigger infection, virulence,infectivity and the ability to produce a latent infection. Nosocomial infectionscan result from endogenous or exogenous flora, with the latter often present onan 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 animateenvironment (the patient) via the hands of healthcare workers (HCWs).
According to Weber and Rutala, "A key concept in considering thehospital environment as an infectious hazard for patients is proof of acausative role of inanimate objects in human disease." They point toseveral levels of proof offered by researcher Frank Rhame:2
In an editorial in Infection Control and Hospital Epidemiology, Weberand Rutala pondered the following: Do patients colonized or infected with VREcontaminate their environment? What is the role of surface contamination in thetransmission of vancomycin-resistant enterocci (VRE)? Is surface contaminationlinked to the transmission of other nosocomial pathogens?3 Theyconcede that in VRE outbreaks, "It often has been difficult to determinewhether cross-transmission occurred due to contaminated common equipment (e.g.,stethoscopes), acquisition of transient hand carriage by healthcare personneldue to direct contact with a colonized or infected patient or acquisition oftransient hand carriage by healthcare personnel due to contact with acontaminated surface. Cross-transmission of VRE occasionally has been linked tocontaminated medical devices, including an electronic thermometer and afluidized bed. Disinfection or removal of the contaminated equipment terminatedthe outbreaks."
Survival of the Fittest
Weber and Rutala point to various studies conducted to analyze the survivalrates of enterococci inoculated onto environmental surfaces. Noskin, et al.,4reported that Enterococcus faecalis survived for five days and Enterococcusfaecium for seven days on countertops. Both species survived on bed railsfor 24 hours without significant die-off, on telephone handpieces for 60minutes, on the diaphragmatic surface of stethoscopes for 30 minutes and ongloved and ungloved fingers for at least 60 minutes. Other investigators havedemonstrated survival of VRE for more than three days on inoculated surfaces5or equipment contaminated by colonized or infected patients.6Survival of 18 hours on pieces of sterile cotton sheets also has beendemonstrated.7
When trying to prevent the transmission of disease, the most importantenvironmental 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-fluidizedbeds (enteroccocus); mattresses (Pseudomonas, Acinetobacter); plaster(Pseudomonas); and urine-measuring devices (Serratia).8
Contamination of the inanimate environment-especially bed rails, bed sheetsand patient gowns -- has been most closely associated with methicillin-resistantStaphylococcus aureus (MRSA), C. difficile andantibiotic-resistant Enterococcus. These microorganisms also have beenfound 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 andits presence has been detected on the following:9
In a study that appeared in the Lancet in 1996, 12 of 29 ventilatedpatients acquired VRE; the resistant strain was isolated from 157 of 1,294environmental cultures. The study concluded that it is impossible to separatepatient-to-patient transmission via HCWs or the contaminated environment, butthat the latter was not a major source of infection.10
"It will be extremely difficult to disentangle the contributions of theanimate and inanimate reservoirs of VRE in leading to transient hand carriage ofVRE by medical personnel," Weber and Rutala write.11"Clearly, proper handwashing with an antimicrobial agent before and aftereach contact with patients or their immediate environment is crucial inpreventing person-to-person transmission of nosocomial pathogens. Unfortunately,compliance with CDC handwashing guidelines has been noted in less than one halfof the instances in which it is indicated.12 For this reason,additional contact precautions have been recommended, including wearing gloveswhen entering the rooms of patients with VRE. We believe there is sufficientevidence to state that inanimate surfaces likely play a role in the transmissionof VRE. Supportive evidence includes environmental cultures demonstratingwidespread surface contamination in rooms of many patients colonized or infectedwith VRE and experimental evidence that VRE can survive on environmentalsurfaces for hours and that hands can become colonized with VRE via patient orenvironmental surfaces."
What infection control experts conclude from these various studies is thatthe inanimate environment may contribute to the spread of some pathogens such asVRE, but that the precise contribution is probably 0 percent to less than 25percent. It has been found that non-critical surfaces play a minor role in thetransmission of most pathogens, although surface contamination may occur withsome pathogens and hands may become colonized. Most experts believe thatmeticulous disinfection and handwashing can prevent most transmission ofdisease.13
"Even though surface contamination may play a role in diseasetransmission, changes in routine disinfection only are unlikely to reducedisease transmission because recontamination of the patient environment likelyis rapid," Weber and Rutala write.14 "Preliminary studiessuggest that current protocols for terminal cleaning may not eliminate VRE fromenvironmental surfaces.15 In conclusion, we believe that widespreadenvironmental contamination with VRE is likely in the rooms of colonized orinfected patients. Good handwashing and use of recommended barrier precautionsare indicated to prevent cross-transmission of VRE. There is no evidence thatchanging routine cleaning protocols is likely to alter the level of surfacecontamination. However, terminal cleaning protocols may need to be altered.Research efforts should focus on improving compliance by healthcare providerswith currently recommended handwashing and barrier precautions."
The Role of Surveillance in Environmental Reservoirs
Most infection control experts do not believe routine microbiologicsurveillance of the inanimate hospital environment is warranted. "Althoughmany epidemics of nosocomial infections have stemmed from reservoirs ofpathogens in the inanimate hospital environment, the contribution of theenvironment to the acquisition and spread of endemic nosocomial infections hasbeen thought to be insignificant," Rutala says.16
In 1974 the Committee on Infections Within Hospitals of the American HospitalAssociation concluded, "The occurrence of nosocomial infection has not beenrelated 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 specificepidemiologic goal in mind are unnecessary and economically unjustifiable."17
Despite this guidance, a nationwide survey of hospital to infection controlprograms in 1976-1977 revealed 70 percent of respondents were conductingmoderate to extensive microbiologic surveillance of hospital environmentsroutinely.18
To test the relationship between environmental contamination and nosocomialinfections, healthcare professionals launched a study of the University ofWisconsin Hospital that moved from a 56-year-old building into a new and morespacious facility.19 Cultures of fomites and surfaces (including sinkdrains, faucets and inner walls of ice machines) water and air were taken in theold and new hospitals immediately before taking occupancy and after 6 to 12months of occupancy.
The most common nosocomial pathogens, including ancinetobacter species,Enterobacteria and Staphylococcus aureus, were isolated from 17 percentof 276 specimens obtained from the old hospital and in 14 percent of 311cultures from the new hospital immediately before occupancy. After 6 to 12months of occupancy, 11.3 percent of the 311 cultures from the new hospitalyielded common pathogens. Acinetobacter was recovered from cultures in the oldhospital but was found infrequently in the new facility. Pseudomonas also wasrecovered frequently from cultures in the old hospital, particularly from sinkdrains. Staphylococcus aureus was recovered from both facilities almostequally. The study closely watched the nosocomial infections among patientsbefore and after the move and concluded that there was no decline in the rate ofinfection immediately after moving into the new hospital's supposed "lesscontaminated" environment.
According to the study's researchers, "Despite major differences inenvironmental contamination between the old and new hospitals, the incidence ofnosocomial infection in patients remained unchanged. We conclude that organismsin the inanimate hospital environment contribute negligibly to endemicnosocomial infection and that routine microbiologic surveillance of theinanimate environment is not cost effective."20
The study suggests that the increase in environmental contamination in thenew hospital after occupancy can be attributed to organisms originating in thehuman environment of the hospital-from infected patients and from the hands ofHCWs. At any time, at least 40 percent of HCWs carry gram-negative bacilli and10 percent carry Staphylococcus aureus.21
So, is it a chicken-and-egg scenario? Is bacteria transferred onto the handsfrom fomites or vice versa? One of the badges of a healthcare professional is apager, a fomite waiting for an opportunistic bacteria. Curious about the rolepagers play in the transmission of infection, researchers Deepjot Singh, MD,Hanspreet Kaur, MD, William Gardner, MD and Lisa B. Treen, BSMT, assessed thebacterial contamination of pagers in a large community teaching hospital andlevel I trauma center.22
One hundred healthcare workers whose pagers were cultured included housestaff, attending physicians, registered nurses, respiratory therapists andseveral medical students. In the survey, participants were asked about theirservice assignments, how often they cleaned their pagers, what cleaning agentswere used and the time interval since the last cleaning. Pagers were removedfrom their cases and their fronts and four sides were each pressed into agarplates. Pagers were then cleaned with a 70 percent isopropyl alcohol swab andallowed to dry. Cultures were then repeated.
Queries about HCWs' cleaning practices revealed 88 of 100 participants hadnever cleaned their pagers, and two HCWs had cleaned their pagers more than oncein the previous six months. None of the participants reported cleaning theirpagers in the two weeks prior to the study. Those who cleaned their pagersregularly used an alcohol swab.
Culture results revealed that bacterial counts were highest for respiratorytherapists and lowest for house staff. Medical students had the lowest bacterialcounts, however, they represented just three of the 100 participants. Thebacterial carriage rates were highest on pagers carried by HCWs in the emergencydepartment and healthcare personnel from the intensive care unit (ICU) had asignificantly higher bacterial load than those in a general medical unit.
All pagers revealed the presence of coagulase-negative staphylococci and Staphylococcusaureus; other organisms found included Bacillus species, Streptococcusspecies and Candida species. Three pagers belonging to house staff grewmethicillin-resistant Staphylococcus aureus (MRSA). Gram-negative bacilliand enterococci were not found on any of the pagers. The study found thatcleaning the pagers with alcohol resulted in a 94 percent decrease in the totalbacterial colony count and completely eliminated Staphylococcus aureus.
The study conducted by Singh, et al., acknowledges other studies pointing tostethoscopes, electronic thermometers and otoscopes as potential vectors ofnosocomial infections. One study demonstrated the transfer of Micrococcusluteus from inoculated stethoscopes to skin and suggested that otherbacteria could be transferred in this manner.23-24 Another studyrevealed that staphylococcal species were isolated from 21 percent ofstethoscopes in a hospital setting.25
It has been documented that 22 percent of HCWs cleaned their stethoscopesregularly.26 This study was conducted in a 450-bed general hospitalto evaluate the bacterial contamination of stethoscopes, determine bacterialsurvival on stethoscope membranes and study the efficacy of 70 percent alcoholor liquid soap for membrane disinfection. Among the 355 stethoscopes tested, 234carried at least two different bacterial species and 31 carried potentiallypathogenic bacteria. Although some bacteria deposited onto membranes couldsurvive 6 to 18 hours, none survived after disinfection.
The pager study revealed one common thread in so many studies of theenvironment as potential vector: gram-negative bacilli and enterococci were notisolated from fomites because they require a warm, moist environment. Devicessuch as pagers and stethoscopes may not be environments conducive to the growthof gram-negative bacilli. The study did not assess viral contamination, whichthe researchers acknowledge may be transmitted by fomites.27
The researchers concluded: "Hospital pagers are rarely cleaned and areoften 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 percentisopropyl alcohol significantly decreases bacterial load, including pathogenicbacteria. However, their role as vectors of nosocomial infection remainsunproved."