Hearing aid services are sought by patients varying widely across several factors, including age, underlying diseases, socioeconomic status, and history of pharmacologic interventions. Because these factors directly influence the immune system's ability to defend and protect the human body from a variety of potentially infectious micro-organisms, dispensing professionals provide hearing aid services, both knowingly and unknowingly, to individuals with compromised immunity.1-3
Such patients have a heightened susceptibility to micro-organisms that commonly reside within many healthy persons or on various surfaces. While these microbes do not pose a threat to healthy individuals with intact immune systems, even mildly immuno-compromised patients may be at an increased risk of developing opportunistic infections. Opportunistic infections originate from commonplace microbes that take the opportunity to infect a body with a weakened immune system.1 These micro-organisms may lead to a level of infection that ultimately results in serious, life-threatening complications.
In the hearing aid clinic, micro-organisms may be transmitted in a variety of ways. However, contact transmission remains the most common means of cross-contamination and possible disease transmission.3,4 Contact transmission occurs when a clinician or patient touches another individual or object. For example, removing a hearing aid from a patient's ear or accepting a hearing aid from a patient with bare hands may allow inadvertent cross-infection via contact transmission.
If transmission occurs, microbes naturally seek entry into the body by traditional routes, including natural orifices (nose, eyes, and ears) or via the epithelial layer of the skin.4 Given the nature of the practice of hearing aid dispensing and servicing, the concern for cross-infection cannot be overlooked.
Although hearing aids have not been previously identified as a source of infection, providing hearing aid services potentially exposes clinicians and patients to bodily fluids, including ear drainage, blood, and, particularly, cerumen.
Cerumen is not considered an infectious agent unless it is contaminated with blood or mucus. However, due to its color and viscosity, visual detection of blood or ear drainage contaminants cannot be made with predictable accuracy and, therefore, cerumen should be treated as if it is a potentially infectious agent.5
STUDY MEASURES MICRO-ORGANISMS ON HEARING AIDS
Because hearing aids are often contaminated with cerumen and dispensing professionals handle numerous such devices on a daily basis, the potential for cross-infection cannot be ignored. In the absence of previous studies, one purpose of this investigation was to document the presence or absence of bacterial and/or fungal micro-organisms on the surface of custom hearing aids. The second purpose was, when microbial growth was found, to identify the specific bacterial and/or fungal organisms retrieved.
Ten hearing-impaired men (7 men, 3 women), ranging from 61 to 88 years of age (x=75.7 years, sd=8.26), participated in this study. All patients had been previously fit with completely-in-the-ear (CIC), in the canal (ITC), or in-the-ear (ITE) hearing aids at St. Louis University Medical Center's Audiology Clinic. Potential subjects who arrived at the clinic for routine hearing aid follow-up wearing their hearing aid(s) were informed of the proposed study and asked to volunteer, resulting in the sequential selection of subjects.
Once a subject had given informed consent, one hearing aid was removed from his or her ear. In the case of binaural fittings, the participant randomly selected one hearing aid prior to its removal from the ear. The researchers wore non-Latex SensiCare medical examination gloves on both hands to remove and handle the hearing aids and to collect a specimen from each.
Hearing aid specimens were obtained using the Baxter Culturette System (Becton Dickinson & Company). Each culturette system was encased in a container consisting of a plastic tube topped off with a plastic cap (Figure 1).
Each culturette system contained one dry, sterile, rayon-tipped swab anchored to the plastic cap and one ampule of .05-ml modified Stuart's bacterial transport medium consisting of sodium glycerophosphate (1%), sodium thioglycolate (0.1%), calcium chloride dihydrate (0.1%), and water. The culturette system container was wrapped and sealed in sterile packaging.
Two separate cultures were required for each hearing aid for the microbial analysis: a routine culture for bacterial identification and a fungal culture for fungal identification. Before the hearing aid was removed from the subject's ear, two separate culturette system containers were removed from the sealed packaging. The author then removed the hearing aid from the subject's ear using gloved hands. Hearing aid cultures were obtained by aligning the two culturette system swabs and simultaneously swabbing the entire outer surface of the hearing aid with the rayon-tipped portion of two separate sterile culturette systems (Figure 2).
The individual cultures were activated by crushing the ampule, thereby causing saturation of the rayon-tipped swab with the solution in preparation for analysis (Figure 3). The cultures were labeled, placed in a biohazard specimen pouch, and dropped off at the St. Louis University Microbiology Lab for analysis within 15 minutes of specimen collection. The specimens underwent preliminary analysis within 8 hours of collection.
The micro-organisms detected from the routine and fungal cultures are listed in Table 1. Hearing aid specimens are listed numerically in the order obtained, along with the corresponding hearing aid model (CIC, ITC, or ITE) and lab analysis results. The routine culture identified bacterial micro-organisms and associated growth levels for each hearing aid specimen.
As shown in Table 1, light-to-moderate amounts of 10 different bacteria and 3 fungi were isolated from the group of hearing aids swabbed in this study. Each of the 10 hearing aids contained light-to-heavy amounts of at least one bacterium, with Coag Neg staphylococcus recovered from 9 of the 10 hearing aids. The microbial composition of seven of the hearing aids contained two or more independent bacteria, including Coag Neg staphylococcus, Acinetobacter lwoffi, diphtheroids, Lactobacillus, Enterobacter cloacae, Pseudomonas aeruginosa, enterococcus (Streptococcus), and/or S.aureus.
In addition to bacterial growth, 4 of the 10 hearing aids contained light-to-moderate amounts of fungal growth, including Aspergillus flavus (2 hearing aids), Candida parapsilosis (2), and/or the light growth of an unspecified mold (1). When bacterial and fungal composition are considered, 8 of the 10 swabbed hearing aids revealed at least 2 distinct colonies of microbial growth.
The predominant organism recovered from hearing aid surfaces was Coag Neg staphylococcus (9 of 10 hearing aids), although a total of 9 other bacteria and 3 fungi were also recovered.
With the exception of Coag Neg staphylococcus, each hearing aid specimen reflected a unique bacterial composition. For example, while hearing aids 2, 3, and 5 all contained light-to-moderate amounts of Coag Neg staphylococcus, the concomitance of A. lwoffi was found on the surface of hearing aid 2 only. Similarly, diphtheroids and Lactobacillus were also isolated, but only from the surfaces of hearing aids 3 and 5, respectively. Hearing aid 7 was the only hearing aid in which Coag Neg staphylococcus was not detected; however, three unique bacteria (E. Cloacae, P. Aeruginosa, and enterococci) were recovered from its surface, none of which was isolated from the surfaces of the remaining hearing aids.
Taking into account both bacterial and fungal growth, hearing aid 7 also represented the greatest range of microbial composition, revealing the presence of the light growth of three independent bacteria and moderate growth of one fungus.
The general finding of light-to-heavy amounts of microbial growth on hearing aid surfaces is not necessarily unusual. The external auditory canal produces cerumen, a substance characterized by an acidic pH level capable of inhibiting microbial growth.6,7 Because cerumen is physiologically designed to inhibit bacterial or fungal reproduction, the residual presence of the very micro-organisms it is designed to combat is to be expected.
From a microbiologic standpoint, the recovered bacteria and fungi are widely distributed throughout the environment. In normal individuals, Coag Neg staphylococcal microbial flora thrive on skin surfaces and are typically cultured in the external auditory canal, either alone or in combination with other organisms, including diphtheroids or occasional fungal spores.6-8
In this study, staphylococcus was generically identified on the surfaces of the majority of hearing aids. With the exception of the presence of S. aureus, S. capitis, and S. bacillus, the specific subspecies of staphylococcal infection independently recovered from the group of hearing aids remains unknown. Staphylococcus, a bacterium represented by various species and subspecies, is traditionally classified into one of two groups according to whether or not the specific species of staphylococcus produces the blood-clotting enzyme coagulase.9 S. aureus is the only species of staphylococcus found in humans that produces the enzyme coagulase. Therefore, all species of staphylococcus not identified as S. aureus, including S. albus, S. epidermis, and others, may be generically referred to as Coag Neg staphylococcus.9
This investigation examined only a small sample of hearing aids. It was not designed to determine cause and effect; therefore, the data do not identify hearing aids as a vector for the spread of disease. Nevertheless, the preliminary findings do generate several interesting thoughts regarding infection control in the hearing aid clinic.
- First, despite cerumen's role in inhibiting various forms of microbial growth, the external auditory canal is more prone to bacterial infection than other skin surfaces.8 Furthermore, the efficacy of cerumen in inhibiting microbial growth may be uniquely challenged in hearing-impaired populations wearing conventional amplification. A hearing aid or earmold occludes the ear, creating a warm and moist environment even in the presence of a large vent. When the ear canal retains moisture, the pH level of the ear canal is affected, resulting in a neutral or more alkaline levels that is more conducive to bacterial and/or fungal growth.8
- Secondly, although the microbes recovered from hearing aid surfaces are ubiquitous in nature, the hallmark of immunosuppression is characterized by susceptibility of disease-prone individuals to these very same organisms. For instance, Coag Neg staphylococcus is a universal microbe of normal skin and nasopharyngeal flora. Because of its ubiquitous nature, shedding of this bacteria is very common. However, because of its universal nature, it also accounts for a high percentage of hospital-acquired infections by susceptible patients exhibiting varying degrees of immunosuppression.9 While the majority of hearing aids swabbed in this study contained light-to-heavy amounts of Coag Neg staphylococcus, it could be argued that since this bacterium is found universally, it should not be considered a potential source of cross-infection. On the other hand, arguments could also be made that there is a risk of cross-infection.
With the exception of Coag Neg staphylococcus, found on 9 of the 10 hearing aids, the data showed that each hearing aid had a unique microbial composition. From this perspective, the potential for cross-infection becomes a more compelling issue.
For instance, the clinician handling hearing aid 7 with unwashed, bare hands who subsequently handles hearing aid 8 with the same unwashed, bare hands could cross-contaminate hearing aid 8 with the microbial content of hearing aid 7. When hearing aid 8 is re-inserted into the patient's ear, that patient's ear canal will be exposed to the microbial content from hearing aid 7. If the patient has some level of immunocompromise, the otherwise ubiquitous microbes from hearing aid 7 contaminating hearing aid 8 might, under some conditions, become opportunistic, exposing the patient to potentially pathogenic bacteria and/or fungi. Since this particular study was not designed to show cause and effect, the described scenario remains theoretical.
AREAS OF FUTURE RESEARCH
This study provided preliminary data indicating the presence of moderate amounts of bacterial and/or fungal growth on hearing aid surfaces. However, conclusions regarding the significance of the findings and possible clinical implications must be supported with further research. The small pool of hearing aids obtained from a relatively homogeneous group of geriatric patients limits the generalizability of the findings. The subjects in this particular study were adult patients who ranged from 61 to 88 years of age. Since hearing aid services are sought by pediatric patients as young as 6 months, the presence of microbial growth on earmold and/or hearing aid surfaces in younger patients may be significantly different from that initially reported in this geriatric patient group.
Comparison of microbial growth across different socioeconomic groups is an additional factor to investigate since the social environment in which a person lives greatly influences the incidence of certain infectious organisms. For example, the incidence of Hepatitis B is significantly higher in lower socioeconomic populations than in mid- or high-income level households.9,11 The inclusion of a larger pool of subjects differing across age and socioeconomic status would provide a more accurate assessment of hearing aid-related microbial growth.
With the expansion of sample groups, future research should focus on determining if hearing aids are a potential vector for disease via direct or indirect cross-contamination. For this determination, information must be gathered regarding the knowledge of, attitudes toward, and practice of hearing aid-related infection-control procedures in clinics where hearing aids are dispensed. This information is important because infection rates in the clinic and other treatment facilities can be reduced or eliminated by the implementation of appropriate infection-control measures.12
PREVENTING DISEASE TRANSMISSION
Protection against inadvertent transmission of disease from patient to patient, clinician to patient, and patient to clinician must be approached from a preventive standpoint.
The first step is to determine if hearing aids pose a risk in potential disease transmission. Preliminary data reported in this study indicate the presence of both bacterial and fungal microbial growth on hearing aids surfaces. Table 2 lists common complications and clinical manifestations of these bacteria and fungi. A larger scale study would provide further insight into the types and amounts of microbial growth detectable on hearing aid and hearing aid-related surfaces.
A second step in controlling the spread of disease in hearing aid healthcare facilities is the collection of information regarding current infection-control practices specific to the practice of hearing aid dispensing. The collection of both laboratory and survey data will make it possible to determine the need for hearing aid-specific infection control, and, if such a need is shown, provide information on which to base profession-specific educational programs in hearing aid infection control.
Until such information is collected and analyzed, professionals dispensing hearing aids may wish to follow general infection-control principles in their practice. Infection control involves a systematic effort to manage the environment to minimize exposure to micro-organisms that may make the clinician or patients ill.13
While the following is not comprehensive in scope, the first important element required for successful infection control is the development of the mindset that each patient seeking hearing aid services is a potential carrier of an infectious disease.3–5,12 Secondly, hand washing is a critical component to controlling cross-infection and dispensers should wash their hands before and after each patient.
Third, Latex or non-Latex gloves should be worn prophylactically when the risk of encountering infectious substances is high. Within the context of the hearing aid clinic, it is recommended that gloves be used during cerumen-management procedures and for purposes of handling hearing aids/earmolds. For more information on infection control, please refer to the available literature.3–5,12