Due to the cross-industry content, literature was obtained through reputable research databases (e.g., PubMed, Web of Science, and Dimensions) that index high-quality journals from a broad range of fields, including the physical sciences (e.g., mycology, biochemistry, biology, engineering, and physics) and applied sciences (e.g., public health, healthcare management, medicine, and economics). Search terms included (but not limited to) “Candida auris,” “C. auris,” “antimicrobial resistance,” “nosocomial spread,” “C. auris surveillance,” “UVC disinfection,” “healthcare facility disinfection,” and “global fungal disease burden.” Additional literature was identified by carefully reviewing appropriate articles’ citations and relevant conference presentations. Our review critically examines the evidence base for the feasibility of UVC LED disinfection and waste management of C. auris.
2.1. Global disease Threat of C. auris
Fungal diseases account for a significant global burden of morbidity and mortality. Fungal infections (including skin, nails, and hair) are estimated to impact approximately one billion people, with associated mortality rates accounting for over 1.5 million deaths annually [
58]. Nearly all forms of yeast are from the
Candida genus, though many are not responsible for infection or have only superficial infection capacity [
59]. Ringworm, nail fungus, yeast infections (e.g., vaginal candidiasis), and thrush (e.g., oral
Candida infections) are among the most common fungal infections [
60]. Candidiasis is an infection caused specifically by a
Candida species and can present in many infection sites; however, there are over 200 specific species [
61]. The annual incidence of oral and esophageal candidiasis is estimated to be approximately 3.3 million, with the global burden of recurrent vulvovaginal candidiasis being approximately 134 million each year. Invasive candidiasis is unlike oral and vaginal candidiasis because it infects the bloodstream, brain, heart, eyes, bones, and other internal parts of the body. The annual incidence of invasive candidiasis (from any form of
Candida spp.) is estimated to be approximately 750,000 [
58]. Candidemia, a common healthcare-associated infection, is a specific and dangerous blood infection caused by
Candida isolates with adverse outcomes [
62].
C. auris is a particularly resilient branch of the
Candida species that rapidly develop multi-drug resistance with morphisms specific to regional development or genetic clade [
63]. To date,
C. auris has been reported in over 47 countries worldwide, representing all continents except Antarctica [
64].
C. auris strains have been categorized into different genomic clades: I (southern Asia), II (eastern Asia), III (Africa), IV (South America), and V (Iran), each with independent emergence, which is visualized in
Figure 1.
C. auris clades are revealed by genome analysis, PCR amplification of genetic loci, or mass spectrometry [
65]. Despite the first isolation in 2009 (ear canal), retrospective sample testing revealed the presence of
C. auris as far back as 1996 in South Korea [
12]. More than 740 isolates of
C. auris have now been identified [
23,
63,
66,
67,
68,
69,
70,
71,
72,
73,
74]. Multidrug-resistant and pan-drug-resistant
C. auris isolates are increasingly detected worldwide [
75,
76].
The rising ambient air temperatures (i.e., emerging climate change), combined with changes in avian migration patterns, farm activities, and increased urban dwellers, have created an environment that supports thermotolerant fungi like
C. auris [
67,
77]. Since first being identified in Japan,
C. auris has become a globally transmitted pathogenic infection that often leads to invasive candidemia and invasive candidiasis of the heart and central nervous system [
78,
79]. In 2016, the Centers for Disease Control and Prevention (CDC) identified the first reported case of a patient in the United States with
C. auris, discovered via a misidentified isolate collected in 2013 [
80].
C. auris has been identified in several infection sites and bodily fluids, including blood, urine, bile, ear canal, nares, axilla, skin, and, in rare instances, the oral, esophageal, and gut mucosa [
63].
C. auris has gained notoriety due to its increasing resistance to common antifungal agents and the possibility that it may be the first fungal disease related to emerging climate change [
67,
77]. Many
C. auris strains have a high minimum inhibitory concentration (MIC) towards antifungal drug classes and common disinfectants, contributing to the challenge of decolonization and the treatment of infections [
51,
63]. Its pathogenicity has been associated with virulent traits like the production of proteases, lipases, mannosyltransferases, oligopeptides, siderophore-based iron transporters, and biofilm formation. These virulent traits assist
C. auris in invading, colonizing, and acquiring nutrients from the host [
11]. Further,
C. auris has the capability of transforming into a persistent yeast capable of surviving under unfavorable conditions [
11,
61,
81,
82].
Once
C. auris colonizes and progresses to invasive candidiasis or candidemia, several molecular mechanisms can evade the action of antifungals, leading to resistance to agents like amphotericin B and the Azole and Echinocandins classes of drugs. Azole (e.g., fluconazole) resistance is associated with the overexpression of drug efflux pumps belonging to ATP Binding Cassette (ABC) and Major Facilitator Superfamily (MFS) transporters and encoding alterations of the ergosterol synthesis pathway (overexpression of ERG11, and point mutations in ERG11, Y132F or K143R). The Echinocandin resistance in
C. auris is shown to be attributable to mutations of FKS1, a gene that codes the enzyme responsible for the key fungal cell wall component, β(1,3)D-glucan. Single nucleotide polymorphisms in genes related to the ergosterol synthesis pathway leading to altered sterol composition and potential amino acids substitution in the FUR1 gene (i.e., F211I) have been linked to
C. auris resistance to polyenes (e.g., amphotericin B) and nucleoside analogs (e.g., flucytosine) respectively [
61,
83,
84,
85,
86,
87,
88]. In addition to the multidrug resistance,
C. auris can survive on surfaces, including human skin, for extended periods, contributing to high mortality rates (30-60%) in healthcare settings [
63,
64,
89,
90]. Difficulties in pathogen identification and disinfection have also led to increased transmission and delayed infection management [
11,
61,
81,
82].
While antifungals may be used to treat invasive candidiasis due to C.
auris, multidrug-resistant and pan-drug-resistant isolates are rapidly being identified across the globe [
78]. The near-simultaneous emergence of multi-drug resistant
C. auris on multiple continents, as well as the associated high mortality rate, make
C. auris a significant global threat [
91]. The changing climate trending towards warmer global temperatures is not enough to fully explain the rapid ability to develop resistance to antifungals [
67]. More concerning is that many MDR
C. auris outbreaks have no direct epidemiological links, indicating they are developing new resistance in each cluster [
92]. In 2022, the World Health Organization listed
C. auris on the ‘WHO fungal priority pathogens list’ in the critical priority group, urging global action on three priority areas: (1) surveillance, (2) research & development and innovation, and (3) public health interventions [
78]. In 2018, the CDC made
C. auris a nationally notifiable infectious disease [
93].
The global incidence rate of
C. auris infections cannot currently be established due to a lack of uniform surveillance systems, few epidemiological outbreak studies, and limited diagnostic capability [
78]. Conventional laboratories often misidentify
C. auris as one of several similar isolates, such as other
Candida species like
C. haemulonii,
C. famata, and
C. sake, as well as
Rhodotorula glutinis,
R. mucilaginosa, and
Saccharomyces species [
25,
94]. While accurate incidence rates are challenging to estimate, individual studies and health systems across the globe have evaluated incidence through outbreak investigations. Studies across Asia, Europe, the Middle East, Africa, Australia, and the Americas have examined colonization, progression to IC and candidemia, and specific antimicrobial resistance [
12,
79,
91,
95,
96,
97,
98,
99,
100,
101]. Du et al. (2020) estimate that there are over 400,000 candidemias (bloodstream infections) each year across all species, with a global mortality rate higher than 40%; however, other studies have estimated mortality rates up to 60% [
102].
In the United States, the CDC reported 1,747 confirmed clinical cases across 26 states and DC, with 95% of cases occurring in population-dense states (i.e., New York, Illinois, New Jersey, California, and Florida) by the end of 2020 [
80]. This rapid escalation continued in the US, with over 3200 active cases accumulated between 2019 – 2021, which rose from a 45% case increase in 2019 to a 95% case increase in 2021, with a three-fold increase in AMR cases [
103]. Following this trend, the CDC reported 2,377 clinical cases and 5,754 screening cases of
C. auris in 2022 [
20]. These rates are widely thought to be under-reported due to a lack of robust surveillance systems and non-uniform screening protocols [
15].
While incidence rates may be underreported or unreliable, disease burden has been tracked more frequently. The median length of hospital stay for adult patients identified with
C. auris candidemia was 46-68 days (70-140 days for pediatric patients) (WHO, 2022). Researchers and hospital investigation teams more frequently report mortality rates; however, rates related to IC and candidemia vary significantly across the globe as noted in
Table 1. Since the emergence of
C. auris, mortality rates reported for candidemia have been quite high, nearing 50% of diagnosed infections [
63]. Further complicating accurate surveillance and monitoring is that misidentification of
Candida species also extends to mortality reporting errors. Mortality rates are often reported only for candidemia or IC, without species attribution [
104].
2.1.1. Vulnerable Populations
While invasive candidiasis and other infections from
C. auris are not thought to pose a significant threat to healthy people even if colonization occurs, vulnerable populations are often unprotected and unable to successfully clear the infection without intensive therapies [
110]. Invasive candidiasis and candidemia are nosocomial infections that can disproportionately impact the critically ill, immunocompromised, elderly, and patients with extensive comorbidities or a history of frequent antimicrobial therapy [
12,
23,
63,
78,
111]. Patients with indwelling medical devices (e.g., central venous catheters), patients using parenteral nutrition, patients on mechanical ventilation, and hospital admissions longer than 10-15 days are among the most at risk for adverse outcomes [
78]. Underlying respiratory and/or cardiovascular illness, vascular surgery, prior antifungal exposures, and low APACHE II score (ICU-based severity-of-disease classification system) are considered significant risk factors associated with
C. auris candidemia and poor prognosis [
63,
112]. Nosocomial outbreaks of invasive candidiasis (
C. auris-specific) in intensive care (ICU) settings are common, especially where colonization on non-human hosts has been previously identified [
94]. In an analysis of 27 ICUs in India, where 1,400 candidemia cases were reported, over 5% were attributed to
C. auris [
112]. Rudramurthy et al. (2017) also found that patients with
C. auris candidemia had longer ICU admissions prior to diagnosis than other microbial infections, indicating that nosocomial spread contributed to fungal outbreaks in India [
78]. It is estimated that unless substantial actions to address AMR infections are taken, nearly 10 million people will die annually by 2050 [
113,
114].
Annually, over 2 million children die globally before their first month of life [
115,
116], with infection as one of the top three most prevalent causes (along with prematurity-related complications and intrapartum-related complications) [
116,
117].
Candida species are responsible for the greatest number of neonatal invasive fungal infections [
117]. The most common
Candida species affecting pediatric populations are
C. albicans, C. parapsilosis, C. glabrata, and
C. krusei [
118]; however,
C. auris is now impacting pediatric populations as well. In a prospective cohort study of hospitalized infants (<60 days postnatal age with sepsis) in Low and Middle-Income Countries (LMICs) at 19 hospitals across 11 countries, researchers found
C. auris to be the third most commonly reported pathogen [
3,
4,
117,
118,
119]. In South Africa,
C. auris was among the 5th most common
Candida species responsible for candidemia [
120]. In India, of 273 neonates from three hospitals with neonatal invasive candidiasis (NIC) cases, investigators isolated
C. auris in 2.2% of the cases, highlighting the vulnerability and susceptibility of infants in LMICs [
121].
The crude mortality rate associated with NIC varies significantly, unsurprisingly, with higher rates disproportionately impacting LMICs compared to high-income countries (HIC) (8.9%-75% in LMIC and 12%-37% in HIC) [
117]. Risk factors for neonates developing NIC or candidemia include preterm birth, older infants and children with ICU stays, post-surgical stays, underlying malignancies, malnourishment or requiring parenteral nutrition, post-solid organ transplantation, an underlying renal disease requiring hemodialysis, central venous catheter placement, and requiring respiratory support [
118,
119]. A case study in Italy found that an extremely low birth weight preterm neonate born via vaginal delivery from a
C. auris colonized mother was colonized within only a few hours after birth [
122]. Though limited by the ability to clearly determine if the colonization route was the birth canal or the ICU environment, this case does highlight the heightened risk for already vulnerable infants to
C. auris [
122].
2.1.2. Economic Impact
AMR is a significant and global threat to public health and the successful clinical management of microorganisms [
2,
6]. Economic simulations run by the World Bank suggest that by 2050, with an optimistic (i.e., low) AMR impact, the annual global gross domestic product (GDP) could fall by approximately 1.1%, with GDP shortfalls exceeding
$1 trillion annually after 2030. Less optimistic simulations (i.e., high AMR impact) predicted a 3.8% decline in annual GDP by 2050, with an annual shortfall of
$3.4 trillion by 2030 [
123]. The predicted impact of AMR is likely to disproportionately impact low-income countries and increase the rate of poverty [
123]. The total economic burden of fungal diseases and AMR in the US is thought to be dramatically underestimated [
59,
124]. The financial burden can be estimated as a total burden or pared down to direct costs, loss of productivity costs, premature death costs, costs per patient, and costs per hospitalization. In the absence of a standard reporting mechanism, economic burden is not always consistent across reports.
Table 2 provides an overview of the estimated global economic burden for all AMR diseases, fungal diseases, and candidiasis/candidemia specifically.
2.3. Current Healthcare Environmental Infection Control Standard Procedures
The global spread of
C. auris has been attributed to the easy transmission through direct or indirect contact on high-touch surfaces, air, and wastewater, the ability to survive outside of a human host, and the ability to sustain long periods of desiccation, biofilm formation, and high thermal tolerance [
21,
41,
63,
89,
121,
135,
136,
137,
138,
139]. Not only is
C. auris highly resilient to hostile environments, but studies suggest that it is also adaptive to environmental stress, creating near-impossible parameters for disinfection [
63]. In India, ICU patients who were not colonized with
C. auris at the time of admission were later colonized during their stay [
132]. After a
C. auris outbreak at a London hospital, researchers found that
C. auris was not isolated from any patient prior to their admission to the ICU [
100]. Once the patient’s skin is colonized, transmission can proceed via skin-to-skin contact with individuals beyond the healthcare setting [
140]. Appropriate infection control protocols include identifying
C. auris colonization and infections and genomic analysis to assess for AMR. However, a critical factor in infection management is adequate and scalable disinfection [
104]. Given the broad range of colonization sites, disinfection and environmental health facility management is critical for water, air, and surface. Due to the cross-sector siloes, the protocols for point-of-service (e.g., patient and bed-specific cleaning in rapid rooming flips) disinfection across applications, larger healthcare facility environmental health management, and waste management are often ineffective and inefficient.
According to several pathogenic surveillance agencies (i.e., CDC, European Centre of Disease Prevention and Control, Pan American Health Organization, World Health Organization, Public Health England, and Centre for Opportunistic, Tropical and Hospital Infections in South Africa), infection prevention and control of
C. auris in healthcare settings includes proper hand hygiene, transmission-based precautions (i.e., patient and room precautions to limit exposure), cleaning and disinfection, uniform screening and surveillance practices, and enhanced communication [
26,
104,
111]. These generic and nonspecific disinfection procedures all lack the specificity for
C. auris. This strategic gap in environmental
C. auris mitigation to prevent nosocomial spread is apparent [
39]. Each component of the infection prevention protocol is critical; however, this review focuses specifically on disinfection as a primary prevention method. Environmental disinfection and cleaning (hand hygiene products are included here; however, policies and methods of hand washing are not) in patient care and high-traffic environments is challenging and covers patient and room turnover, daily cleaning practices, mobile and high-touch equipment disinfection (e.g., blood pressure cuffs, glucometers, stethoscopes, crash carts, etc.) [
26]. Standard protocols are limited by the required complex procedures to reach efficacy, availability of disinfecting agents, and end-user education. Another significant limitation is the non-standardized use of “no-touch” disinfection, such as UVC LED technology. Reliance on standard contact and air precautions for patient care of colonized individuals is not a sustainable or widely effective method. Enhanced disinfection and waste management is necessary for a robust infection prevention and control policy.
2.3.1. Water
C. auris can survive and spread through wastewater as well. A recent study in Nevada positively detected the dangerous fungus in nearly 80% of effluent samples with over 90% positivity rates near healthcare facilities [
141]. Testing and surveillance of wastewater, particularly in effluent sewer sheds near healthcare facilities, can help identify and track potential outbreaks and trigger early warning alarms for public health action. Lower-resourced regions and LMICs without proper water infrastructure may also reuse wastewater after only superficial disinfection. If water treatment and disinfection strategies in these areas do not fully inactivate
C. auris, public health could be compromised by transmission of the pathogen through daily WASH activities. The documented capability to produce biofilms can complicate wastewater system testing and surveillance and require more nuanced disinfection of water supplies [
41,
141]. The most common methods of water decontamination are chemicals (i.e., chlorination and ozonation). However, these methods generate persistent residual carcinogenic by-products (such as chlorine or bromate) [
142,
143,
144]. Furthermore, these methods have led to new resistant microorganisms and affected the organoleptic properties of water [
143,
144]. No specific healthcare setting infection prevention and control protocols were found, including an absence of water testing protocols for
C. auris isolates as a function of outbreak surveillance.
2.3.2. Air
Airborne transmitted pathogenic infections, which occur via droplets or aerosol, are also a common concern [
145,
146,
147]. Coughing, sneezing, and even talking may lead to pathogen transmission, which became a critical concern during the COVID-19 pandemic. Researchers have also found a positive relationship between decontamination effectiveness and airflow conditions [
148]. In poorly ventilated environments, indoor air has lower convection, leading to the environmental accumulation of pathogens and increasing the likelihood of infection [
149].
C. auris can spread through contamination of air handling units (AHUs) [
150]. Fungal colonies can be transmitted via aerosolized particulates with both active and passive air samples. In Tehran, air samples tested positive for the presence of fungi (
C. auris was not included in the testing protocol), indicating low air quality and the need for enhanced filtration and air purification [
151]. However, the lack of discriminant air particulate testing and non-uniform air sampling renders the rate of aerosolized
C. auris transmission unknown. Public Health England’s efforts to address
C. auris include a targeted effort to understand the transmission pathways via the aerosolized spread, particularly in healthcare settings; however, data from this ongoing study has yet to be reported [
152]. No standard air purification protocols or procedures for monitoring and disinfecting air handling systems for
C. auris were found.
2.3.3. Surface
C. auris can survive on surfaces for more than three weeks (wet or dry surfaces), with colony growth (up to 1 log increase) possible on wet wood surfaces [
41,
104,
153]. Among the surfaces from which
C. auris has been isolated in healthcare facilities include mattresses, bedside tables, bed rails, chairs, windowsills, ventilators, thermometers, pulse oximeters, IV poles, and ECG leads [
21,
26,
63,
100,
132,
154].
C. auris colonization and survival can also spread to sinks, bathrooms, cleaning buckets, computers, phones, and doors with relative ease [
67]. In fact, an outbreak in Brazil during the height of the COVID-19 pandemic (March 2020) and increased infection prevention and control awareness was traced back to colonized axillary thermometers reused across patients without sufficient disinfection [
154].
In surface applications, the current chemical disinfection protocols range widely by site and sector and are impacted by the MICs associated with
C. auris isolates. Common disinfection wipes containing quaternary ammonium compounds (QACs) used for high-touch surfaces or in floor detergent mixtures are ineffective on the fungal strain, allowing transfer and spread of
C. auris colonies to other areas [
104,
155]. Only a few hospital-grade disinfectants are certified by the Environmental Protection Agency (EPA) to kill
C. auris (List P), and no registered products have been developed specifically for
C. auris [
104,
156]. Three of the most common hospital disinfectants (QACs, Iodine-based, and chlorine-based) have only minimal effectiveness unless strict protocols are followed, including follow-up with ethanol-based gel sanitizers after wet-to-dry cleaning. Disinfectants without sporicidal claims were not able to inactivate
C. auris [
155].
While chlorine-based products were considered the most effective for superficial disinfection, strict cleaning protocols must be followed [
26,
39,
51,
104,
133,
157]. Even slight deviation from the established protocol diminishes effectiveness tremendously, and use in patient care areas is limited due to the caustic chemical properties and respiratory irritation [
104,
157]. All high-touch items and multi-use equipment, personal protective equipment, and employee items require disinfection after every use; however, when these standards are not followed, nosocomial spread increases [
158]. Chemical-based cleaning agents also require accurate concentrations tailored to the specific surface for cleaning and cannot be used across varying surfaces with efficacy [
12]. In environments where strict adherence to complex cleaning protocols requires nuanced chemical concentrations and specific wet and dry times before effectiveness, the window for error widens. Despite adherence to standard infection prevention and control procedures, a significant, high-mortality
C. auris outbreak was identified in a European tertiary care hospital [
159]. Healthcare facilities facing growing patient boarding challenges, rapid turnover of fatigued and burned-out staff, and continual cost cuts can lead to seemingly innocuous shortcuts in environmental disinfection procedures. However, even small deviations amplify the nosocomial spread of
C. auris and contribute to AMR in patients who become colonized in healthcare facilities. Switching to single-patient-use materials is recommended where feasible [
111]; however, this is costly and not always practical in lower-resourced facilities where the reuse of even single-use equipment is common. Healthcare facilities overburdened with patients and have limited human and resource capacity often use less than ideal infection control procedures, leading to
C. auris outbreaks [
67,
158].
Further complicating surface colonization and resistance to disinfection is the proclivity for biofilm formation. Biofilms can form around the exterior of fungal (and other microbial) colonies on plastics, steel, poly-cotton, and other high-touch surfaces, including on dampness-prone skin niches (e.g., sweat in axillary regions) [
41,
160]. Biofilms may promote multi-drug resistance as the film protects
C. auris from hostile environments, including dehydration and common germicides [
41,
98,
104]. In healthcare settings, biofilm development contributes to the overall nosocomial spread and pathogenicity, particularly among medical equipment, including those with direct internal mucosal exposures (e.g., urinary and intravenous catheter tubing) [
41,
63,
161]. When biofilm formation is present, common hospital-grade disinfectants are even less effective. Ledwoch and Maillard (2018) tested 12 commercial wipes and hypochlorite disinfectants on
C. auris biofilms on stainless steel. They found that over 50% failed to decrease survival or transferability, and up to 75% failed even to delay regrowth [
162].
For skin colonization, common chlorhexidine-based soaps are not widely effective through standard hand washing procedures, which may be due to biofilm formation [
104,
134]. Additional steps using alcohol-based sanitizers are needed to reach maximal disinfection from the skin [
104]. When using alcohol-based sanitizers, at least one minute of wet contact time is necessary to achieve disinfection; however, the standard hand sanitation time for healthcare professionals is often under 15 seconds [
163]. Even using optimal conditions, chlorohexidine (0.5%-4.0%) and other common alcohol disinfectants often achieve less than a 3-log
10 reduction. There are no efficacy studies that demonstrate inactivation of
C. auris colonization on skin with only 15 seconds of contact time [
163]. No matter what chemical agent is used for disinfection, terminal cleaning should be completed at least twice daily in addition to per-patient disinfection practices [
12], which adds another layer of time and capacity complexity to the infection prevention protocols.
Other less common disinfecting agents have been used with varying effectiveness. Farnesol, which is a quorum-sensing molecule, has been used to inhibit biofilm formation, similar to its properties in
C. albicans, and has the capacity to reduce the expression of multi-drug resistance genes [
164]. Ozone disinfection units have also been used for beds and linens with some success; however, they require long exposure times [
51].
2.3.4. Waste Management
Regulated medical waste (RMW) or healthcare waste is not governed by a single regulating body, and therefore, waste management protocols vary widely by facility and region. Over time, common medical waste management strategies have included incineration, steam, microwave irradiation, mechanical, chemical, and pyrolysis [
165,
166]; however, disinfection and sterilization of healthcare waste have now become more common [
167]. Disinfection of surgical waste is paramount for disrupting and preventing the spread of infectious diseases, such as pathogenic MDR microbes. Disease outbreaks across several countries and facilities have been traced back to an organizational failure to comply with established guidelines for medical waste [
167]. Standard disinfection of medical waste using chemical disinfectants has serious challenges, including hazardous operating conditions and limited germicidal efficacy. Without proper disinfection of infectious waste, transmission across healthcare facilities and among waste workers and communities is likely [
168]. Another notable challenge in LMICs is that increased diagnostic testing has generated a rise in healthcare waste that was not accounted for during implementation planning. Many of the facilities and even governments of LMICs were not prepared to be held accountable for adequate waste management strategies, strongly calling for strategies that required lower resources, costs, and technical expertise [
169].
Fundamental principles for the appropriate management of hazardous waste to safeguard public health and environmental protection have been established through several international agreements, including [
170]:
The Basel Convention on the Control of Transboundary Movements of Hazardous Waste and Their Disposal minimizes the generation of hazardous wastes, the treatment of waste close to where it was generated, and the transboundary movement of hazardous waste.
The Bamako Convention is a treaty with well over a dozen signatories that bans the importation of hazardous wastes into Africa.
Polluter Pays Principle - the producer of waste is legally and financially liable for disposing of waste in a manner safe for people and the environment.
Precautionary Principle - When risk is uncertain, it must be regarded as significant.
Proximity Principle - Hazardous waste must be treated and disposed of as close as possible to where it was produced.
As improved healthcare access and technological advancements have grown exponentially, the need for enhanced waste management has also risen. Infectious disease outbreaks have driven the need for field testing of medical waste to identify and disrupt pathogen spread. Field indicators have been used in West African nations (Liberia & Guinea) to improve the efficacy of chlorine-based disinfection against Ebola, while modern technology (e.g., genome sequencing of contaminants for targeted disinfection) continues to be applied for contemporary standards [
171]. Some facilities in Uganda and India have even designed smart waste bins to assist in the proper segregation and disinfection of infectious waste [
172,
173].
Incomplete disinfection and variation in medical waste management procedures can significantly contribute to the spread of infectious diseases. Within the context of national healthcare systems, active governmental intervention can help establish and operationalize a successful and sustainable healthcare waste management system [
42]. Components of the development of effective and safe healthcare waste management systems must include 1) healthcare waste management planning at the national level and at healthcare facilities, 2) waste minimization, reuse, and recycling protocols, 3) waste segregation, 4) safe storage, and transport, and 5) treatment and effective disposal of waste [
42]. While these strategies are not specific to the disinfection of
C. auris, a broad application of microbial disinfection of healthcare waste should be applied to serve in multi-layered protection procedures.
2.4. UVC LED Disinfection of C. auris in Healthcare Settings
UVC technology is rapidly advancing, as are the many use cases, with many potential opportunities in healthcare settings. The evidence confirms that UVC can inactivate up to 99.99% of microbial pathogens, including highly resilient
C. auris. Conventional UVC disinfection uses low-pressure mercury lamps emitting light at the wavelength of 253.7 nm; however, the energy dose required to kill or inactivate resilient pathogens (e.g.,
C. auris) often rises quickly beyond the safe exposure levels for humans [
40]. The recent emergence of UVC light-emitting diodes (LEDs) as an alternative to mercury lamps is a significant advancement. UVC LEDs can potentially provide as good or better disinfection as traditional mercury lamps but have other advantages beyond efficacy ratings [
174]. Conventional mercury lamps (254 nm peak wavelength) can have high energy demands, short lifespans, cumbersome sizes, and pose potential human exposure hazards [
31,
175]. UVC LEDs can emit light at multiple wavelengths between 250 – 280 nm and have lower energy and voltage requirements, smaller sizes, and optics that can be tailored to microbial disinfection [
31,
176,
177].
C. auris is susceptible to UVC inactivation; however, longer exposure times and higher doses of UVC energy are required when compared to other common
Candida species (
C. auris k-values 0.108 to 0.176 cm
2/mJ vs.
C. albicans k-values 0.239 and 0.292 cm
2/mJ). In general, lower k-values were found for isolates expressing AMR properties, indicating a higher dose of UVC is necessary for the inactivation of AMR
C. auris [
53]. Studies have determined the optimal wavelength dose for inactivating
C. auris. As shown in
Table 3, Mariita et al. (2022) found that a peak wavelength sensitivity of 267–270 nm offered higher disinfection performance against multidrug-resistant
C. auris. Giese and Darby (2000) also noted that wavelength sensitivities of 267 and 270 nm showed a similar effect, with the fastest inactivation rate at the average log reduction value (LRV) of 0.13 LRV/mJ
−1/cm
2. A linear regression analysis revealed a significant association between all arrays and their disinfection efficacy at 5, 10, 20, and 40 mJ/cm
−2 while emphasizing the effectiveness of UVC emission wavelengths of 267–270 nm [
54].
The applications for UVC disinfection in healthcare settings are broad. UVC LED technology can be effectively and easily integrated into protocols for water, air, surface, and waste disinfection. In fact, a systematic review of UVC germicidal inactivation found that UVC had a potent effect on microorganisms, including those with AMR, when used as an adjunct to manual chemical cleaning procedures [
179]. UVC disinfection in water and water distribution systems at healthcare sites can ensure that pathogens, like
C. auris, are not recycled within closed systems and can prevent transmission to water treatment facilities where wide distribution would be possible. UVC LED water reactors at treatment facilities can prevent the transmission of pathogens to community and household water sources. In air applications, UVC LED technology can be applied to air handling units and circulated air systems to enhance the purification of recirculated air. Pathogens can colonize inside air ducts, become dislodged, and then spread through circulated air and deposited on surfaces where skin contact can escalate. UVC disinfection of in-duct systems will purify and disinfect forced air, while in-room units will disinfect the circulating air between two UV sources. In surface application, mobile UVC units may provide terminal cleaning enhancements for all surfaces (particularly high-touch surfaces). Mobile units may be autonomous robots or personnel-monitored units but can disinfect manually pre-cleaned areas with a high degree of efficacy.
2.4.1. Water
UVC LED technology is a proven method for disinfecting water and wastewater, which are primary transmission sources of many gastrointestinal pathogens. Several studies have demonstrated the efficacy of UVC LED technology at inactivating microbial pathogens in water and wastewater, though few studies have specifically evaluated
C. auris due to its rapid transmission via surface contact first [
35,
141,
144,
180,
181,
182,
183,
184,
185,
186,
187,
188].
Research has discovered that UVC technology has applications in disinfecting drinking water [
181,
182,
189], rainwater disinfection [
190], and food processing water management [
191]. UVC LED technology in water reactors can inactivate biofilm-bound
Pseudomonas aeruginosa (265nm at UV dose 8 mJ/cm
2) to a 1.3 ± 0.2 log inactivation or LRV [
180], pathogenic bacteria
Aeromonas salmonicida and
Escherichia coli (265nm at UV dose 24 mJ/cm
2 and 28 mJ/cm
2) to a 4.5 log reduction [
35],
Giardia sp. and
Cryptosporidium sp. [
183], and a wide range of other pathogenic microbes [
33,
34,
143,
192]. UVC LED disinfection also reduces the use of harmful chemicals and any associated risks while demonstrating efficacy at eliminating microorganisms, pharmaceuticals, and personal care product residue [
193].
In addition to the disinfection of water for consumption or daily use and solid waste management, UVC may also effectively disinfect wastewater, particularly in and around healthcare facilities. Researchers isolated
C. auris from wastewater, demonstrating an epidemiologic link to healthcare facilities within that wastewater treatment plant’s sewer shed in Southern Nevada, further highlighting the importance of scalable and sustainable pathogen disinfection [
194]. Researchers have specifically found effective UVC disinfection for wastewater reuse [
142,
195,
196] and sewage decontamination [
197]. While chlorine-based disinfection has traditionally been used to disinfect wastewater, concerns about the impact have resulted in UVC disinfection as an alternative [
42]. UVC LED water reactors are effective at eliminating microorganisms in water and wastewater using peak wavelengths between 260-270 nm, with synergistic inactivation at 260|280 nm for
E. coli [
28,
48]. Human norovirus can be effectively inactivated in wastewater by UVC LED water reactors and scaled tertiary wastewater treatment facilities, which can integrate UVC LED Driven Advanced Oxidation Processes (AOP) to decontaminate and disinfect wastewater simultaneously [
184,
185]. Combined UVC LED and AOP disinfection of wastewater has also been effective at reducing medical contaminants. The combined UVC LED + H
2O
2 wastewater treatment system showed efficacy and efficiency for smaller-scale water treatment facilities [
198].
The effectiveness of UVC-based water disinfection depends on several important operating parameters. More opaque liquids reduce UV treatment effectiveness at different levels [
182,
199]. Whereas water circulation and exposure time improve water decontamination effectiveness [
200]. Interestingly, water volume causes a dubious effect on UVC treatment, with insignificant to slightly better performance in lower volumes [
189,
200]. Continuous or pulsed UVC light application provides comparable results [
143]. Depending on the intended final use of the treated water, the protocols must reach different decontamination targets. For drinking water, the treatment should disinfect the surface with at least a 4-log reduction [
182]. The reuse of wastewater, excreta, and greywater, on the other hand, requires at least a 3-log reduction for water disinfection [
182]. Combined UV wavelengths or combining UV treatments with other methods may also have additive effects. Several studies have demonstrated that combining UV wavelengths [
144,
187,
195,
201,
202] and multi-method treatment applications [
142,
196,
203] have demonstrated synergistic effects that have improved the disinfection or decontamination process. Combining UVA and UVC [
144,
187,
202] or UVB and UVC [
186], UVC with other light sources such as excimer lamps [
203], or UVC with chemical oxidants [
196] all lead to synergistic effects.
A systematic review of the literature found that all examined studies achieved some level of decontamination or disinfection. Most studies achieved biological reductions of less than 3 log [
144,
181,
186,
187,
201,
202,
204], and three studies achieved sterilization levels up to 5-6 log reduction [
182,
191,
199]. These results give users confidence in applying UVC for its decontamination capabilities in the water. While UVC light can inactivate microbial pathogens, some pathogens can recover from the UVC decontamination effect. These microorganisms use dark repair and photoreactivation processes to recover from the UVC impact [
146,
186,
195,
205].
2.4.2. Air
UVC LED disinfection is effective at inactivating aerosolized viruses, bacteria, and fungi [
145,
146,
147]. There is variation in how UVC technology is used to purify air of microbial contaminants, ranging from sanitizing the air circulating between UVC sources to disinfecting the surfaces of air handling units, filters, and fans to prevent re-circulation of spores [
40,
206,
207,
208]. Some studies of UVC LED air treatment have demonstrated disinfection of some, but not all, biological indicators [
145,
146,
147], and one study achieved sterilization in part of its biological targets [
148].
Forced air handling systems can incorporate UVC disinfection technology into traditional filters and interior surfaces of the fans and system (in-duct systems) to further purify the air that is dispersed into the environment. While
C. auris is not part of the standard air pathogen testing array, adding UV disinfection to filters increases the log reduction of many harmful pathogens, including
C. albicans [
208]. The log reduction of airborne pathogens is dependent on the highly variable UV dose and standard operating parameters of each in-duct UV disinfection product [
209]. Air purification systems with combined HEPA + UV disinfection technology have been found effective at sanitizing rooms up to 12 m
2 in area [
206]. Stand-alone UV recirculation units (or unitary UV systems) are another common application of UV air disinfection that works similarly to in-duct systems but are more compact and operate at variable air flows. These units draw air from the floor or near other high-concentration areas and then redistribute the cleaned air at breathing height [
208]. A 2-log reduction of
Candida spores was achieved in a 2-point circulating air sanitation cycle; however, when combining surface and air UVC disinfection in a single room, the required dose is unknown [
40]. Mobile air disinfection units have shown effectiveness at eliminating
C. auris from recycled air in healthcare settings using combination systems that disperse ozone into a room before sanitizing the recombinant air with UVC light [
40]. The Khan-Mariita Equivalent Ventilation Model (KM Model) supplements standard mechanical ventilation with UVC air treatment, accounting for many of the previously noted variables (e.g., room size, occupancy, existing ventilation, and targeted air changes per hour) [
207]. In healthcare settings (as with many other environments), it is challenging to circulate only fresh air. Therefore, recirculation of potentially contaminated air is necessary. Utilization of the enhanced KM model that integrates UVC disinfection into standard mechanical ventilation allows for increased energy efficiency, net carbon-zero requirements, and decreased dependency on outside air injection. Broad application of the principle of UVC disinfection of
C. auris on surface and circulated air shines a spotlight on the potential for building systems to integrate the KM Model as a standard ventilation system, in addition to other disinfection protocols [
207].
Muramoto et al. (2021) developed an air purifier that combines UVA/UVC LEDs, a HEPA filter, a honeycomb ceramics filter, and a pre-filter. In this system, the LEDs are responsible for treating the surface of the HEPA filter to decontaminate any microorganisms trapped in it, leading to the faster elimination of floating influenza viruses [
210]. Researchers are also particularly interested in demonstrating how compact systems that are easy to implement in different settings are applicable to UVC LED disinfection [
146,
147,
148,
211]. Nicolau et al. (2022b) found that UVC possessed higher decontamination efficacy than ozone and was capable of achieving synergistic effects when combined with other methods.
2.4.3. Surface
Researchers have also determined UVC decontamination effectiveness on different materials, including various hard surfaces (e.g., carpet or laminate) [
212], food contact surfaces [
213], and recreational ball types [
214]. In addition to the decontamination effectiveness, Wood et al. (2021) evaluated the relative humidity impact on the decontamination effectiveness and compared conventional UVC sources with LEDs. Trivellin et al. (2021) found that UVC light did not cause any visual changes or material degradation following disinfection. A synergistic effect from the combination of UVC treatment with mild temperatures (60 ◦C) was also found [
145].
UVC disinfection is also specifically effective at inactivating
C. auris on hard surfaces, which is common in healthcare settings [
49,
51,
52,
53,
55,
56,
57,
215]. The use of UVC technology as an adjuvant disinfection modality for
C. auris may be an advantageous environmental mitigation strategy [
26]. While there is no standard log reduction requirement for
C. auris, at least a 3-log
10 reduction (99.9% reduction) is suggested to most likely be clinically effective [
57]. A review of several UVC exposure parameters and devices found that UVC was clinically effective against
C. auris when using proper conditions [
56]. In lab testing, a mobile UVC tower equipped with high-performance bulbs at the 254 nm wavelength used for a continuous 7-minute exposure period in a patient-room-sized test chamber demonstrated 99.97% inactivation of
C. auris [
52]. Other lab settings have found UVC technology to be an effective approach to inactivating
C. auris as well [
49,
50,
51,
55]. Maslo et al. (2019) saw a 99.6% reduction after a 10-minute pulsed-xenon UV light exposure cycle at a 2-meter distance from their mobile UV device and 100% elimination after more than 15 minutes of exposure. Chatterjee et al. (2020) reported a 0.8 to 1.19 log reduction of
C. auris when exposed for 30 minutes. However, despite the increased exposure time, isolates from clade III were not susceptible to inactivation in their lab testing [
49]. In 2020, an experimental test at the University of Siena found a 4.43 log reduction of
C. auris after 15 minutes of exposure to a novel UVC chip [
50]. In a modification of the American Society for Testing and Materials (ASTM), six relatively low-cost (<
$15,000 per unit) UVC devices (3 room decontamination devices and 3 UVC box devices) were tested against the suggested clinically effective 3-log
10 reduction of
C. auris. Three of the tested units (one room decontamination and two enclosed boxes) met all criteria for effective decontamination [
216]. In a lab test, UVC exposure (267 – 270 nm) prevented
C. auris biofilm growth on stainless steel and plastic and significantly reduced formation on poly-cotton fabrics [
54]. The field studies branch of the Respiratory Health Division at NIOSH) found UV disinfection of
C. auris was 99.9% effective but required significantly higher UV energy dose than other
Candida species (
C. auris 103–192 mJ/cm
2 vs.
C. albicans 78-80 mJ/cm
2) [
217].
Some studies have reported that
C. auris is resistant to UVC treatment [
218]; however, this seems to be a misnomer. More recent research has determined specific UVC parameters necessary for inactivation, aligning with the fungus’ propensity to respond differently to other common disinfectants and antimicrobial therapies due in part to the higher MIC and rapidly developing AMR [
219]. While UVC inactivation efficacy is reduced when used outside of the recommended parameters, proper use as an adjuvant to other disinfection protocols is advantageous [
51]. Time and exposure parameters, such as irradiance and fluence rate, in addition to clade and strain-specific parameters, are critical variables when determining the most efficacious disinfection protocol. Findings seem to suggest that efficacy is inversely proportional to the distance from the UVC source [
219]; however, some studies achieved inactivation at shorter distances, although later experienced regrowth [
39]. The UV-360 Room Sanitiser (UltraViolet Devices, Inc. Valencia, CA), using four vertical UV lamps (254 nm) in a 360-degree motion sensor cycle, produced maximal inactivation after 30-minute exposure cycles; however, they noted that Japan/Korean strains were most susceptible compared to Venezuela, Spain, and India strains [
219]. Whole room decontamination devices are capable of a 4.57 log reduction in
C. auris when in the direct line of sight but had slightly reduced efficacy (3.96) when only achieving indirect exposure [
57].
2.4.4. Waste Management
Disinfection or reduction of disease-causing microorganisms in medical waste to minimize disease transmission is imperative (WHO, 2014). Since completely destroying all microorganisms is challenging, sterilizing medical and surgical instruments is generally expressed as a 6 log
10 reduction (a 99.9999% reduction) or greater of a specified microorganism [
42]. In addition to thermal, chemical, biological, and mechanical waste-treatment technologies already discussed, UV has also been identified as an effective synergistic waste-treatment modality. Smart waste bins may include UV light for additive disinfection of the interior walls and air circulating inside closed waste units, rendering waste safer to transport within and outside of healthcare facilities [
172]. Other UV waste management strategies include disinfection of waste prior to disposal, such as with mobile boxes designed to disinfect used N-95 masks during the COVID-19 pandemic [
220]. Novel applications for small, mobile waste bins with integrated UVC LED disinfection are also emerging to address contamination before large-scale storage [
221]. While UV has been used to destroy airborne and surface pathogens as a supplement to other technologies, it may be limited in its ability to penetrate closed waste bags. The effectiveness of UVC is likely to depend on the processes and protocols in place, allowing a direct line of sight to the discarded waste [
42].
2.4.5. UVC LED Disinfection Critical Factors
Effective UVC LED disinfection of C. auris and other AMR pathogens relies on several critical factors across applications. Pathogen inactivation in water applications is highly dependent on the turbidity of the water and UV transmittance (or wavelength). Air temperature and humidity are critical factors in UV disinfection of AHUs. Air velocity inside units is also an important factor, as higher velocities result in cooled air. Many critical factors impact surface UV disinfection, including the material, topography, and reflectiveness of the material to be treated. The critical factors associated with surface disinfection are also important considerations in air applications, as the interior surfaces of AHUs are necessary. UVC disinfection in waste management is beholden to many of these critical factors depending on the specific waste and container to be treated