Antifungal resistance is often discussed as though it were a single public health challenge: a fungal pathogen becomes less susceptible to one or more treatments, therapeutic options narrow, and clinicians need better drugs, better diagnostics, or better stewardship. But that framing can obscure an equally important question. How did resistance become a public-health problem in the first place?
Candida auris, azole-resistant Aspergillus fumigatus, and Trichophyton indotineae provide three very different answers. All are emerging fungal pathogens associated with substantial acquired antifungal resistance, but their resistance problems have developed through distinct ecological and epidemiological pathways. C. auris has become closely associated with transmission in healthcare settings. Resistant A. fumigatus can emerge during medical treatment, but resistance can also be selected outside the patient through environmental exposure to azole fungicides. T. indotineae, meanwhile, has been linked to patterns of antifungal use, including inappropriate use of corticosteroid-containing antifungal creams.1
These differences matter because the same endpoint, an infection that has become more difficult to treat, may require very different interventions upstream. Understanding antimicrobial resistance (AMR) therefore requires more than identifying the drug and the resistance mechanism. It also requires asking where selection occurs, how the resistant organism reaches people, whether transmission amplifies the problem, and where intervention has the best chance to interrupt that pathway.
Resistance Can Emerge Along Very Different Paths
Viewed as a sequence, the contrasts among these fungi become clearer. Resistance may first be selected within a treated patient, in an environmental reservoir, or through patterns of drug use in the community. The resistant organism must then reach people, and in some cases it can spread further from person to person or through contaminated environments. Each step creates a different potential point of intervention.
For C. auris, healthcare transmission is central. The organism can colonize people without causing symptoms, persist in healthcare environments, and spread through contaminated surfaces and equipment.2 Resistant A. fumigatus presents a different model. Resistance can develop during prolonged azole treatment, but people can also inhale an organism that acquired resistance in the environment before infection occurred.3 T. indotineae brings the problem into another setting altogether: contagious superficial fungal disease, with resistance concerns linked to antifungal exposure, prescribing practices, and misuse of topical therapies.1,4
The distinctions are not merely taxonomic. They determine where stewardship, surveillance, and prevention need to act.
Candida auris: When Resistant Organisms Spread Through Healthcare
C. auris illustrates how AMR can become intertwined with healthcare transmission. The organism can cause invasive disease, but infection represents only part of the public-health challenge. Patients may also carry C. auris on their bodies without symptoms, and colonized as well as infected individuals can shed the fungus into their surroundings. Contaminated healthcare surfaces and equipment can then contribute to transmission.
That ability to colonize patients changes the epidemiology of resistance. A patient does not necessarily need to develop resistance during antifungal therapy to become part of the resistant population. CDC has documented outbreaks involving resistant strains among people with overlapping healthcare exposures who had not previously received antifungal treatment, supporting transmission of organisms that were already resistant.
Persistence compounds the challenge. Colonization can continue for prolonged periods, while the organism can also persist in the healthcare environment. A resistant isolate therefore has implications beyond the treatment of a single infection. Once resistant C. auris is circulating, controlling AMR also means limiting the movement of those organisms among patients and through healthcare environments.
That makes infection prevention, environmental control, surveillance, and recognition of colonization integral to the response. Appropriate antifungal use still matters because additional resistance can emerge, but prescribing stewardship alone cannot stop the spread of a resistant lineage that is already moving through a healthcare system.
Aspergillus fumigatus: When Selection Happens Outside the Patient
Azole-resistant A. fumigatus moves the focus away from patient-to-patient transmission and toward another question: where is resistance being selected?
There are at least two relevant routes. Resistance can develop during prolonged azole treatment in a patient. But A. fumigatus also encounters azole compounds outside medicine. Agricultural azole fungicides are sufficiently similar to medical azoles that environmental exposure can select resistant fungal populations. A person may subsequently inhale an A. fumigatus isolate that was already resistant before it entered the body.3
This environmental route changes the logic of stewardship. In the conventional clinical model, reducing unnecessary antimicrobial exposure within patients is intended to reduce the selective pressure that favors resistant organisms. With A. fumigatus, part of that relevant pressure may operate elsewhere.
The molecular evidence helps connect those environments. Resistance genotypes, including TR34/L98H and TR46/Y121F/T289A, have been associated with environmental fungicide selection, and genomic evidence has supported an agricultural origin for at least some pan-azole-resistant isolates causing human infection.1,5
That does not mean every azole-resistant A. fumigatus infection begins in agriculture. Resistance can also emerge during medical therapy. The more important point is that clinically consequential resistance can have more than one origin, including one outside the healthcare system.
Antifungal stewardship in hospitals and clinics can address medical azole use, but it cannot by itself eliminate selection occurring in environmental reservoirs. Resistant A. fumigatus therefore makes the One Health dimension of fungal AMR particularly visible. Human treatment, environmental antifungal exposure, and fungal ecology can intersect before the patient ever reaches a hospital.
Trichophyton indotineae: When Drug Use Helps Reshape a Common Infection
T. indotineae highlights another route entirely. Instead of an invasive fungal infection associated with healthcare transmission or an environmental mold acquired through inhalation, it causes contagious superficial dermatophytosis. Its emergence shows how resistant fungal disease can become a public-health concern in ordinary community care as well as in hospitals.1,6
The organism is frequently associated with terbinafine resistance, commonly involving mutations in the squalene epoxidase gene, and cases have now been reported in more than 40 countries.
Patterns of antifungal use are central to the concern. CDC identifies inappropriate antifungal prescribing, misuse of over-the-counter topical antifungals, inadequate adherence, and use of topical antifungal–corticosteroid combination products among practices that may contribute to resistant ringworm.4 Underregulated use of high-potency corticosteroid-containing antifungal creams has been cited as a likely contributor to the emergence of T. indotineae.1
That association should not be reduced to the claim that one product category alone caused the problem. The evidence instead points toward a broader stewardship failure in which drug exposure, treatment practices, and community transmission can interact.
Diagnosis further complicates the response. Routine laboratory methods often cannot reliably distinguish T. indotineae from closely related dermatophytes, and advanced molecular methods are generally required for dependable identification.4,6 That diagnostic limitation complicates recognition and surveillance of T. indotineae.
T. indotineae therefore pushes antifungal stewardship beyond decisions about systemic therapy for severely ill patients. Community prescribing, access to topical treatments, drug combinations, adherence, diagnostic capacity, and recognition of treatment failure all become part of the resistance problem.
Why One Antifungal-Stewardship Strategy Cannot Solve All Three
The appropriate response in each case begins with the same question: where is the process generating or amplifying resistance actually occurring?
For C. auris, resistant organisms can spread through healthcare settings, making infection prevention, screening, environmental control, and surveillance essential.
For A. fumigatus, relevant selective pressure may extend beyond human medicine. Appropriate prescribing addresses one route to resistance, but environmental exposure to azole fungicides creates another.
For T. indotineae, the problem reaches into community antifungal use, where inappropriate prescribing, over-the-counter misuse, corticosteroid-containing combinations, adherence, and diagnostic limitations can all shape the conditions in which resistant disease emerges and persists.
A stewardship program aimed only at reducing total antifungal use would miss much of what distinguishes these threats. The relevant target may instead be transmission within a healthcare facility, selection in an environmental reservoir, or patterns of treatment and access in community care.
Antifungal stewardship therefore has to account for the epidemiology of resistance, not simply the volume of prescribing. Its goal is to reduce the pressures and practices that favor resistance while preserving effective treatment, and those pressures can operate in very different places.
Surveillance Has to Follow the Pathway Too
Surveillance faces the same problem. A system designed around the manifestation of resistance that mattered for one pathogen may miss the next one.
For C. auris, the blind spot is colonization. Surveillance focused only on symptomatic infection would overlook people who carry the organism without symptoms but can still participate in healthcare transmission.
For resistant A. fumigatus, the challenge is that clinically important resistance may reflect selection occurring either within patients or in the environment before infection. Clinical isolates therefore represent only one part of the resistance pathway.
For T. indotineae, surveillance can falter at an even earlier step: identifying the organism correctly. Routine laboratory methods often cannot reliably distinguish it from closely related dermatophytes, creating a basic obstacle to recognizing resistant disease and tracking its spread.
Detecting colonization, recognizing resistance selected outside the patient, and correctly identifying an emerging dermatophyte are fundamentally different tasks. Useful surveillance has to be built around the epidemiology of the organism it is trying to detect.
What the Next Resistant Fungus Might Teach Us
The comparison among C. auris, azole-resistant A. fumigatus, and T. indotineae matters partly because of how different they are. Their clinical manifestations, ecological niches, routes of acquisition, and resistance pathways vary substantially, yet each has emerged as a significant antifungal-resistance concern.
That diversity makes it difficult to assume that the next resistant fungal threat will resemble the last. Looking only for resistance emerging during treatment may miss selection occurring in the environment. Concentrating only on invasive infections may overlook resistant superficial disease spreading through communities. Focusing only on infected patients may underestimate organisms for which colonization contributes to transmission.
Current World Health Organization guidance accordingly treats fungal disease and antifungal resistance as problems requiring coordinated attention to surveillance, diagnostics, stewardship, research, treatment access, health systems, and environmental drivers.7
The larger lesson from these three fungi is that resistance is shaped not only by organisms and drugs but also by ecosystems, treatment practices, transmission routes, and human behavior. The pathway that produces and propagates resistance determines where control efforts have the best chance to interrupt it.
