A strain of ringworm that resists the standard antifungal pill has now been identified in 29 countries, according to a study published in Emerging Infectious Diseases, a journal of the Centers for Disease Control and Prevention. Laboratory identifications of the fungus tripled over the period examined.
Ringworm is one of the most common skin problems in medicine. It causes the itchy ring-shaped rash familiar to anyone who has had athlete's foot or jock itch, and it usually clears with a cream or a short course of pills. That is why this strain matters. It looks like the common thing and does not behave like it.
For a household, the practical version of this story is narrow. Nobody needs to change how they do laundry this week. But a rash treated as ringworm that has not improved after weeks of standard treatment is now a reason to go back to a clinician rather than buy another tube of cream.
A Global Map Assembled from Laboratory Submissions
The study was led by researchers based at Pitié-Salpêtrière University Hospital in Paris, working with colleagues in Europe and North America. It was published in a CDC journal, but it was not a CDC investigation, a point worth keeping straight.
The team used the Mass Spectrometry Identification project, a free shared database hosted by the hospital's parasitology and mycology laboratory that lets diagnostic laboratories submit fungal samples for identification against a reference library covering roughly 1,700 species. The researchers examined submissions from more than 500 users across 54 countries that tested positive for fungi between 2022 and 2025. Trichophyton indotineae turned up in 29 countries, including the United States, and its share of all Trichophyton identifications in the database rose from 1.2 percent to 3.6 percent. The authors wrote that their results reveal "a marked global increase in T. indotineae isolation."
Two limitations shape how far those numbers can be pushed, as the published study abstract makes clear. This is laboratory surveillance, not population surveillance, so it counts samples that reached a participating lab rather than infections that occurred. And detections rise when testing capacity rises. The database was upgraded in 2022 to distinguish this species reliably, so part of the increase reflects better tools rather than more disease.
A Common Rash That Stops Responding to the Standard Pill
Trichophyton indotineae was reclassified as its own species in 2020, having previously been grouped with related fungi. Its defining feature is genetic mutations that confer resistance to terbinafine, the first-line oral antifungal for skin infections.
Infections caused by it tend to be more severe than typical ringworm. According to CDC's clinician brief on emerging ringworm, the rash often covers large areas of the body and is difficult to treat. It spreads the same way ordinary ringworm does, through skin-to-skin contact and through shared towels, bedding, clothing, and hairbrushes.
Diagnosis is the bottleneck. Culture-based techniques used by most clinical laboratories typically misidentify this species as Trichophyton mentagrophytes or Trichophyton interdigitale, and genomic sequencing is needed to tell them apart. Patients are often treated for weeks or months on the assumption that a standard antifungal will work before anyone orders testing that would show otherwise.
CDC lists several factors that may be driving resistance generally, including topical antifungal and corticosteroid combination products, inappropriate antifungal prescribing, misuse of over-the-counter antifungals, and patients not completing prescribed courses.
North America Is a Thin Slice of the Data
The geographic breakdown deserves scrutiny before anyone reads a global map as a risk map. The large majority of identifications came from European laboratories, and North American users made up a small fraction of the network.
That does not mean the fungus is a European problem. It means Europe is where the testing is. Regions with widespread infection and limited access to mass spectrometry, including parts of Africa and South and Southeast Asia, are almost certainly undercounted. The strain is widespread in South Asia, and clinicians are advised to consider a patient's travel history when a severe or treatment-resistant rash appears.
In the United States, the first two confirmed cases were identified in New York City in early 2023 after standard testing initially pointed to a different species. CDC has published an account of how the first US cases were confirmed using advanced molecular detection. The surveillance picture here remains genuinely thin. CDC's clinician brief states that antifungal susceptibility testing for dermatophytes is not widely available and that the United States does not require reporting. Cases are reported voluntarily to an emerging diseases registry maintained by the American Academy of Dermatology.
That means no reliable US case count exists, and any figure presented as one should be treated with caution. Detections continue to be reported in new countries, including a first detection reported in Brazil.
Rashes That Are Not Clearing and When to Ask About Testing
Most ringworm is still ordinary ringworm, and most of it still responds to standard treatment. The signal to watch for is not the rash itself but its failure to improve.
Reasonable prompts for a return visit include a rash that has not improved after several weeks of prescribed or over-the-counter antifungal treatment, one spreading across large areas of the trunk, limbs, or groin, one that keeps recurring after treatment ends, or a persistent rash following travel to South Asia. Patients in those situations can ask whether fungal culture or susceptibility testing is appropriate, and clinicians are advised to refer widespread rashes to a dermatologist.
Two cautions are worth naming. Steroid creams can make dermatophyte infections worse and are commonly applied to an itchy rash before a diagnosis is confirmed. And treatment courses for resistant infections can run weeks to months, so stopping early because the rash looks better is one of the ways resistance is thought to develop.
Household prevention is unglamorous and effective. Do not share towels, bedding, razors, or hairbrushes with someone who has an active rash, wash and dry shared linens thoroughly, and keep affected skin covered where practical.
Clinicians who need help can contact their state or local health department, which can reach CDC's fungal diseases team for testing assistance, and the American Academy of Dermatology maintains diagnostic and treatment resources developed with CDC input, along with the registry for reporting suspected resistant cases.
What comes next depends largely on laboratory capacity. Until reporting requirements or testing access change, the honest position is that nobody knows how common resistant ringworm is in the United States, and the most useful thing a patient can do is refuse to accept a rash that will not clear as normal.
Key Questions Answered
What is Trichophyton indotineae? A species of fungus that causes ringworm and frequently carries mutations making it resistant to terbinafine, the standard oral antifungal. It was recognized as a distinct species in 2020.
Where has it been found? Laboratory identifications came from 29 countries in this study, with the large majority of samples from Europe, where testing capacity is concentrated. It is widespread in South Asia and has been confirmed in the United States.
Is this a new outbreak? No. It is an expanding surveillance picture for a pathogen documented over the past decade. Rising detections partly reflect improved laboratory identification tools introduced in 2022.
How does it spread? Through skin-to-skin contact and through contaminated items including towels, bedding, clothing, and hairbrushes. It spreads the same way common ringworm does.
When should someone see a doctor about a rash? When a suspected ringworm rash has not improved after several weeks of standard treatment, covers large areas, keeps returning, or follows travel to a region where the strain is common.
Can it still be treated? Yes. Resistant infections generally require itraconazole rather than terbinafine, sometimes for up to 12 weeks. Treatment decisions belong to a clinician, and courses should not be stopped early.
How many US cases are there? No one knows. Resistant ringworm is not a reportable condition nationally, and susceptibility testing is not widely available, so official national case counts do not exist.
