Drug-resistant Trichophyton indotineae has now been identified in 29 countries. Its resistance to first-line terbinafine, resemblance to other dermatophytes, and expanding global transmission are creating new challenges for diagnosis, treatment, surveillance, and antifungal stewardship.
The CDC confirms that an emerging highly contagious skin fungus that causes ringworm has now been identified in 29 countries.1 The fungus, called Trichophyton indotineae, does not respond to terbinafine, a standard first-line oral medication used to treat superficial fungal infections. Transmitted through skin contact, or contact with infected material or pets, T indotineae infection manifests as erythematous, scaly, and itchy plaques that can cover large areas of skin around the groin, gluteal area, trunk, or face.2
T indotineae was first described in India in 2016 when dermatologists began reporting a spike in difficult-to-treat ringworm among patients. However, a retrospective examination of symptoms and the classical presentation of patients revealed that the first cases of T indotineae infections occurred in 2008. Initially, cases of terbinafine-resistant ringworm were linked to a strain of fungus in the T mentagrophytes complex.
In 2020, T indotineae was classified as its own separate entity.3 The first US case was reported in 2023. Now spread internationally to 29 countries, T indotineae presents a growing public health concern and a reminder that drug resistance is a global threat.
Three things stand out about T indotineae: its resistance to terbinafine, how difficult it is to distinguish it from other fungi, and its unstoppable march across the globe. Terbinafine failure in treating T indotineae is due to the fungus having acquired genetic mutations in its squalene epoxidase (SQLE) gene. Terbinafine acts primarily by inhibiting squalene epoxidase, an enzyme necessary for the synthesis of fungal cell membranes. With the SQLE gene mutated, terbinafine can no longer bind to its original binding sites, which makes it ineffective.
T indotineae cannot be diagnosed clinically using signs and symptoms alone. That is to say: a doctor cannot look at a lesion and say “This is T indotineae” as attempting to do so may lead to a misdiagnosis that results in treatment failure, prolonged patient distress, continued transmission, and unnecessary exposure to drugs that may not even work.4
This dilemma also presents itself in the laboratory. Because T indotineae closely resembles other fungi such as T interdigitale and T mentagrophytes, regular lab tests may be unable to distinguish it. More advanced and effective tests, like DNA sequencing of the internal transcribed spacer, are often needed. DNA sequencing is, however, too expensive, labor-intensive, and too slow for routine infection control or clinical practice.
A quicker and more effective option for identifying T indotineae, which proves suitable for infection control and clinical practice, is matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry.
MALDI-TOF relies on how fungal proteins absorb, emit, or reflect light in their own unique way. It uses the mass spectrometry identification (MSI) application, a tool developed in France and available for free. This tool contains a large database of 17,563 spectra corresponding to 1,700 fungal species. It enables the identification of rare species, such as T indotineae.
Following a major upgrade in March 2022, MSI now has greater than 95% sensitivity in identifying T indotineae and correctly differentiating it from T interdigitale and T mentagrophytes.5,6 T indotineae, and these other fungi are differentiable based on 2 specific mass peak pairs. Because the application is free and available online, it enables a real-time international network of laboratories across 27 countries.
Work has been done to increase the special database. As a result, there has been a 3-fold increase in T indotineae identifications in the past 3 years, according to the CDC. Identifications rose from 237 spectra in March 2022 to 1,692 in December 2025. In a study
Around 89.4% of total T indotineae detections
T indotineae is spread through international travel and migration, and local community transmission amongst non-travelers. As people migrate internationally, T indotineae is brought from endemic regions to new countries. The introduction of T indotineae into China has now been well confirmed.7 In Germany, many infections have been confirmed in patients without a relevant travel history, which points towards transmission between locals. 8
It may also be spread through sexual contact and contact with domestic animal vectors. T indotineae has been found in dogs in India, Iran, and Egypt. Sexual transmission has been documented in France, leading researchers to believe that sexual transmission may require more attention than it is currently given.9, 10
Curbing the spread of T indotineae worldwide will involve individuals directly as well as health authorities. There is a need for better surveillance, clinical monitoring, and travel monitoring. There is also a need to change the way antifungals are used indiscriminately in certain countries.
When an infection with T indotineae is confirmed, clinicians should discontinue terbinafine and consider second-line options, such as itraconazole. Clinicians should note that “ringworm lesions that are progressive upon referral or appear to worsen even when treatment is being applied effectively, might be suffering from antifungal resistance. Physicians should consider this possibility if a case appears suspicious,” says
And should there be a recurrence, patients must be counseled to take medications as prescribed and to prevent the spread through proper personal hygiene.
The public must also be educated about this new fungus and told how to protect themselves, such as practicing good personal hygiene, limiting body contact with strangers or people who present with skin lesions, and seeking medical evaluation if they have a ringworm that won't go away.
“If you or your child has ringworm and their primary treatment does not work, speak with your doctor. A second-line therapy may be needed,” says
T indotineae can affect anyone, even people with an intact immune system. And it may be worse in people who are immunocompromised.
Health authorities and facilities must use real-time surveillance tools, such as MSI, to identify infections and track drug resistance, thereby helping prevent outbreaks. The public must change its attitude toward the careless use of antifungals. But more importantly, this is a call for greater pharmacovigilance and antimicrobial resistance stewardship for antifungals and beyond.
