Two severe cases of ringworm identified at a clinic in the US city of New York have reportedly failed to respond to a common antifungal treatment, according to a case study published by the Centers for Disease Control and Prevention.

It's unlikely the two cases are directly linked – with one patient developing the infection in the US in the summer of 2021 and the other in Bangladesh in the summer of 2022 – raising concerns over a potentially hidden outbreak of drug-resistant dermatitis.

Ringworm is caused by any one of several dozen species of fungus, and is not – as its name suggests – a worm. Also known as tinea or dermatophytosis, it threads its way through the skin or under the nails, irritating the tissues into producing red, itchy rashes.

As the infection widens it often leaves an area of healthy looking skin in its center, appearing as an inflamed ring of infection.

Though rarely fatal, ringworm infections aren't pleasant, and can develop into more painful lesions that risk secondary bacterial infections if untreated.

In the two New York cases, the culprit was initially mistaken for a ubiquitous form of ringworm called Trichophyton mentagrophytes, which is often picked up from pets like cats, dogs, and rabbits.

A close look at the infecting microbe's genes told a different story, revealing it was a relatively recently discovered species known as Trichophyton indotineae.

Though the two fungi are closely related, T. indotineae carries protective mutations in a gene for the enzyme targeted by a first-line antifungal medication called terbinafine. Taken in pill form, the drug is administered for a variety of infections caused by fungi that infect the skin and nails.

Resistance to terbinafine was first identified in strains of yet another species of tinea in India, back in 2014. In 2020, a study on similar infections in two patients uncovered the same resistance gene in the new tinea species, one that has since been retrospectively identified as having already spread as far as Europe as long back as 2011.

Yet these two cases are the first to be spotted in the US, sparking fears that it could be evidence of the strain's expansion into a new population.

The first patient, a 28-year-old pregnant woman, attended a clinic in the December of 2021 over concerns for her persistent, itchy rash that covered her neck, abdomen, pubic region, and buttocks. She had no other medical conditions, hadn't recently traveled overseas, and couldn't recall encountering anyone else with a tinea infection.

Following the birth of her child, the patient was put on a course of oral terbinafine, which was discontinued after two weeks due to seeming ineffective. The rash eventually cleared after four weeks of further treatment with a different antifungal medication.

The second patient was aged 47 and reportedly had no major medical conditions. She developed her rash while she was in Bangladesh visiting family, some of whom were similarly infected. After a four-week course of the medication failed to provide any relief, she was provided with an alternative treatment that eased symptoms.

Given the fact T. mentagrophytes is so widespread and looks virtually identical to the new drug-resistant strain, health authorities are eager to let clinics know they ought to keep an open mind in cases where ringworm looks particularly nasty or stubborn to treatment.

"Health care providers should consider T. indotineae infection in patients with widespread tinea, particularly when eruptions do not improve with first-line topical antifungal agents or oral terbinafine," the CDC advises in its report.

The centers also encourage the consideration of sound 'antimicrobial stewardship', a term that describes efforts to optimize the use of therapies that target agents of infection.

The rise of resistance in bacteria reducing the effectiveness of antibiotics has become a significant concern in medicine, yet the possibility of fungi sharing their own secrets to warding off pharmaceutical treatment is just as alarming. Maybe even more so, given the difficulty in finding novel antifungals in the first place.

However, T. indotineae looks set to be an inconvenience more than a catastrophe. Fortunately, options for the treatment seem to work, even if they aren't without their own challenges.

Yet the very existence of resistance factors that can be passed on to other species is a concern authorities can't afford to take lightly.

This brief was published in the CDC's Morbidity and Mortality Weekly Report.