Ringworm, also called tinea or dermatophytosis, is a common fungal infection of the hair, skin, or nails caused by dermatophyte fungi. Ringworm spreads easily by skin-to-skin contact with infected animals or people and by fomites (everyday objects or surfaces that may be contaminated). Usually, ringworm skin infections are minor ailments that can be treated successfully with topical antifungal medications or oral terbinafine, a first-line oral antifungal drug for ringworm, in cases of widespread infection.

However, ringworm caused by Trichophyton indotineae, a frequently antifungal-resistant fungus, can cause widespread, highly inflamed lesions on the face, trunks, arms, legs, or groin. Trichophyton indotineae infections generally do not improve when treated with most topical antifungals or terbinafine. Antifungal-resistant ringworm is difficult to treat and can affect anyone, including people with healthy immune systems. Outbreaks of antifungal-resistant ringworm are occurring in the Indian Subcontinent, and cases have been detected throughout Asia and Europe. 

Dermatologists and public health officials in New York City have reported cases of severe ringworm caused by Trichophyton indotineae and fungal nail infections resistant to terbinafine. In July 2023, an analysis from a reference laboratory testing dermatophyte isolates from several U.S. states and Canada found that 19% of the isolates were resistant to terbinafine. A study from France described severe ringworm cases potentially transmitted during sex among men who have sex with men. To date, there have been no similar published reports of ringworm transmitted through sexual contact in the United States. Additional data are needed to understand the extent of severe and antifungal-resistant ringworm in the United States.