A Case of Tinea Nigra Caused by Annulohypoxylon sp. between Toes and on Toenail

Abstract

We report the case of tinea nigra caused by Annulohypoxylon sp. between the right 2nd, 3rd, 4th, and 5th toes and on the right 4th toenail of a 93-year-old man. Clinically, it was a non-inflamed, linear, patchy, black-brown pigmented spot that was more black-toned than those in the reported cases. At the time of recurrence of the nail plate lesion, a pale brown linear pigmented spot with a mosaic-like appearance was observed via dermoscopy. Microscopic examination from interdigit and nail plate revealed numerous black-brown hyphae. In the culture of the scales, the colonies turned from black to white villi at each passage. In the slide culture, only hyphae grew ; the spores and conidia did not grow. Hortaea werneckii and Venezuelan endemic Stenella araguata were ruled out. Genetic analyses of the ITS-D1/D2 region from the culture samples led to an identification of Annulohypoxylon sp. There have been no cases of infection in humans with plant rot fungus species. The possibility of contamination was ruled out because of obvious clinical features of tinea nigra, and only Annulohypoxylon sp. could be isolated and cultured. We hypothesized that the infection was from the living environment. To the best our knowledge, this is the first case in the world of tinea nigra caused by Annulohypoxylon sp. Although an antifungal agent was applied topically, relapse was observed in both the toe web and the nail plate. We recommend that tinea nigra caused by this species should be regarded as an incurable disease.