Sporothrix brasiliensis Atypical Dimorphism in Tissue.

Sporotrichosis, caused by the thermodimorphic fungus Sporothrix brasiliensis, is an emerging zoonotic infection in Brazil and other Latin American countries. Typically, the parasitic form in host tissue is yeast; however, we report three cases in immunocompromised patients exhibiting simultaneous yeast and mycelial structures in biopsies. Identification of S. brasiliensis was confirmed through calmodulin gene sequencing, with phylogenetic analysis supporting species-level classification. Direct examination and histopathology revealed both budding yeast cells (3-8 μm) and hyphae, a rare morphological phenomenon previously unreported for this species in humans. This atypical dimorphism may be influenced by local tissue conditions, such as oxygen exposure and lower temperatures, and has significant diagnostic implications. Recognition of polymorphic forms is essential for pathologists and mycologists, highlighting the evolving histopathological and diagnostic challenges in sporotrichosis.