Cutaneous Protothecosis: Reported Human Cases in Vietnam and Diagnostic Challenges.

Protothecosis, a rare opportunistic infection caused by Prototheca species, achlorophyllous microalgae, is increasingly recognized but remains underdiagnosed due to its rarity and nonspecific clinical presentation. We report two cases of human protothecosis in Vietnam, occurring in a 63-year-old and 81-year-old female presenting with chronic, painful, ulcerative lesions on upper extremities. Despite initial misdiagnosis and empirical antifungal and antibacterial treatments, histopathological examination revealed characteristic Prototheca sporangia, leading to the definitive diagnosis. These cases underscore the importance of considering protothecosis in atypical cutaneous infections, particularly when patients fail to respond to empirical antibacterial or antifungal therapies, and highlight diagnostic and treatment challenges.