Protothecosis is a rare infection caused by achlorophyllic algae that are members of the genus Prototheca. They are ubiquitous in nature in organic material. The clinical manifestations can be acute or chronic and local or disseminated. The disease is classified as cutaneous, causing bursitis or disseminated/systemic, affecting both immunocompetent and immunosuppressed patients, with more severe and disseminated infections occurring in immunocompromised individuals. Prototheca wickerhamii and Prototheca zopfii are the most frequent organisms reported in humans. Diagnosis is made by observing asexual sporangia (thecas) on histopathological examination of tissue. Medical and surgical treatment should be considered. Ketoconazole, itraconazole, fluconazole, voriconazole, posaconazole, and amphotericin B are the most commonly used antifungals. Voriconazole and amphotericin B are highly effective against Prototheca spp. Treatment failure is not uncommon because of the comorbidities that limit the therapeutic outcome.

We report the case of a Mexican lupus patient who developed proximal white subungual onychomycosis on multiple toenails. Microsporum canis was the causative agent. Both nail invasion by M. canis and proximal white subungual onychomycosis are rare conditions indicative of an underlying immunodeficiency.

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