Prototheca wickerhamii Causing Toe Infection Presenting Initially as Gouty Arthritis: A Rare Case Report.

Prototheca, a genus of opportunistic pathogenic microalgae, can cause protothecosis in humans and animals, manifesting as cutaneous lesions or disseminated/systemic infections. This report describes a rare case of Prototheca wickerhamii toe infection in a 78-year-old Chinese male, presenting initially as gouty arthritis. The patient, who worked in fish farming with frequent water exposure, had a history of herpes zoster and hypertension. For three years, he experienced recurrent episodes of erythema, swelling, and pain in the bilateral first metatarsophalangeal joints without identifiable triggers. Symptoms worsened over the past month, leading to hospital admission with suspected gout. Physical examination revealed a 1 cm ulcer on the right foot with a dark red granulation tissue base, extending 4 cm deep. The surrounding skin was erythematous, swollen, exudative, and markedly tender. Laboratory findings showed normal serum uric acid (354 μmol/L) but elevated neutrophil percentage (82.1%). Magnetic resonance imaging (MRI) suggested possible gouty arthritis in the right foot, but no urate crystal deposition was evident. Five days later, re-evaluation revealed significantly elevated interleukin levels (IL-6 >2500 pg/mL, IL-8 157.29 pg/mL), indicating a robust new inflammatory response, with no significant improvement in local symptoms. Concurrently, deep tissue sampling and microbiological investigations (including histopathology, blood agar culture, fluorescent staining, Matrix-Assisted Laser Desorption/Ionization - Time Of Flight (MALDI-TOF) mass spectrometry) and Internal Transcribed Spacer (ITS) sequencing confirmed P. wickerhamii infection. Antifungal therapy was switched to itraconazole, resulting in symptom improvement within five days, and the patient was discharged. This case highlights the variable, masquerading, and potentially severe clinical presentations of P. wickerhamii infection. It underscores the importance of heightened awareness for microbiological screening in cases of suspected "gout" with persistent deep ulcers, particularly in individuals with relevant occupational or environmental exposure histories, especially with normal serum uric acid. Deep tissue sampling for definitive diagnosis is crucial to avoid empiric corticosteroid therapy, which may exacerbate opportunistic infections.