Diagnostic difficulty identifying Apophysomyces trapeziformis septic arthritis in a patient with multiple myeloma

INTRODUCTION: Mucormycosis is a rare fungal infection, but can cause substantial morbidity and mortality in both immunocompromised and immunocompetent patients. Apophysomyces is a mucormycetes species ubiquitous in nature, particularly in soil, decaying wood and other organic matter. Apophysomyces is known to cause cutaneous fungal infections, particularly after penetrating trauma. Septic arthritis is a rare clinical manifestation. CASE PRESENTATION: We describe a case of Apophysomyces trapeziformis causing septic arthritis of the knee of a patient with multiple myeloma. He was treated multiple times for bacterial septic arthritis with minimal improvement. Surgical tissue specimens finally grew mucoraceous mould, and DNA sequencing and morphological assessment of spores identified the mould as A. trapeziformis. The patient was treated with amphotericin B and posaconazole, but ultimately required an above-the-knee amputation for definitive treatment. CONCLUSION: This case illustrates the need to evaluate for fungal infection in a persistent septic arthritis that is culture negative and refractory to empiric antibiotics, particularly in an immunocompromised individual. It also shows the importance of a thorough social history and adequate tissue specimens for culture.