Answer: Prosthetic knee septic arthritis due to Histoplasma capsulatum. The tuberculate macroconidia with thick-walled, radial, and finger-like projections are suggestive of Histoplasma capsulatum. Dimorphism was demonstrated at 37°C (yeast-like colonies) and at 30°C (floccose mold-like colonies). Sequencing of the internal transcribed spacer (ITS) region and comparison with known sequences in the NCBI BLAST database (http://blast.ncbi.nlm.nih.gov/Blast.cgi) gave a 99% identification of Histoplasma capsulatum.
Histoplasma capsulatum is a dimorphic fungus which grows as a multicellular mold form at 30°C in culture but as a budding single-celled yeast form at 37°C. Only sporadic cases have been reported in the South East Asian region (1), including Singapore, where our patient lived (2). Sepedonium is an environmental organism that can resemble H. capsulatum morphologically; however, it is not dimorphic (3).
The first case of prosthetic joint arthritis due to H. capsulatum reported involved an 84-year-old nursing home resident in North Carolina who presented with pain and a fluctuant mass at the hip joint 16 years after a total hip arthroplasty (4). She was placed on a lifelong course of oral itraconazole solution, with no recurrence of infection after 3 years of antifungal therapy. A second case report of H. capsulatum prosthetic joint infection involved a 77-year-old male with right knee pain and a draining sinus tract 13 years after his total knee arthroplasty. He underwent a two-stage procedure. Eight months later, he developed worsening knee pain. The knee aspirate grew H. capsulatum. The infected prosthesis was removed, and the patient was treated with itraconazole for 9 months before reimplantation (5).
Our patient was treated with oral itraconazole solution at 100 mg twice daily. Follow-up arthrocentesis culture at 4 months was negative. He underwent revision surgery and implantation of prosthesis and was being maintained on itraconazole for 8 months at the time of submission of the manuscript for this article, with no recurrence of symptoms.
See https://doi.org/10.1128/JCM.00207-18 in this issue for photo quiz case presentation.
ACKNOWLEDGMENT
We acknowledge the National University Hospital Department of Microbiology for performing the ITS sequencing.
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