Fungal Bone and Joint Infections

Fungal osteoarticular infection (FOI) is uncommon, but its incidence is increasing due to the growing number of patients at risk. It affects both immunocompetent and immunocompromised patients. The most common etiological agents are Candida and Aspergillus species. Vertebrae are the most frequently involved bones. Biopsy of the involved bone is often required to make a definitive diagnosis of FOI. Histological examination is a key tool for the diagnosis of FOI by establishing fungal invasion of tissues and vessels as well as the host reaction to the fungus. The tissue reaction varies with the fungal agent, the immune status of the host, and the chronicity of the infection. Microscopically, FOIs display a large spectrum of lesions including (1) a nonspecific inflammatory response, (2) suppurative inflammation, (3) necrotizing inflammation, (4) pyogranulomatous inflammation, (5) granulomatous and necrotizing inflammation, and (6) inflammatory fibrosis. Although the morphological characteristics of fungi in tissue sections may be specific, ancillary techniques may increase the sensitivity of histological examination. Histochemical stains remain the most used ancillary tools for the histological diagnosis of FOI. Alternatively, several techniques (immunohistochemistry, in situ hybridization, and nucleic acid amplification) have been developed to identify specific agents in tissue sections.