16783 Prompt identification of primary cutaneous nocardiosis utilizing immunohistochemical staining: A case report

Abstract

A 72-year-old man with a history of renal transplantation (maintained on mycophenolate, tacrolimus, and prednisone) sustained a laceration to the right forearm after a fall onto gravel. Three weeks later, he reported onset of a painful, pustular rash of the right extensor arm. Before admission, he did not respond to outpatient clindamycin or ceftriaxone. HSV/VZV PCR swabs, blood cultures, and tissue fungal stain were negative. Preliminary hematoxylin and eosin evaluation showed extensive dermal neutrophilic infiltrate. Acid-fast bacilli (AFB) culture smear showed 2+ AFB with initial concern for a rapidly growing nontuberculosis mycobacterium. However, tissue gram and acid-fast stains showed rare, equivocal and weakly staining filamentous structures within dermal abscesses, which were highlighted and confirmed by immunohistochemical (IHC) staining. Consequently, the patient was diagnosed with primary cutaneous nocardiosis, and the antimicrobial regimen was altered, resulting in gradual clinical improvement on linezolid and amoxicillin-clavulanate. The presented case demonstrates the diagnostic challenge of distinguishing Nocardia from mycobacteria, as both grow on the same media, show similar colony morphology, and demonstrate acid-fast staining. Unfortunately, features of Nocardia, such as acid-fast staining and filamentous cell morphology, can be equivocal. We utilized a mycobacterium tuberculosis IHC stain with cross-reactivity to Nocardia to rapidly confirm our suspicion. In the current case, the utilization of an IHC stain allowed for prompt presumptive identification of Nocardia (initially thought to be NTM) and subsequent timely alteration of antimicrobials prior to confirmation of Nocardia brasiliensis via culture.