[Intramuscular abscess and multiple lymphadenitis due to Nocardia asiatica infection requiring differential diagnosis from smoldering adult T-cell leukemia/lymphoma].

A 72-year-old male engaged in horticulture developed a progressively enlarging painful mass in the left upper arm. The first biopsy revealed no malignancy or pathogenic bacteria. He completed an adequate course of antibiotics for infectious lymphadenitis, but showed no apparent improvement. Computed tomography demonstrated systemic lymphadenopathy, suggesting malignant lymphoma. Monoclonal integration of the HTLV-1 proviral genome in peripheral blood cells was confirmed by Southern blot analysis, but repeated lymph node biopsies identified no malignant cells. Based on these findings, a diagnosis of smoldering adult T-cell leukemia/lymphoma was made. After prolonged culture of the specimen, Nocardia sp. were detected and 16S rRNA sequencing confirmed infection by Nocardia asiatica. Combination therapy with sulfamethoxazole-trimethoprim and minocycline was started, and the systemic lymphadenopathy improved within 3 months. Although Nocardia asiatica infection manifesting as systemic lymphadenitis is extremely rare, it should be considered as a possibility in immunocompromised patients to ensure an accurate diagnosis.