Abstract

Mycobacterium malmoense is an emerging non-tuberculous mycobacterial (NTM) pathogen that usually causes extra-pulmonary disease particularly in significantly immunosuppressed patients. Recent reports from Scotland and the Netherlands, however, document an increasing proportion of pulmonary NTM infections due to Mycobacterium malmoense, indicating a rising incidence in parts of Northern Europe-though still predominantly associated with pulmonary disease. We report a 78-year-old woman from north-eastern Germany with chronic tenosynovitis caused by Mycobacterium malmoense under minimal long-term prednisone therapy. Diagnosis was established by histopathology, culture, and hsp65 gene sequencing. Although up to 40% of reported Mycobacterium malmoense infections are extrapulmonary, nearly all described tenosynovitis cases have occurred in children or in profoundly immunosuppressed adults. This case therefore represents an exceptionally rare clinical constellation, demonstrating that even minimal immunosuppression can permit infection with this slow-growing NTM. It further underscores the diagnostic complexity, along with the absence of clear guideline recommendations regarding optimal therapy duration, highlighting the need for individualized, multidisciplinary management.