Extrapulmonary Mycobacterium malmoense infection in a patient with minimal immunosuppression

Mycobacterium malmoense is an emerging non-tuberculous mycobacterial (NTM) pathogen that usually causes extrapulmonary disease, particularly, in significantly immunosuppressed patients. Recent reports from Scotland and the Netherlands, however, document an increasing proportion of pulmonary NTM infections due to M. malmoense, indicating a rising incidence in parts of Northern Europe-although still predominantly associated with pulmonary disease. We report a 78-year-old woman from north-eastern Germany with chronic tenosynovitis caused by M. malmoense under minimal long-term prednisone therapy. Diagnosis was established by histopathology, culture, and hsp65 gene sequencing. Although up to 40% of reported M. 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.