Mycobacteria scrofulaceum

Scrofula in a child under five is usually caused by non-tuberculous mycobacteria, chiefly M. scrofulaceum and Mycobacterium avium complex, acquired from soil, water, or food rather than from a person; in adults and in high-prevalence settings the same clinical picture is more often tuberculous. The node enlarges painlessly over weeks, the overlying skin becomes violaceous and thin, and it may rupture to form a chronically draining sinus. Systemic symptoms are absent, the chest radiograph is normal, and the tuberculin skin test is negative or only weakly positive, which helps distinguish it from tuberculous adenitis. M. scrofulaceum is a scotochromogen, pigmented whether or not it is exposed to light. Complete surgical excision is both diagnostic and curative and is preferred to incision and drainage, which risks a persistent fistula; antimycobacterial drugs are reserved for incompletely excised or recurrent disease. The IFN-gamma release assay is helpful because, unlike the tuberculin test, it is not confounded by BCG or most non-tuberculous mycobacteria.