Mycobacterium preseptal cellulitis as a clue to underlying disseminated Bacillus Calmette-Guérin-osis

Preseptal cellulitis is a common pediatric infection, typically caused by bacterial pathogens such as Staphylococcus aureus and Streptococcus species. However, in rare cases, atypical pathogens, including mycobacteria, may be implicated, particularly in immunocompromised individuals. We report a case of a five-month-old male infant who presented with respiratory distress, multiple pustular skin lesions, and right-sided preseptal cellulitis. The presence of an ulcerated Bacillus Calmette-Guérin (BCG) vaccination scar raised suspicion of mycobacterial involvement. Incision and drainage of the preseptal abscess was performed, and polymerase chain reaction testing of the aspirated pus confirmed Mycobacterium tuberculosis complex. Further genetic evaluation revealed X-linked severe combined immunodeficiency (X-SCID), establishing a diagnosis of disseminated BCGosis. The infant was promptly initiated on anti-tubercular therapy with isoniazid, rifampicin, and ethambutol, leading to resolution of preseptal cellulitis. This case underscores the importance of considering mycobacterial infections in preseptal cellulitis, particularly in infants with immune dysfunction and an ulcerated BCG scar. Such rare presentations may serve as sentinel events for unrecognized primary immunodeficiencies.