In a case report, researchers described an 18-year-old female patient diagnosed with digital actinomycosis following surgical removal of a painless mass on the ring finger.

Actinomycosis is a chronic bacterial infection caused by Actinomyces species, anaerobic organisms that typically colonize the oral and gastrointestinal tracts. Infection most commonly affects the cervicofacial region. Involvement of the fingers is rare, and isolated digital cases are uncommon, particularly in young patients.

The researchers detailed that the patient presented with a 2-month history of progressive swelling on the dorsum of the ring finger. The lesion measured approximately 1 x 1 cm, was firm and nontender, and showed no skin discoloration or lymphadenopathy. Laboratory testing, including complete blood count, was within normal limits.

Radiography demonstrated a poorly defined lytic lesion of the middle phalanx with cortical erosion and mild periosteal reaction. Imaging findings raised concern for chronic infection, inflammatory disease, or neoplasm. Differential diagnoses included chronic osteomyelitis, atypical mycobacterial infection, nocardiosis, foreign-body granuloma, and bone tumors.

The mass was excised without prior antibiotic therapy. Histopathologic examination revealed sulfur granules—basophilic aggregates of branching filamentous bacteria—within a background of suppurative inflammation and fibrosis. Periodic acid–Schiff staining highlighted the filamentous colonies, confirming their characteristic architecture. Ziehl-Neelsen staining was negative for acid-fast bacilli, helping exclude mycobacterial infection. No evidence of malignancy was identified.

Although culture can confirm Actinomyces infection, it may be difficult because of the organism’s slow growth and susceptibility to contamination. In this case, histopathology provided definitive diagnosis.

Following complete surgical excision, the patient recovered without recurrence at 4 months. Actinomycosis is typically treated with prolonged penicillin therapy, often combined with surgical management in localized disease.

The case highlighted the importance of considering actinomycosis in the differential diagnosis of chronic digital masses with lytic bone changes. Identification of sulfur granules on routine hematoxylin–eosin staining, supported by Periodic acid–Schiff staining and exclusion of acid-fast organisms, remains central to diagnosis. Early recognition can prevent misdiagnosis as malignancy or other chronic infections and guide appropriate treatment.

Source: Clinical Case Reports