An unusual presentation of a large lower extremity abscess caused by Aggregatibacter aphrophilus: A case report

Aggregatibacter aphrophilus (A. aphrophilus) is a rare cause of severe infections, including periodontitis, brain abscess, and endocarditis. While typically oral commensals, its involvement in extremity soft tissue abscesses is uncommon, posing diagnostic challenges and often mimicking tumors. This report describes a rare A. aphrophilus infection presenting as a large, spontaneously draining lower extremity abscess without trauma history.

A 63-year-old male with untreated diabetes presented with posterior knee pain. Initial MRI suggested a biceps femoris tendon hematoma, treated conservatively. Because swelling, skin infiltration, and spontaneous drainage progressed, a soft tissue tumor was suspected, leading to referral to our hospital. MRI showed a small area of increased signal intensity in the proximal tibial bone marrow, with an enhancing, large heterogeneous extrarticular mass in surrounding soft tissue. Needle biopsy and tissue culture from the debrided abscess identified A. aphrophilus. The patient was successfully treated with piperacillin, requiring five weeks of hospitalization for wound management.

This case highlights the diagnostic complexity of A. aphrophilus infection when presenting atypically in soft tissues. Initial misdiagnosis as a hematoma and subsequent tumor suspicion underscore considering rare bacterial etiologies in persistent or draining lesions, especially in immunocompromised patients. A. aphrophilus may require specific culture or molecular methods for identification. Prompt diagnosis and targeted antibiotic therapy were crucial.

A. aphrophilus should be considered in the differential diagnosis of large, spontaneously draining lower extremity soft tissue lesions, even without a history of trauma. This case emphasizes microbiological culture for diagnosing atypical infections and preventing delayed treatment.