B73-06 Empyema Necessitans Secondary to Thoracic Actinomycosis: A Diagnostic Challenge

Abstract Actinomyces species are gram-positive filamentous bacteria that are part of the normal oral, gastrointestinal, and female genitourinary flora. Aspiration of Actinomyces into the thoracic cavity can lead to thoracic actinomycosis, most often caused by Actinomyces israelii. This infection typically affects immunocompetent hosts. Risk factors for infection include emphysema, chronic alcohol use, and poor oral hygiene. Due to its indolent course and tendency to invade across tissue planes, thoracic actinomycosis is frequently mistaken for malignancy. Recognizing features consistent with Actinomyces is therefore critical to avoid diagnostic delays. We report a case of thoracic actinomycosis complicated by empyema necessitans. A 63-year-old man with emphysema and prior cervical spinal fusion presented to the hospital with a two-week history of a rapidly enlarging painful chest wall mass. Physical exam revealed a tender but non-erythematous six-by-three centimeter chest wall mass. Chest imaging demonstrated bilateral apical consolidations with left-sided cavitation, cortical breakthrough of the right first and second ribs, and gas tracking along spinal fixation hardware. These findings were initially concerning for malignancy, though there was no mediastinal or hilar lymphadenopathy. Aspiration of the mass revealed purulent fluid and gram stain showed gram-positive cocci and filamentous rods. Cultures grew Streptococcus constellatus, MSSA, and Candida species, but not Actinomyces. However, anatomic pathology revealed the presence of sulfur granules, reinforcing clinical suspicion for actinomycosis. Next-generation sequencing was performed and confirmed Actinomyces israelli as the implicated pathogen, most consistent with the clinical syndrome of empyema necessitans. Empyema necessitans is a rare complication of empyema characterized by invasion into the chest wall, predominantly caused by Mycobacterium tuberculosis or Actinomyces israelii. The absence of recent dental procedures or ongoing alcohol use rendered this case atypical. Co-isolation of Candida from the abscess represents another distinctive feature, as concurrent infection with Actinomyces has not been previously reported. While Actinomyces has been shown to inhibit Candida growth within the oral cavity, this case demonstrates that co-infection may occur in deep thoracic disease. This case underscores the importance of including Actinomyces in the differential diagnosis of thoracic lesions with radiologic features mimicking malignancy. Culture-based diagnosis is difficult, as growth may be inhibited by faster-growing pathogens within polymicrobial abscesses. Tissue invasion without lymphadenopathy should raise suspicion for infection so that prompt antimicrobial therapy can be initiated. This abstract is funded by: None