BACKGROUND Actinomycosis is a clinically significant but uncommon infectious disease caused by anaerobic commensals of Actinomyces species, and the incidence of thoracic empyema is rare. We report an extremely rare case of empyema caused by Actinomyces naeslundii (A. naeslundii). CASE REPORT A 39-year-old man presented to our hospital with fever and dyspnea. He had massive pleural effusion and was diagnosed with a left lower-lobe abscess and left thoracic empyema. Thoracic drainage was performed and Ampicillin/Sulbactam was administered for 3 weeks. Four years later, the patient presented with back pain, and chest X-ray showed increased left pleural effusion. After close examination, malignant pleural mesothelioma was suspected, and computed tomography-guided needle biopsy was performed, which yielded a viscous purulent pleural effusion with numerous greenish-yellow sulfur granules. A. naeslundii was identified through anaerobic culture. Thoracoscopic surgery of the empyema cavity was conducted, and Ampicillin/Sulbactam followed by Amoxicillin/Clavulanate was administered for approximately 6 months. No recurrence has been observed for 1 year since the surgical procedure. CONCLUSIONS Actinomyces empyema is a rare condition, and this case is the second reported occurrence of empyema caused by A. naeslundii. The visual identification of sulfur granules contributed to the diagnosis. Long-term antibiotic therapy plays a crucial role in treatment.
