Pulmonary Actinomycosis

Definition and Organism

Epidemiology

• Actinomycosis is now rare, with an incidence of approximately 1:100,000 in developed countries - declining steeply since the antibiotic era.
• Thoracic disease accounts for approximately 15% of all actinomycosis cases.
• Human colonization by Actinomyces is nearly universal (mouth, colon, vagina); progression to clinical disease is uncommon.
• Males are affected more frequently than females; the disease spans all age groups but peaks in the 4th-6th decades.
• Predisposing factors include poor dental hygiene, aspiration risk states, alcoholism, immunosuppression, prior lung disease (bronchiectasis, lung abscess), and bronchial obstruction (tumor, foreign body). (Fishman's Pulmonary Diseases and Disorders, pp. 2440-2441)

Pathogenesis

1. Causes a slowly progressive pneumonic consolidation beginning as a small peripheral nodule
2. Crosses tissue planes - a hallmark feature - spreading contiguously to the pleura, mediastinum, and chest wall, disregarding normal anatomical barriers
3. Forms dense, "woody" fibrotic lesions with surrounding suppuration
4. Produces sulfur granules - small yellow granules visible in pus - which are microcolonies of organisms embedded in calcium phosphate

Clinical Presentation

• Tuberculosis (fever, night sweats, weight loss, hemoptysis)
• Lung malignancy (chest wall mass, weight loss, dense consolidation)
• Fungal infection or other chronic pneumonia

• Chest wall sinuses - draining tracts discharging sulfur granules
• Rib involvement - periosteal proliferation or frank bone destruction
• Empyema necessitans - rare but characteristic
• Mediastinitis (from esophageal perforation or direct spread) (Fishman's Pulmonary Diseases and Disorders, pp. 2441-2442; Murray & Nadel, p. 3839)

Radiographic Features

• Peripheral consolidation, mass, or nodule - typically in lower lobes
• Air-space opacification that crosses fissures
• Pleural thickening or effusion adjacent to the parenchymal lesion
• Chest wall involvement in advanced cases

• Early: small peripheral nodule progressing to consolidation or mass
• Consolidation often has hypoattenuated areas (necrosis) or frank cavitation
• In bronchiectatic form: cylindrical bronchiectasis, bronchial wall thickening, mucus impaction, centrilobular nodules
• Transfissural spread - extension across interlobar fissures - is a characteristic CT sign
• Periosteal reaction or rib destruction on bone windows suggests advanced chest wall involvement

Diagnosis

1. Clinical suspicion - chronic pneumonia with chest wall extension, draining sinuses, or transfissural spread
2. Sulfur granules - yellow granules visible in sputum, pus, or pleural fluid (macroscopic finding); on microscopy they show a central mass of gram-positive filaments with peripheral "clubs" (Splendore-Hoeppli phenomenon)
3. Microbiological culture:
   • Anaerobic cultures of sputum, BAL fluid, pleural fluid, or tissue biopsy
   • Growth is slow (5-7 days minimum); cultures must be held for 2-4 weeks
   • Contamination by oral flora makes sputum cultures unreliable; bronchoscopic protected specimens or CT-guided biopsy are preferred
   • Definitive diagnosis: demonstration of Actinomyces israelii on anaerobic culture
   • Growth can be misinterpreted as normal flora - a common pitfall
4. Histopathology - most reliable method; shows sulfur granules surrounded by neutrophils, granulation tissue, and fibrosis
5. Bronchoscopy - to exclude malignancy and obtain protected specimens; endobronchial actinomycosis (rare form) can appear as a polypoid mass
6. 16S rRNA gene sequencing - increasingly used for definitive species identification in culture-negative cases

Treatment

• High-dose IV penicillin G (18-24 million units/day) for 2-6 weeks (induction phase) - given parenterally until clinical improvement
• Followed by oral amoxicillin (500 mg three times daily or 1 g twice daily) for a total duration of 6-12 months
• Prolonged therapy is essential to prevent relapse due to the organism's propensity to form fibrotic barriers that limit antibiotic penetration

• Doxycycline
• Erythromycin / other macrolides
• Clindamycin
• Imipenem (severe/refractory cases)
• Cephalosporins (first/second generation)
• Metronidazole and aminoglycosides are NOT effective

• Surgery is generally NOT the primary treatment
• Indications for surgery: resection of destroyed lung tissue, drainage of empyema, excision of chest wall sinuses, or when diagnosis cannot be established without thoracotomy
• Combination of prolonged antibiotics + surgical drainage when chest wall or empyema involvement is present
• Murray & Nadel emphasizes: pleural actinomycosis should be treated with high doses of penicillin for prolonged periods, with management of pleural effusion similar to any bacterial pneumonia

Key Differentials

Summary Points for Exam

1. Actinomycosis = "great mimic" of TB, malignancy, and fungal infection
2. Causative agent: Actinomyces israelii (gram-positive, anaerobic filamentous bacterium)
3. Pathogenesis: aspiration of oropharyngeal commensals
4. Hallmark findings: sulfur granules, transfissural spread, chest wall sinus formation
5. Recurrence after short-course antibiotics is a clinical clue
6. Diagnosis: anaerobic culture + histopathology (sulfur granules); bronchoscopy/CT-biopsy preferred over sputum
7. Treatment: high-dose IV penicillin then oral amoxicillin for 6-12 months total
8. Murray & Nadel: >50% of thoracic cases have pleural involvement; pleural fluid may be frank pus (PMN-predominant) or serous (lymphocyte-predominant)
9. Chest CT: consolidation with cavitation, pleural thickening adjacent to lesion, periosteal reaction of ribs in advanced disease
10. Surgery reserved for diagnosis, drainage, or resection of destroyed tissue; prolonged antibiotics are the cornerstone