= chronic suppurative infection characterized by formation of multiple abscesses, draining sinuses, abundant granulation tissue ← mucosal disruption + low tissue oxygen tension

• Organism: Actinomyces israelii / naeslundii / odontolyticus / viscosus / meyeri / gerencseriae / eriksonii, gram-positive non–acid-fast anaerobic pleomorphic small branching filamentous bacterium with proteolytic activity, superficially resembling the morphology of a hyphal fungus (Gömöri methenamine silver stain-positive filaments); closely related to mycobacteria
  • Actinomycotic infections are polymicrobial!
• Pathogenesis: trauma / surgery / foreign body → disruption of mucosal barrier → bacterial entry into deep tissues → fibrotic lesion spreading beyond fascial planes → abscess formation centrally → draining sinus tracts extending from abscess to skin / adjacent organs

Spread:

1. contiguous: production of proteolytic enzymes allow crossing of normal anatomic barriers
2. hematogenous

NO lymphatic spread ← size of organism!

Histo:

1. mycelial form in tissue as yellow tangled filaments of actinomyces = DIAGNOSTIC sulfur granules seen as round / oval horseshoe-shaped basophilic masses with a radiating fringe of eosinophilic clubs
2. rod-shaped bacterial form = opportunistic pathogens that normally inhabit oropharynx (dental caries, gingival margins, tonsillar crypts) + GI tract + female genital tract

At risk: very poor dental hygiene, immunosuppressed patient, prolonged use of IUD, bisphosphonate therapy

Location: mandibulofacial >intestinal >lung

Clinical types:

1. Mandibulo- / cervicofacial actinomycosis (50–65%)
   • Origin: odontogenic
   • At risk: poor dental hygiene, recent dental extraction, dental caries, oromaxillofacial trauma, chronic tonsillitis, otitis, mastoiditis, osteonecrosis from irradiation / bisphosphonate therapy
   • draining cutaneous sinuses
   • “sulfur granules” in sputum / exudate = colonies of organisms arranged in circular fashion = mycelial clumps with thin hyphae 1–2 mm in diameter
   • destruction of mandible (most frequent bone involved) around tooth socket = osteomyelitis of mandible
     Site: angle of jaw, submandibular region, cheek, submental space, masticator space, temporomandibular joint
   • NO new-bone formation
   • spread into soft tissues at angle of jaw + into neck ignoring normal fascial planes
   • NO / few reactive regional lymph nodes
   • acute form:
     • soft-tissue swelling / painful pyogenic abscess / mass
   • subacute to chronic form:
     • painless indurated mass ± spread to skin often accompanied by draining sinus tracts
2. Pleuropulmonary / thoracic actinomycosis (15–30%)
   Cause:
   1. aspiration of infected material from oropharynx
   2. hematogenous spread
   3. direct extension into mediastinum from cervicofacial infection (extremely rare)
   4. transdiaphragmatic / retroperitoneal spread
   
   Predisposed: alcoholics
   Histo: masses of PMN leukocytes containing round actinomycotic / sulfur granules surrounded by a rim of granulation tissue
   • Lung
     • draining chest wall sinuses ← spread through fascial planes
     • Predisposed: areas of parenchymal destruction and bronchiectasis ← prior TB / other organisms (= tendency of actinomyces for invasion of devitalized tissue)
     • enhancing extensive transfissural chronic segmental airspace consolidation:
       • hypoattenuating areas + peripheral enhancement (= lung necrosis)
       • often adjacent pleural thickening
       • Site: usually unilateral + lower lobe predominance
     • multiple small cavitary lesions with ringlike enhancement (= abscesses)
     • fibrotic pleuritis
     • chronic pleural effusion / empyema (in >50%)
     • rarely acute airspace pneumonia ← postobstructive endobronchial actinomycosis
       DDx: carcinoma, TB, bacterial / fungal pneumonia
   • Vertebra + ribs
     • destruction of vertebra with preservation of disk + small paravertebral abscess without calcification (DDx to tuberculosis: disk destroyed, large abscess with calcium)
     • thickening of cervical vertebrae around margins
     • destruction / thickening of ribs
3. Abdominopelvic / ileocecal / abdominal actinomycosis (20%)
   • Cause: appendicitis, colonic diverticulitis; penetrating trauma, gut surgery; prolonged IUD use >2 years (25% of IUDs become eventually colonized with serious infections in 2–4%)
   • Location: initially localized to cecum / appendix
   • fever, leukocytosis, mild anemia
   • weight loss, nausea, vomiting, lower abdominal pain
   • chronic sinus in groin, vaginal discharge
   • yellow / brown sulfur granules on cervical Papanicolaou smear
   • concentric bowel wall thickening:
     • adjacent cystic / solid mass
     • surrounding invasive soft-tissue strands
     • strong enhancement of solid portions ← extensive dense fibrosis = HALLMARK of actinomycosis
   • fold thickening + ulcerations (resembling Crohn disease)
   • rupture of abdominal viscus (usually appendix)
   • fistula formation
   • rarely regional lymphadenopathy
   • usually NO / minimal ascites
   
   Cx:
   1. abscess in liver (15%), retroperitoneum, psoas muscle, pelvis, tuboovarian abscess (containing yellow “sulfur granules” = 1–2-mm colony of gram-positive bacilli)
   2. hydronephrosis ← compression of ureter by pelvic abscess
4. Mixed organs (10%)
   • Tubular bones of hands
     • destructive lesion of mottled permeating type
     • cartilage destruction + subarticular erosive defects in joints (simulating TB)
   • CNS (2–3%), skin, pericardium

Dx: anaerobic culture; species-specific antibodies

Rx: high doses of penicillin G + surgical débridement

DDx: malignancy, chronic granulomatous disease (TB, fungal infection)