Microbiology

Actinomycosis is caused by anaerobic or microaerophilic bacteria, primarily of the genus Actinomyces (e.g., A. israelii, A. naeslundii, A. odontolyticus), that colonize the mouth, colon, and vagina. Most infections are polymicrobial, but the role of other species in the pathogenesis of the disease is unclear.

Epidemiology

Actinomycosis is associated with poor dental hygiene, prolonged use of intrauterine contraceptive devices (IUCDs), and treatment with immunosuppressing drugs (e.g., anti-tumor necrosis factor α agents, glucocorticoids, bisphosphonates).

Pathogenesis

After disruption of the mucosal barrier, resident Actinomyces can infect locally and spread contiguously in a slow progressive manner, ignoring tissue planes. The hallmark of actinomycosis is the development of single or multiple indurated lesions with fibrotic walls often described as “wooden.” Central necrosis of lesions with neutrophils and sulfur granules is virtually diagnostic of the disease.

Clinical Manifestations

• Oral-cervicofacial disease: Infection starts as a soft-tissue swelling, abscess, mass, or ulcerative lesion, often at the angle of the jaw, with occasional contiguous extension to the cranium, cervical spine, or thorax. Pain, fever, and leukocytosis are variable. Radiation therapy and particularly bisphosphonate treatment are associated with actinomycosis of the maxilla and mandible.
• Thoracic disease: The pulmonary parenchyma and/or pleural space is usually involved. Chest pain, fever, and weight loss are common.
  • Radiographic studies demonstrate a mass lesion or pneumonia. Cavitary disease may occur, and >50% of pts have pleural thickening, effusion, or empyema.
  • Lesions cross fissures or pleura and may involve the mediastinum, contiguous bone, or the chest wall (empyema necessitatis). In the absence of these findings, the disease is often mistaken for a neoplasm or for pneumonia.
• Abdominal disease: The diagnosis is challenging given that abdominal disease may not present clinically until months or years after the initial event (e.g., appendicitis, diverticulitis, bowel surgery) and that any abdominal organ or region can be involved.
  • The disease usually presents as an abscess, mass, or lesion fixed to underlying tissue and is often mistaken for cancer.
  • Sinus tracts to the abdominal wall, perianal region, or other organs may develop and mimic inflammatory bowel disease. Recurrent disease or a wound or fistula that fails to heal suggests actinomycosis.
• Pelvic disease: Pelvic actinomycosis is often associated with IUCDs that have been in place for >1 year; other foreign bodies (e.g., surgical mesh) have also been implicated. The presentation is indolent.
  • Pts have fever, weight loss, abdominal pain, and abnormal vaginal bleeding or discharge. Endometritis progresses to pelvic masses or tubo-ovarian abscess.
  • When there are no symptoms and Actinomyces-like organisms are identified on Papanicolaou-stained specimens, the pt should undergo close follow-up but the IUCD does not have to be removed. Symptomatic pts should probably have their IUCD removed and-if advanced disease is excluded-receive a 14-day course of empirical antibiotics for possible early endometritis.
• Miscellaneous sites: Actinomycosis can involve musculoskeletal tissue, soft tissue, or (rarely) the CNS. Hematogenous dissemination, most commonly to the lungs and liver, can occur.

Diagnosis

Actinomycosis should be considered when a chronic progressive process with mass-like features crosses tissue boundaries, a sinus tract develops, and/or the pt has evidence of a refractory or relapsing infection despite short courses of antibiotics.

• Aspirations, biopsies, or surgical excision may be required to obtain material for diagnosis.
• Microscopic identification of sulfur granules (an in vivo matrix of bacteria, calcium phosphate, and host material) in pus or tissues helps establish the diagnosis; however, additional histopathologic and microbiologic studies are required to distinguish actinomycosis from mycetoma and botryomycosis, in both of which sulfur granules develop.
• Anaerobic cultures usually require 5-7 days to become positive but may take 2-4 weeks; even a single antibiotic dose can affect the yield of cultures.