Actinomycosis

Definition

Actinomycosis is an uncommon, subacute to chronic bacterial infection that induces both suppurative and granulomatous inflammation. Localized swelling with suppuration, abscess formation, tissue fibrosis, and draining sinuses characterize this disease. The infection spreads contiguously and often forms draining sinuses that extrude characteristic, but not pathognomonic, “sulfur granules.” Infections of the oral and cervicofacial regions are the most common, but any site in the body can be infected, including the thoracic region, abdominopelvic region, and central nervous system (CNS). Musculoskeletal, cutaneous, or disseminated disease is rare but does occur. Actinomycosis sometimes mimics other diseases, particularly malignancy.

Epidemiology

Actinomyces spp are members of the endogenous mucous membrane flora in the oral cavity, lower gastrointestinal tract, bronchi, and female genital tract. No external environmental reservoir, such as soil or straw, has been documented, nor has person-to-person transmission of pathogenic Actinomyces spp been demonstrated. Although infection can occur in all age groups, it is rarely seen in children or patients older than 60 years. Most cases are encountered in individuals in the middle decades of life. A male-to-female infection ratio of 3:1 is reported in most series. The explanation for this ratio is the higher prevalence of poor oral hygiene and oral trauma in men. The annual reported incidence in the United States is fewer than 100 cases. However, because of the fastidious nature of the organism, many cases are undiagnosed, and the true incidence is probably much higher.

Oral and cervicofacial diseases are commonly associated with dental caries and extractions, gingivitis and gingival trauma, infection in erupting secondary teeth, chronic tonsillitis, otitis or mastoiditis, diabetes mellitus, immunosuppression, immunodeficiency, malnutrition, and neoplastic disease. Pulmonary infections generally arise after aspiration of oropharyngeal or gastrointestinal secretions and have been reported in patients with underlying lung disorders, such as emphysema, chronic bronchitis, and bronchiectasis. Gastrointestinal infection frequently follows loss of mucosal integrity, such as with surgery, trauma, foreign bodies, perforated appendix or diverticulitis, neoplasia, foreign bodies, and emergency colonic surgery. Extended use (>2 years) of intrauterine contraceptive devices (IUDs) increases risk for the development of actinomycosis of the female genital tract.

Pathobiology

Actinomyces spp are agents of low pathogenicity and require mucosal barrier disruption to cause disease. Actinomycosis usually occurs in immunocompetent persons but may affect individuals with diminished host defenses. Risk factors include corticosteroids, bisphosphonates, leukemia with chemotherapy, human immunodeficiency virus (HIV), alcoholism, solid organ transplant, and local tissue damage caused by trauma, recent surgery, or irradiation.

Other bacterial species that are frequently copathogens with Actinomyces spp may assist in the spread of infection by inhibiting host defenses and reducing local oxygen tension. Once the organism is established locally, it may spread progressively to surrounding tissues. The infection tends to spread without regard for anatomic barriers, including fascial planes and lymphatic channels. The end result is a chronic, indurated, suppurative infection (usually with draining sinuses and fibrosis, especially in pelvic and abdominal infection). The fibrotic walls of the mass before suppuration are “wooden” in nature and may be confused with a neoplasm. Hematogenous spread can be fulminant but is rare.

Actinomyces spp grow in microscopic or macroscopic clusters of tangled filaments surrounded by neutrophils. Plasma cells and multinucleated giant cells are often observed with lesions, as are large macrophages with foamy cytoplasm around purulent centers. When visible, these clusters are pale yellow and exude through sinus tracts; they are called sulfur granules. These granules (1 to 2 mm in diameter) are made of aggregates of organisms and contain calcium phosphate. A central purulent loculation surrounds the granules. Their centers have a basophilic staining property, with eosinophilic rays terminating in pear-shaped “clubs.” One to six granules can be present per loculation, and up to 50 loculations can be present in a lesion. Multicenter giant cells can be seen as well.