Actinomycosis is a chronic suppurative or granulomatous bacterial infection caused by branching, non–spore-forming, gram-positive bacteria belonging to the genus Actinomyces. Actinomyces spp. are strict or facultative anaerobes. Their cellular morphology is variable, usually diphtheroid or ﬁlamentous, although bacillary and coccoid forms may also occur. Actinomyces are true bacteria; their ﬁlaments are narrower than fungal hyphae; and unlike the latter, they readily fragment into bacillary forms. In addition, Actinomyces spp. reproduce by binary ﬁssion and not by spore formation or budding as in fungi. They are part of normal oral ﬂora within gingival crevices and tonsillar crypts. Their prevalence is increased with poor oral hygiene, in periodontal pockets, in dental plaques, and on carious teeth.
Actinomycosis is a cosmopolitan, sporadically occurring endogenous infection. Males are 1.5 to 3.0 times more likely to have the disease than females. Actinomyces israelii is the usual cause of actinomycosis, although several other species can occasionally cause human disease. Clinical types of actinomycosis and their incidence include cervicofacial (55%), pulmonary (20%), abdominal (20%), and disseminated (5%). A hallmark of actinomycosis is the tendency to spread through anatomic barriers, including fascial planes, and the development of multiple sinus tracts. Cervicofacial infection usually follows tooth extraction or other trauma to the oral mucosa and is characterized by a ﬁrm indurated mass in the region of the jaw (“lumpy jaw”) that often suppurates and gives rise to multiple cutaneous ﬁstulas. Poor oral hygiene is important as a predisposing factor. Other predisposing conditions include diabetes, immunosuppression, malnutrition, and local tissue damage.
Pulmonary actinomycosis may result directly from a cervicofacial focus or from extension through the diaphragm from an intraabdominal lesion. As a rule, it is secondary to aspiration of the organism from the mouth, and generally the lower lobes are involved. Initial symptoms include mild fever and cough with purulent sputum. With abscess formation, the sputum may become blood streaked. If it is not treated, the infection often spreads to the pleura and through the thoracic wall, causing subsequent empyema, soft tissue abscesses, and multiple draining sinuses. The clinical and radiographic signs of pulmonary actinomycosis are similar to those of nocardiosis, tuberculosis, and other lung disorders.
Abdominal infection most often occurs after appendectomy or bowel perforation, which may be either traumatic or spontaneous. Abdominal actinomycosis is more common in the cecum and appendix, where it is frequently associated with sinus formation. In disseminated actinomycosis, virtually any organ may be involved.
Actinomyces spp. form characteristic sulfur granules in infected tissue but not in vitro. Pus should be placed in a sterile Petri dish and examined with a hand lens against a dark background for these granules. If present, these granules are yellowish white to white ﬁrm ﬂecks that vary in size from a barely visible speck to 5 mm in diameter. Under an oil immersion lens, Gram stain of the granules reveals an internal tangle of delicate gram-positive ﬁlaments with a rosette of peripheral clubs. Similar granules may be formed by other microorganisms such as Nocardia brasiliensis, Streptomyces madurae, and Staphylococcus aureus. However, these other granules lack the rosette of peripheral clubs.
Diagnosis of actinomycosis is conﬁrmed by culture and occasionally by histopathologic evidence of Actinomyces infection. Actinomyces spp. are slow-growing, fastidious organisms, which require an enriched medium, anaerobic or microaerophilic conditions, and up to 14 to 21 days for optimal growth. Therefore, the clinical microbiology laboratory should be informed if Actinomyces spp. are suspected. Actinomyces spp. are almost invariably isolated as part of a polymicrobial ﬂora. However, the signiﬁcance of these coexisting bacteria in the pathogenesis of actinomycosis is unclear, and these pathogens do not need to be speciﬁcally treated with antibiotics.
Management of patients with actinomycosis often requires prolonged courses of antibiotics and surgical intervention in complicated cases. High-dose penicillin is the treatment of choice for actinomycosis. For mild infections without signiﬁcant suppuration or ﬁstulous tracts, oral penicillin V or amoxicillin for 2 to 6 months is recommended. Acceptable alternatives to penicillin include the tetracyclines, erythromycin, and clindamycin. In patients with more severe cervicofacial actinomycosis that requires surgery, intravenous penicillin G for 4 to 6 weeks followed by oral penicillin V for 6 to 12 months is recommended.
In addition to antibiotics, surgical intervention may be required for excision of necrotic tissue or recalcitrant ﬁbrotic lesions and for drainage of extensive abscesses with marsupilation of persisting sinus tracts. Prognosis is excellent with a combined medical-surgical approach. However, recurrences can occur, so prolonged observation of patients after treatment is recommended.