Actinomycosis is an indolent, slowly progressive infection caused by anaerobic or microaerophilic bacteria, primarily of the genus Actinomyces, that colonize the mouth, colon, and vagina. Mucosal disruption may lead to infection at virtually any site in the body. In vivo growth of actinomycetes usually results in the formation of characteristic clumps called grains or sulfur granules. The clinical presentations of actinomycosis are myriad. Common in the preantibiotic era, actinomycosis has diminished in incidence, as has its timely recognition. Actinomycosis has been called the most misdiagnosed disease, and it has been said that no disease is so often missed by experienced clinicians. Thus this entity remains a diagnostic challenge.
Three clinical presentations that should prompt consideration of this unique infection are (1) the combination of chronicity, progression across tissue boundaries, and mass-like features (mimicking malignancy, with which it is often confused); (2) the development of a sinus tract, which may spontaneously resolve and recur; and (3) a refractory or relapsing infection after a short course of therapy, since cure of established actinomycosis requires prolonged treatment. An awareness of the full spectrum of the disease, prompting clinical suspicion, will expedite its diagnosis and treatment and will minimize the unnecessary surgical interventions, morbidity, and mortality that are reported all too often.
Actinomycosis is most commonly caused by A. israelii. A. naeslundii, A. odontolyticus, A. viscosus, A. meyeri, and A. gerencseriae are established but less common causes. Most if not all actinomycotic infections are polymicrobial. Aggregatibacter (Actinobacillus) actinomycetemcomitans, Eikenella corrodens, Enterobacteriaceae, and species of Fusobacterium, Bacteroides, Capnocytophaga, Staphylococcus, and Streptococcus are commonly isolated with actinomycetes in various combinations, depending on the site of infection. The contribution of these other species to the pathogenesis of actinomycosis is uncertain.
Comparative 16S rRNA gene sequencing has led to the identification of an ever-expanding list of Actinomyces species and to the reclassification of some actinomycetes as Arcanobacterium. Increasing data support the Actinomyces species A. europaeus, A. neuii, A. radingae, A. graevenitzii, A. turicensis, A. cardiffensis, A. houstonensis, A. hongkongensis, A. lingnae, and A. funkei as well as two former Actinomyces species now classified as Arcanobacterium (A. pyogenes and A. bernardiae) as additional causes of human actinomycosis.
Actinomycosis has no geographic boundaries and occurs throughout life, with a peak incidence in the middle decades. Males have a threefold higher incidence than females, possibly because of poorer dental hygiene and/or more frequent trauma. Factors that have probably contributed to the decrease in actinomycosis incidence since the advent of antibiotics include improved dental hygiene and the initiation of antimicrobial treatment before the disease develops fully. Individuals who do not seek or have access to health care, those who have an intrauterine contraceptive device (IUCD) in place for a prolonged period (see “Pelvic Disease,” below), and those who receive bisphosphonate treatment (see “Oral-Cervicofacial Disease,” below) are probably at higher risk.
Pathogenesis and Pathology
The etiologic agents of actinomycosis are members of the normal oral flora and are often cultured from the bronchi, the gastrointestinal tract, and the female genital tract. The critical step in the development of actinomycosis is disruption of the mucosal barrier. Local infection may ensue. Once established, actinomycosis spreads contiguously in a slow progressive manner, ignoring tissue planes. Although acute inflammation may initially develop at the infection site, the hallmark of actinomycosis is the characteristic chronic, indolent phase manifested by lesions that usually appear as single or multiple indurations. Central necrosis consisting of neutrophils and sulfur granules develops and is virtually diagnostic. The fibrotic walls of the mass are typically described as “wooden.” The responsible bacterial and/or host factors have not been identified. Over time, sinus tracts to the skin, adjacent organs, or bone may develop. In rare instances, distant hematogenous seeding may occur. As mentioned above, these unique features of actinomycosis mimic malignancy, with which it is often confused.
Foreign bodies appear to facilitate infection. This association most frequently involves IUCDs. Reports have described an association of actinomycosis with HIV infection; transplantation; treatment with infliximab, glucocorticoids, or bisphosphonates; and radio- or chemotherapy. Ulcerative mucosal infections (e.g., by herpes simplex virus or cytomegalovirus) may facilitate the development of actinomycosis.
Actinomycosis occurs most frequently at an oral, cervical, or facial site, usually as a soft tissue swelling, abscess, or mass lesion that is often mistaken for a neoplasm. The angle of the jaw is generally involved, but a diagnosis of actinomycosis should be considered with any mass lesion or relapsing infection in the head and neck (Chap. 31). Radiation therapy and especially bisphosphonate treatment have been recognized as contributing to an increasing incidence of actinomycotic infection of the mandible and maxilla (Fig. 163-1). Otitis, sinusitis, and canaliculitis (most commonly due to Propionibacterium propionicum) also can develop. Pain, fever, and leukocytosis are variably reported. Contiguous extension to the cranium, cervical spine, or thorax is a potential sequela.
Bisphosphonate-associated maxillary osteomyelitis due to A. viscosus. A sulfur granule is seen within the bone. (Reprinted with permission from NH Naik and TA Russo. © 2009 University of Chicago Press.)
Thoracic actinomycosis usually follows an indolent progressive course, with involvement of the pulmonary parenchyma and/or the pleural space. Chest pain, fever, and weight loss are common. A cough, when present, is variably productive. The usual radiographic finding is either a mass lesion or pneumonia. On CT, central areas of low attenuation and ringlike rim enhancement may be seen. Cavitary disease or hilar adenopathy may develop. More than 50% of cases include pleural thickening, effusion, or empyema (Fig. 163-2...