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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.

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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.

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Etiologic Agents

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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.

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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.

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Epidemiology

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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,” ...

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