Dermatologic Aspects of Actinomycosis Workup

Updated: Dec 11, 2020
  • Author: Juanita Duran, MD; Chief Editor: Dirk M Elston, MD  more...
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Approach Considerations

High clinical suspicion of actinomycosis should be followed by concurrent histological and microbiological evaluation of tissue, diagnostic imaging, and antibiotic treatment. Surgical interventions may be required depending on tissue extension and severity of disease.


Laboratory Studies

Collection, transport, and storage of specimens

Specimens suitable for culturing Actinomyces include aseptically collected peripheral blood, tissue, aspirates, and fluids (eg, cerebrospinal, synovial, pleural). Collections from mucocutaneous sites are challenging owing to the risk of contamination with commensal microbiota in the vicinity. The use of swabs is discouraged. Anaerobic transportation techniques should be used to carry and deliver the specimens to the laboratory. [34]

Direct examination

Microscopic examination for the diagnosis of actinomycosis has been a major tool for years. Evaluation of sulfur granules (not present in all infections) revealing a mass of gram-positive, branching filaments is suggestive, although not pathognomonic, of the disease. [34]

Culture findings in actinomycosis

Actinomyces usually produce small, relatively nondescript, clear-to-white colonies. Some species show a characteristic morphology described as "molar tooth" appearance. [34]

Cultures may reveal a polymicrobial isolate, including anaerobic streptococci, fusiform bacilli, Haemophilus species, and gram-negative bacilli. Obtaining individual susceptibilities in this scenario is discouraged as it may not be clinically helpful. The susceptibility of bacteria should be determined when a given organism is isolated in pure culture or is predominant. [34]

Performing cultures for actinomycosis

Appropriate culture media, including sheep's blood agar and Brucella agar at 37°C, should be used. Most Actinomyces are aerotolerant, although anaerobic culture incubation is recommended for optimal recovery. The media may be examined after 24 hours of incubation in an anaerobic chamber; however, waiting for 48 hours is advised. [34]

Initial characterization of the isolate involves aerotolerance (growth in air vs air plus 5% carbon dioxide), gross morphology, hemolysis, pigmentation, and fluorescence under long-wave ultraviolet illumination (365-nm wavelength). Additional rapid tests for initial grouping include catalase production, nitrate reduction, and motility. Biochemical testing had been historically used for the phenotypic characterization and speciation, but it is particularly unreliable for the identification of many Actinomyces. The assessment of Gram stain morphology from the colonies usually shows gram-positive bacilli/rods (nonbranching). The filamentous, branching morphology is mostly seen in histological samples. Most Actinomyces are aerotolerant and catalase and urease negative, and the majority show no pigment production. [34]

Speciation may be pursued by larger laboratories where MALDI-TOF mass spectrometry is available. An additional tool to obtain specific and accurate answers up to the species level is the use of 16S rRNA gene sequencing. [14, 15, 16]

Susceptibility testing methods and breakpoints for anaerobes, such as Actinomyces, are based on minimum inhibitory concentrations; disk diffusion is not acceptable for this organisms.

Hematologic study results in actinomycosis

Hematologic studies may reflect the infectious/inflammatory process, with an elevated white blood cell count and an increase in positive acute-phase response proteins. Owing to the chronic nature of the infection, it is not uncommon to see associated anemia.


Imaging Studies

The radiologic findings of actinomycosis are variable and depend on the primary site involved by the infection, extension, and duration of disease. Different diagnostic imaging tools such as radiography, axial computed tomography (preferably contrast-enhanced), and magnetic resonance imaging are routinely used to evaluate these lesions, according to their location.

In orocervicofacial disease, it is not uncommon to see an actinomycotic lesion as a mass with focal necrosis and cysts, with relatively homogeneous enhancement (the latter most likely due to abundant granulation tissue and fibrosis). Although the infectious process may evolve to compromise the bone, causing osteomyelitis in the adjacent mandible or maxilla, such a finding is not commonly seen. Maxillary involvement (less frequent) usually presents with a soft mass and underlying osteomyelitis, with cutaneous or maxillary sinus fistulas. Facial actinomycosis can spread into the neck, and even the mediastinum and thorax, ignoring fascial planes. Regional reactive lymphadenopathy in association with the infectious process is rare. [6]

Actinomycosis of the thoracic region may cause bronchopulmonary, pleural, chest wall, and/or mediastinal involvement (rare). Some of the imaging findings include chronic segmental airspace consolidation with necrotic areas showing peripheral enhancement, often accompanied by adjacent pleural thickening. Pleural thickening and effusion may be seen in association with parenchymal disease in more than half of cases. Transdiaphragmatic infection (from thorax to abdomen, and vice versa) has been described. [6]

Abdominopelvic actinomycosis preferentially involves the ileocecal region of the bowel, extending to surrounding structures across fascial and connective-tissue planes. There is concentric bowel wall thickening, with an adjacent mass and prominent inflammatory infiltration, with dense and extensive fibrosis. The ovary and fallopian tubes are the most commonly affected structures in pelvic actinomycosis. [6]

Musculoskeletal, central nervous system, and other less common sites are rarely compromised by Actinomyces. In the presence of radiologic findings consistent with granulomatous, markedly fibrotic, and focally necrotic lesions extending aggressively through different planes of tissue, actinomycosis should be considered as a differential diagnosis.