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Actinomycetoma with negative culture: a therapeutic challenge

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Actinomycetoma with negative culture: a therapeutic challenge
Austin Liu BS1, Jennifer L Maender MD1, Neil Coleman MD2, Sylvia Hsu MD1, Ted Rosen MD1
Dermatology Online Journal 14 (4): 5

1. Departments of Dermatology, Baylor College of Medicine, Houston, TX.
2. Department of Pathology, Baylor College of Medicine, Houston, TX


Nocardia, Actinomyces, and Streptomyces species comprise the three broad classes of organisms that are causative for bacterial mycetoma. Although culture and molecular laboratory studies can usually identify the precise etiologic agent in bacterial mycetoma, occasionally these methods fail to clarify this situation. We report a classic clinical case of this infectious disease where usual diagnostic methods failed to identify the responsible organism, and discuss the empiric approach to such cases.


Mycetoma is a chronic granulomatous infection affecting the skin, subcutaneous tissues, and bone. Although worldwide, this infection is considered endemic in certain foci in tropical and subtropical areas. The causative agents include bacteria (actinomycetoma) and fungi (eumycetoma), and causative organisms are thought to gain entry to the skin by traumatic inoculation. The classic clinical presentation begins with painless papulonodules that slowly progress to cutaneous and subcutaneous edema and sinus tract formation with extrusion of prurulent material containing the characteristic grains. Although the diagnosis can be suggested by both clinical features and histopathologic examination of a representative skin biopsy, definitive diagnosis requires positive tissue culture with identification of the specific etiologic pathogen. This process can be cumbersome because of stringent growth requirements and frequent contamination of the specimens, as well as the small numbers of viable organisms present in a long-standing, very inflammatory lesion. Further testing with molecular modalities, such as PCR, ELISA, and immunoassays, can be employed to confirm the causative agent when repeated culture results are negative. However, the limited availability of these specialized laboratory tests, as well as uncertain knowledge as to their precise sensitivity and specificity, make them less useful than one might hope. We report herein a case of mycetoma that posed diagnostic problems and discuss our approach to such a patient.

Clinical synopsis

A 51-year-old male presented to our county-charity hospital dermatology clinic with a twelve year history of painless papules and nodules located on the right forearm. He reported that the first lesions appeared on his wrist and spread slowly, in a proximal fashion, to involve the skin above his elbow. He complained of recent weakness of the ipsilateral distal extremity. Past prurulent drainage was disclosed historically, leading to resolution with hypertrophic cicatrix formation. The patient emigrated from central Mexico approximately 20 years prior to presentation. He worked as an upholsterer but could not recall a specific instance of trauma to the affected area. His past medical history included diabetes mellitus, hypertension and hyperlipidemia.

Upon review of his records, it was discovered that the patient was initially seen in at the same institution four years previously (2003), at which time an inconclusive skin biopsy demonstrated only focal ulceration and fibrosis. Special tissue stains were negative, and aerobic, anaerobic, fungal and acid fast tissue cultures were all negative. He was lost to follow-up until 2005, when he returned to clinic complaining of progression of skin lesions. A repeat biopsy now revealed a characteristic dermal "grain" amidst a mixed inflammatory infiltrate that proved to contain partially acid fast, branching, filamentous bacteria. However, multiple tissue cultures were negative. Nonetheless, oral therapy was initiated empirically with double-strength trimethoprim-sulfamethoxazole (TMP-SMX) twice daily for a presumed diagnosis of actinomycetoma caused by Nocardia species. Unfortunately, after 1 month of this treatment, he was again lost to follow-up.

Figure 1Figure 2
Figure 1. Right arm with multiple firm erythematous papules and nodules with extensive scarring from sinus tract formation
Figure 2. The skin shows a diffuse mixed inflammatory infiltrate within the dermis. The inflammation is predominantly lymphohistiocytic with focal abscess formation. The inset shows an eosinophilic granule exhibiting a Splendore-Hoeppli phenomenon with a background of neutrophils. (Hematoxylin & Eosin, 4X and 100X)

Figure 3Figure 4
Figure 3. Filamentous and beaded Gram-positive bacteria are seen within the granule. (Tissue gram stain, 100X)
Figure 4. Filamentous bacteria show acid-fast positivity, consistent with Nocardia sp. (Fite stain, 100X)

On physical exam at his most recent presentation (2007), the patient had edema and scarring of the right arm with multiple firm erythematous papules and nodules. (Fig. 1) He had mildly decreased grip strength on the right with maintenance of full range of motion. Shotty lymphadenopathy of the right axillary vault was palpable. Routine hematological and biochemical laboratory tests were all normal; HIV status was negative; and ESR was elevated at 93mm/hr. A skin biopsy was consistent with prior results, revealing a single grain containing partially acid fast, branching, filamentous bacteria among a background of mixed cellular inflammation. (Figs. 2, 3, 4) Multiple tissue cultures were repeated and again failed to yield organisms. After consultation with infectious-disease specialists, the patient was asked to restart twice daily, double-strength TMP-SMX, with a projected treatment course of 9-12 months. Computed tomography of the right arm demonstrated extensive subcutaneous edema and enlargement of the musculature of the forearm with enhancement, suggestive of severe myofasciitis of the anterior compartment. No discrete abscesses or bony lesions were appreciated. He was referred to the orthopedic surgeons for possible decompression surgery.


Mycetoma is not endemic in the United States; however, because of increase in world travel and the influx of foreign immigrants, it is becoming much more common to encounter patients exhibiting this unusual disease. Although both eumycetomas and actinomycetomas have been reported in the United States, actinomycetomas are much more frequently seen because Nocardia species predominate in North and South America [1, 2, 3]. In the Western Hemisphere, Mexico has the highest incidence of actinomycetoma, followed by Venezuela and Argentina [1]. Nocardia brasiliensis, the most common cause of actinomycetoma in Mexico, is often encountered in patients living in U.S. border states such as Texas [3].

Actinomycetoma is caused by the actinomycetes which include the genera Nocardia, Actinomyces, and Streptomyces. The members of the genus Nocardia are filamentous gram-positive bacteria that are ubiquitous soil saprophytes. They gain entry into the skin through traumatic inoculation causing localized infection that is focally aggressive but does not typically disseminate. Early diagnosis and treatment can dramatically affect morbidity associated with this condition. Thus, it is important for clinicians to be aware of this disorder's clinical presentation and methods available for confirming the diagnosis. Clinically, patients experience formation of erythematous papulonodules with drainage of prurulent material and sinus tract formation slowly spreading in a sporotrichoid fashion. Ultimately fibrosis and destruction of underlying soft tissue and bone will ensue. Severe edema and regional lymphadenopathy are common as the disease progresses.

Beyond the clinical appearance, histopathologic analysis of affected skin is a critical step. A reaction pattern of granulomatous inflammation, abscess formation, and fibrosis is typical but non-diagnostic. The identification of characteristic "sulfur granules" or "grains," which contain the infectious organisms, on examination of smears of the discharge or tissue biopsy confirms the diagnosis of mycetoma. However, further study of the granules is necessary to elucidate the causative organism. Grains from eumycetomas are large in comparison to actinomycetomas with an average measurement of 0.5-2.0 mm and 20-100 μm respectively. Eumycetomas can produce both black and white colored grains that can be visualized without a microscope; most grains from actinomycetomas are white to yellow in color and are not easily seen with the naked eye. Within the granules, eumycetomas create septate hyphae 4-5 μm thick that are PAS positive and gram stain negative. By contrast, the granules of actinomycetoma contain fine, branching filaments 1 μm thick that are PAS negative and gram stain positive. Acid fast staining with either Fite or Ziehl-Neelson helps to further differentiate the etiology when a diagnosis of actinomycetoma is suspected because Nocardia species are weakly acid fast, unlike Actinomyces and Streptomyces species. Additionally, the grains of actinomycetomas are often surrounded by a peripheral, radial deposition of intensely eosinophilic material termed a Splendore-Hoeppli reaction. These histopathologic features are extremely helpful because the background histologic appearance of the tissue surrounding the grains can look almost identical in both eumycetoma and actinomycetoma [4, 5].

Precise identification of the organisms requires confirmatory cultures. Guidance of appropriate treatment in the face of either persistent or resistant disease can depend on sensitivity testing obtained from positive culture growth. Unfortunately, culture of mycetomas can be problematic due to stringent growth requirements and frequent contamination by other bacterial organisms. As in the case of our patient, multiple cultures can provide no growth even in the face of proven histopathologic disease [6]. This can result from many factors, including a paucity of grains within the tissue from either prior partial treatment or robust immune response to the infection. Sampling error then becomes an issue, as cultures will not yield growth when taken from tissue not containing grains. Punch biopsy may provide inadequate material for culture necessitating deep excisional biopsy to increase the likelihood of obtaining tissue containing the infectious organism. In addition, the patient may present late in the disease when fibrosis predominates over prurulent drainage, thereby making microbiologic evaluation all the more difficult.

Recently, newer methods of organism identification have become available and may be employed in culture negative patients. Serodiagnosis with ELISA can be used to evaluate levels of IgM antibodies against the causative agent; serodiagnosis is not always diagnostic due to variable levels of humoral response to infection [7]. PCR can lead to species identification via amplification of a 16s rDNA species specific gene sequence [8, 9]. These molecular methods provide alternative means of organism identification, they are not readily available in many labs, and studies evaluating the sensitivity and specificity of their use in large clinical populations is lacking.

Imaging techniques including plain films, computed tomography, and MRI allow the clinician to determine the degree of involvement of underlying structures. Although imaging findings are neither specific nor diagnostic of mycetoma, they provide valuable information that helps to determine the aggressiveness of treatment. The likelihood of mycetoma extension to bone increases with a longer duration of disease. Because of the indolent nature of the condition, it is not uncommon for patients to wait several years before seeking medical attention.

Treatment regimens vary according to the organism causing the infection. Eumycetomas are usually much more challenging to treat as they often respond poorly to pharmacologic therapy with antifungal agents. Surgical intervention is often required to debride infected tissue with wide margins to prevent recurrence. Actinomycetomas, however, are generally much more responsive to drug therapy, especially if treatment is initiated early in the course of the disease. Sulfonamides have been the mainstay of treatment for Nocardial actinomycetoma for over 50 years. The duration of treatment is dictated by the clinical response to medication, but reported cure rates are 60-90 percent with a mean duration of therapy greater than one year [10]. Second-line agents include minocycline, augmentin, amikacin, cefuroxime, ceftriaxone, quinolones, imipenem, and linezolid [11, 12, 13]. Involvement of underlying bone often requires more aggressive measures, including surgical debridement or even amputation of the affected areas. Because of the associated potential morbidities, empiric treatment based on clinical and histological findings alone should be initiated, even if specific microbiologic confirmation cannot be obtained [14].


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