(See also Overview of Fungal Infections.)
Bacteria, primarily Nocardia species and other actinomycetes, cause more than half the cases. The remainder are caused by about 20 different fungal species. When caused by fungi, the lesions are sometimes called eumycetoma.
Mycetoma occurs mainly in tropical or subtropical areas, including the southern US, and is acquired when organisms enter through sites of local trauma on bare skin of the feet or on the extremities or backs of workers carrying contaminated vegetation or other objects. Men aged 20 to 40 are most often affected, presumably because of trauma incurred while working outdoors.
Infections spread through contiguous subcutaneous areas, resulting in tumefaction and formation of multiple draining sinuses that exude characteristic grains of clumped organisms. Microscopic tissue reactions may be primarily suppurative or granulomatous depending on the specific causative agent. As the infection progresses, bacterial superinfections can develop.
The initial lesion of mycetoma may be a papule, a fixed subcutaneous nodule, a vesicle with an indurated base, or a subcutaneous abscess that ruptures to form a fistula to the skin surface. Fibrosis is common in and around early lesions. Tenderness is minimal or absent unless acute suppurative bacterial superinfection is present.
Infection progresses slowly over months or years, gradually extending to and destroying contiguous muscles, tendons, fascia, and bones. Neither systemic dissemination nor symptoms and signs suggesting generalized infection occur. Eventually, muscle wasting, deformity, and tissue destruction prevent use of affected limbs. In advanced infections, involved extremities appear grotesquely swollen, forming a club-shaped mass of cystic areas. The multiple draining and intercommunicating sinus tracts and fistulas in these areas discharge thick or serosanguineous exudates containing characteristic grains, which may be white or black.
Causative agents can be identified presumptively by gross and microscopic examination of grains from exudates, which contain irregularly shaped, variably colored, 0.5- to 2-mm granules. Crushing and culture of these granules provides definitive identification. Exudate specimens may yield multiple bacteria and fungi, some of which are potential causes of superinfections.
(See also Antifungal Drugs.)
Treatment of mycetoma may be required for > 10 years. Death may result from bacterial superinfection and sepsis if treatment is neglected.
In infections caused by Nocardia, sulfonamides and certain other antibacterial drugs, sometimes in combination, are used.
In infections caused by fungi, certain potential causative organisms may be at least partially sensitive to amphotericin B, itraconazole, or ketoconazole (not available in the US), but some are resistant to all antifungal drugs. Relapses occur after antifungal therapy in most patients, and many patients do not improve and even worsen during treatment, indicating the often refractory nature of the infection.
Surgical debridement is necessary, and limb amputation may be needed to prevent potentially fatal severe secondary bacterial infections.