Mycetoma: Eumycetoma and Actinomycetoma

Morphologic Classification of Mycetoma: Eumycetoma and Actinomycetoma

The morphologic classification of mycetomas has not changed since the first description of the disease. Two main groups, caused by multiple etiologic agents, are described.

Eumycetomas , or eumycotic grains, are caused by true fungi. Two types of grains are formed: black grains (caused by dematiaceous/pigmented fungi, the most frequent type of eumycetoma) and white grains (caused by hyaline/nonpigmented fungi). More than 18 eumycetoma-causing species have been identified. Black grains are always of fungal origin, whereas a white grain may be caused by either fungi or bacteria. Table 14-1 summarizes the morphologic features of the most frequent causative agents of eumycetomas.

Actinomycetomas, or actinomycotic grains, are caused by exogenous actinomycetes. The term actinomycetoma is preferred over actinomycotic grains, because use of the latter might cause confusion with a true mycosis. Furthermore, actinomycetoma should not be confused with disease caused by endogenous actinomycetes. More than eight species of aerobic bacteria have been reported to cause actinomycetomas. Table 14-2 summarizes the morphologic features of the most frequent causative agents.

This classification of mycetoma is of critical importance for determining the treatment approach to be used, because the drugs used for eumycetomas are radically different from those used for actinomycetomas.

Prevalence

Geographic distribution is an important criterion to be considered when entertaining a diagnosis of mycetoma. Mycetoma is primarily a tropical disease found in countries with a warm, dry climate. The endemic zone (the Mycetoma Belt) is located at a latitude close to 15º North and includes India, Sahelian Africa (Senegal, Mauritania, Mali, Niger, Chad, Sudan, and Somalia), and Mexico. Sporadic cases are observed outside the endemic zone, in North and Equatorial Africa, Southeast Asia, the Middle East, the Caribbean, and Central and South America. Mycetoma is also occasionally reported in countries with a temperate climate (United States, Romania, Bulgaria), and these are considered to be local or indigenous cases.

Areas that are endemic for mycetoma have a characteristically hot and dry climate, with a short rainy season of 1 to 4 months and annual rainfall totals of 100 to 1000 mm. The alternation of wet and dry seasons seems to favor the disease. Rainfall influences the distribution of etiologic agents, and certain species predominate in particular geographic areas. For example, S. somaliensis predominates in the desert zones of Africa, whereas A. pelletieri predominates in parts of West Africa that receive 500 to 800 mm of rain annually. Nocardia spp. and fungal agents giving rise to white grains ( P. boydii , Acremonium spp., Fusarium spp.) are isolated in wetter parts of Africa, such as the Ivory Coast and the Democratic Republic of Congo, outside the Mycetoma Belt.

Worldwide, actinomycetoma is more frequent than eumycetoma, accounting for 63% of the 2500 mycetoma cases reviewed by McGinnis. However, the distribution of etiologic agents differs radically between endemic areas. For example, in Mexico, Nocardia brasiliensis and Actinomadura madurae account for 96% of all isolates from mycetomas. In contrast, eumycetoma is more frequently encountered in Sahelian Africa, accounting for 44% to 64% of cases, most of which are caused by Madurella mycetomatis .

Etiology

The main causal organisms are environmental fungi and bacteria (exogenous actinomycetes) that have been isolated from the soil and plants (see Tables 14-1 and 14-2 ). Thorns and plant fragments may be found in histologic sections of lesions, suggesting that they play a role in transmission of the etiologic agent to humans. Inoculation may occur following wounding with thorns, splinters, or tools. Most of the patients come from rural areas, where they work outside as farmers, animal breeders, carpenters, or builders. The infection is more frequent in men than in women, with a sex ratio of 4:1, and tends to occur in young adults (between the ages of 20 and 40).

Clinical Signs and Symptoms

Mycetoma principally affects the extremities, with the lower limbs affected in 70% to 80% of cases, most of which involve the foot ( Figures 14-1 and 14-2 ) and leg. In most case series from India and Africa, foot involvement is reported in more than 60% of cases. The second most frequently affected site is the hand, observed in 10% of cases, but other sites have been reported, including the back, thigh, knee, neck, scalp, buttock, and perineum. However, these percentages may differ between endemic areas. In Mexico the percentage of cases of mycetoma of the foot is lower, whereas the frequency of infections of the leg and back is higher.

The incubation period from inoculation to clinical manifestation is thought to last from several months to years. Mycetoma typically begins as a small subcutaneous nodule, the size of a groundnut, or an induration of the skin of exposed areas (see Figure 14-1 ). The infection progresses slowly and insidiously, involving the surrounding subcutaneous tissue. The affected area gradually increases in size, with accompanying local inflammation. The patient may report a history of trauma or an injury sustained several months earlier in that site that has now healed. Some patients may report being pricked by a thorn, but the reliability of these reports is not always evident. As the infection progresses, abscesses develop and the swelling gradually increases, resulting in stretched, shiny skin over the lesion with hypo- or hyperpigmentation, punctuated with draining sinuses. The sinuses and fistulas appear after several months of progression and may open within the swelling or at a neighboring site (see Figure 14-2 ), releasing grains or granules. Grains may range from 0.2 to 5 mm in diameter and may be black, white, gray, yellow, pink, or red. Despite the presence of extensive, voluminous tumors, lesions often remain painless for long periods, possibly explaining why patients wait so long before seeking medical assistance.

Pain appears with osseous involvement and may also be due to secondary bacterial infection. After several years of progression, the infection may extend to deeper structures, and complications are common at this stage. The most frequent complication, bone involvement, is observed principally in the foot and hand, but surprisingly, a few spontaneous fractures have also been described caused by bone involvement. The bone involvement is usually characterized by nocturnal pain and associated with a poor prognosis. Extension to the bone may also be correlated with adjacent arthritis, particularly of the metatarsus or knee. No hematogenous spread has been reported, but a lymphatic inflammatory reaction has been seen in a few cases (less than 5%) and must be distinguished from secondary extension of the infection to draining lymph nodes. Other complications are rare but include deep organ involvement (liver, spleen, peritoneum), local invasion of the perineum, neck mycetoma, paraplegia in cases of intraspinal mycetoma, and, in exceptional cases, pleurobronchial fistulas.

Eumycetoma and actinomycetoma differ in several ways. Eumycetoma follows a slow time course and the associated swelling is generally well demarcated. A few abscesses and draining sinuses are observed and bone involvement occurs at a late stage of disease. Conversely, actinomycetoma is typically more aggressive and develops more rapidly than eumycetoma. Numerous abscesses and draining sinuses are observed. Bone involvement and lymphatic dissemination are more frequent in actinomycetoma, particularly when caused by Nocardia spp. or A. pelletieri .

Diagnosis

The diagnosis is often suspected on clinical grounds such as macroscopic features of the lesion and the geographic origin of the patient. In endemic areas, swollen lesions on the extremities with sinuses and fistulous tracts discharging serosanguineous fluid are highly suggestive of the infection, particularly if grains are visible in the discharge. However, clinical diagnosis may be difficult in limited lesions (termed minimycetoma ). In addition, diagnosis cannot be affirmed in the absence of a biopsy, which is frequently not performed in cases of cystic infection.

A definitive diagnosis of mycetoma can be made on the basis of histology or culture of the microorganism. However, the culture of grains, pus, or biopsy specimens is currently the only method for obtaining the definitive identification of the infecting fungus or bacterium to genus and species level, as several grains display similar patterns. To perform culture, discharged grains are collected by histology. If no grains are macroscopically visible, the direct examination of pus may reveal microscopic grains (e.g., Nocardia spp. grains measure 25 to 150 μm in diameter). Sabouraud dextrose agar (SDA) slants containing chloramphenicol, with or without cycloheximide for eumycetoma, are inoculated with clinical samples. For actinomycetes and Nocardia , Sabouraud medium with cycloheximide can be used, but some authors recommend the Loewenstein or Bennett medium. Cultures should be incubated at 37° C and 25° C, because some species grow poorly at temperatures above 30° C. Indeed, the maximum temperature at which growth occurs is a diagnostic criterion. Cultures should be extended for 4 to 6 weeks before declaring a negative result, as some species grow slowly.

Identification is based on both macroscopic and microscopic characteristics of the cultured isolate. Macroscopic characteristics include growth rate, color, texture, and the presence or absence of pigment diffusing in the agar. For several species, particularly for black-grain mycetoma agents, sporulation does not occur or is delayed on standard mycologic media. Subcultures on specialized (e.g., nutrient-poor) media are therefore often necessary to obtain the asexual or sexual reproductive features required for identification to species level. Colonies of Madurella mycetomatis grow better at 37° C; they are gray or black and display diffusion of a dark brown pigment. Madurella grisea grows at 30° C. Its colonies are gray or black and produce no diffusing pigment. Microscopically, some species have an essentially sterile mycelium (Madurella grisea), whereas characteristic diagnostic structures are obtained for other species (Pyrenochaeta romeroi, Leptosphaeria senegalensis) . For white-grain mycetoma agents ( Pseudallescheria boydii, Fusarium spp., Acremonium spp.), microscopic identification is generally easier, at least to the genus level.

Streptomyces somaliensis usually generates white or yellow colonies within about 1 week. A. madurae colonies are whitish or pink, and A. pelletieri colonies are red. Colonies of Nocardia asteroides develop in 1 to 3 weeks. They are small, cerebriform, and initially yellow or pink, subsequently becoming orange. These criteria are generally sufficient for preliminary identification, and laboratory methods based on biochemical criteria, such as the hydrolysis of agar and casein, are seldom used. Definitive identification to the species level requires the careful and reliable use of cultural and morphologic patterns. For more details, readers are referred to specialty texts on this topic.

Molecular biology tools performed primarily on culture isolates such as PCR with sequencing, and more recently mass spectrometry, are of great value when precise identification cannot be achieved by standard mycologic techniques.