What is mycetoma?
Mycetoma is a chronic infection of the skin, subcutaneous tissue and sometimes bone characterised by discharging sinuses filled with organisms. It is generally found on the foot, where it is sometimes given the name watering can foot. Mycetoma is also known as Madura foot.
What causes mycetoma?
Mycetoma may be due to several fungi (when it is called eumycetoma) or actinomycetes (actinomycetoma). Actinomycetes are bacteria that produce filaments, like fungi. Both the fungi and the actinomycetes are found in soil and plant material in tropical regions.
The organism is inoculated into the skin by a minor injury, for example, a cut with a thorn when barefoot. It is not endemic in New Zealand but mycetoma is occasionally diagnosed in native Pacific Islanders.
The most common fungi to cause mycetoma with black grains are:
- Leptosphaeria senegalensis (Africa)
- Madurella grisea (Africa, South and Central America)
- Madurella mycetomatis (Worldwide)
- Pyrenochaeta romeroi (Africa, South America)
The most common fungi to cause mycetoma with white grains are:
- Acremonium species (Africa, Middle East)
- Aspergillus nidulans (Africa, Middle East)
- Noetestudina rosatii (Africa)
- Pseudallescheria boydii (Worldwide)
The most common actinomycetes to cause mycetoma with white/yellow grains are:
- Actinomadura madurae (Worldwide)
- Nocardia asteroides (Worldwide)
- Nocardia brasiliensis (Central America)
Brown or red grains occur in mycetoma due to:
- Actinomadura pelletieri (Africa)
- Streptomcyes somaliensis (North Africa, Middle East).
What are the clinical features of mycetoma?
Mycetoma is more common in men than women, particularly those aged 20 to 50. It generally presents as a single lesion on an exposed site and may persist for years. Two thirds arise on the foot.
- It starts as a small hard painless lump under the skin.
- It grows slowly but eventually involves underlying muscles and bones.
- The middle of the lesion caves in, ulcerates and discharges pus, which contains grains.
- Eventually, sinus tracts (holes) develop which also discharge pus and grains.
- The surface skin is scarred and pale.
- Considerable deformity often makes it difficult to walk.
- Mycetoma may cause no discomfort but it often itches or burns.
- Secondary bacterial infection is common.
Differential diagnosis of mycetoma
Myceetoma is occasionally confused with other skin conditions such as:
- Other fungal infections such as chromoblastomycosis
- Bacterial infections such as osteomyelitis, atypical mycobacterium infection, tuberculosis, leprosy and syphilis
How is mycetoma diagnosed?
The diagnosis of mycetoma depends on identifying grains. These are obtained using a needle and syringe to extract material from a soft part of the lesion under the skin or by collecting pus. Occasionally a skin biopsy is necessary, which shows characteristic histopathological features of mycetoma and may reveal the organisms.
Laboratory tests for mycetoma
In mycetoma, the colour of the grains may suggest the likely diagnosis; black grains suggest a fungal infection, minute white grains suggest nocardia and red grains are due to Actinomadura pelletieri. Larger white grains or yellow-white grains may be fungal or actinomycotic in origin.
Microscopy using potassium hydroxide (KOH) confirms the diagnosis and type of mycetoma.
- Actinomycotic grains contain very fine filaments.
- Fungal grains contain short hyphae (branched filaments) that are often swollen
Several agar plates are cultured at 25–30 degrees celcius and 37 degrees celcius for up to six weeks. Fungi grow more quickly than actinomycetes.
What is the treatment for mycetoma?
Mycetoma does not resolve without active treatment.
Actinomycetoma responds well to treatment with appropriate antibiotics but they are required for months or years. The sinuses dry up, swelling and tenderness improves and the grains disappear. Deformity may persist.
Single or combination treatment is used:
- Streptomycin injections
- Oral cotrimoxazole (Apo-Sulfatrim®, Bactrim®, Septrin®, Trimel®, Trisul®)
- Amikacin (Amikin®)
- Rifampicin (Rifidin®)
- Minocycline (Minomycin®, Minotabs®).
Eumycetoma is more difficult to treat.
- Surgery to remove the affected tissue completely. These may mean amputation if bone is involved.