Tinea Nigra (Black Skin Fungus) Causes, Symptoms, Treatment

What is tinea nigra?

The word “tinea” refers to fungal infection, and the term “nigra” means black. Tinea nigra refers to a skin infection, which turns the affected skin areas into black or dark brown patches. The soles of the feet and the palms of the hand are the most frequently affected areas in tinea nigra infections. Not everybody is equally prone to develop a tinea nigra infection, for example it is more common in children than in adults and among girls than boys.

Tinea infections are a group of superficial, fungal infections of the skin that are technically referred to as dermatophytosis (commonly known as the ringworm). These infections are usually limited to the outermost layers of the skin, and do not affect the tissue layers underneath. Tinea nigra is no exception. However, tinea nigra is a relatively uncommon infection, compared to the common fungal skin infections such as tinea cruris (fungal infection of the groin), tinea pedis (fungal infection of the feet), and tinea corporis (fungal infection of the body).

Read more on ringworm.

This is because the fungal species that cause other tinea infections are very common in the environment, whereas the species of fungi that causes tinea nigra is relatively rare. Although both tinea nigra and tinea pedis affect the feet, it should be noted that unlike tinea nigra, tinea pedis does not affect the soles of the feet. Tinea pedis is also known as a ringworm infection due to the typical round ring-like lesion and does not cause a dark black spot like tinea nigra.

Tinea nigra is not a serious skin condition. Unlike other tinea infections, tinea nigra produces relatively benign signs and symptoms. Skin inflammation and itching that are the hallmarks of many other fungal skin infections, are not usually present in tinea nigra. This leads to less scratching of the skin, and secondary bacterial infections and permanent scarring rarely occur in this condition. Effective treatments are also available for curing tinea nigra. However, like other fungal skin infections, the treatment period may span many weeks or months.

Signs and Symptoms

Most fungal skin infections are characterized by an intensely itchy skin rash. Tinea nigra is an exception. In most cases, the infected person may not even be aware of the condition till the characteristic brown and black skin rashes appear. These skin rashes affect only the outermost layer of the skin.

Although the skin on any part of the body may show signs of tinea nigra, the skin on the palms of the hands and the soles of the feet are the most commonly affected. The signs and symptoms of tinea nigra do not appear immediately after infection with the causative fungi. There is typically an incubation period of about 2-7 weeks after initial infection before any signs of tinea nigra become obvious.

Unlike in other tinea infections, the skin rash in tinea nigra does not dispaly any redness (technically known as erythema), scaling or pain. The skin rash that is typical of tinea nigra has the following features:

  • Flat skin rash that may occur in a variety of shapes, including round, oval and irregular.
  • Multiple lesions are rare. Solitary lesions are most common.
  • Skin discoloration in the affected regions may vary from light brown to black, depending on the native skin pigmentation of the affected individual.
  • Rash has a clearly defined border, which separates it from the surrounding unaffected skin areas.
  • Skin rash grows slowly, expanding over a period of many weeks or months. The size of the skin rash may increase from a few millimeters to several centimeters over a long period.

Skin rashes of tinea nigra are most common on the palms and soles. However, they may be present in other parts of the body, such as the skin on the chest and the neck.

Read more on itchy feet.

Causes of Tinea Nigra

The fungus, Phaeoannellomyces werneckii, is the most common causative agent of tinea nigra. Phaeoannellomyces werneckii is also known as Exophiala werneckii (former classification), Hortaea werneckii (recent classification), and Cladosporium werneckii.

This fungus is a geophilic brown mold that dwells in compost, soil, and on wood. Phaeoannellomyces werneckii is usually found in the humid regions of the world, such as tropical and subtropical Africa, Australia, Southeast Asia, and South America. However, this fungi is not restricted exclusively to these places, and can be found worldwide.

Other causative fungal species include Stenella araguata and Cladophialophora saturnica. These fungal infections usually affect people who sweat a lot (such as those suffering from hyperhidrosis). It also affects people who are frequently in contact with compost and soil, where the causative fungi of tinea nigra reside.

Diagnosis of Tinea Nigra

Diagnosis of tinea nigra can usually be made upon observation of the typical skin rash of this condition. Confirmation of the diagnosis may be done by scraping the affected skin region on KOH mount and microscopically examining it. The KOH lyses the non-fungal matter in the scrapings, allowing better visualization of the fungi. A fungal culture may also be done to confirm the diagnosis.

It is important to confirm the diagnosis of tinea nigra because the typical dark skin patches that appear in this condition can also be caused by other diseases. Such clearly defined, dark skin patches are also caused by hyperpigmentation, moles, melanoma, chronic skin diseases and coloring dyes.

Treatment of Tinea Nigra

Tinea nigra is not a serious skin condition. Effective topical treatments are available that can cure this condition. The following are some of the main treatment options:

Topical antifungals (such as ketoconazole, and selenium sulfide containing shampoos) can be very effective in treating the skin rashes of tinea nigra. However, the treatment period is long, and may require application of topical antifungals for 3 to 4 weeks. In chronic cases, the treatment period may extend even longer.

To make the topical antifungals more effective in their action on the causative fungi, the affected skin areas can be scraped with a scalpel blade before application of the antifungal medication. However, scraping of the skin should only be done by a trained medical professional.

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