Tinea nigra is an infection that attacks the skin’s uppermost layers. It’s caused by a fungus called Hortaea werneckii. The fungus has also gone by the names of Phaeoannellomyces werneckii, Exophiala werneckii, and Cladosporium werneckii.

This fungus is found in the soil, sewage, and decomposing vegetation of tropical or subtropical coastal regions. In particular, these areas include the Caribbean and the coast of South America. Tinea nigra is rare in the United States, but when it’s seen, it’s usually in the hot, humid climates of the Southeast.

The fungus causes painless brown or black patches to grow on palms and feet. Sometimes other areas of the body, such as the neck and trunk, can be affected.

Tinea nigra stems from infection with the fungus Hortaea werneckii. Direct contact with the fungus is necessary for transmission. Shaking the hand of someone with tinea nigra, for example, won’t spread the condition.

The fungus can infiltrate skin via open wounds or cuts. It tends to thrive on wet, clammy, sweaty skin, which is why the palms of the hands and the soles of the feet seem to be common targets for the infection.

Lesions typically appear about two to seven weeks after exposure to the fungus, according to research in Dermatology Online Journal. And while the condition can strike anyone, the journal Anais Brasileiros de Dermatologia reports it’s typically seen in women under the age of 20.

Tinea nigra is largely painless and harmless, but it does produce a few symptoms. They include:

  • A brown or black patch resembling a stain that usually occurs on the palm of the hand or, more rarely, on the sole of the foot. In one study published in Studies in Mycology, 19 of 22 people with tinea nigra had the patches on their palms while only three had them on their feet.
  • The patch is generally flat, with defined borders.
  • The darkest area of the patch is at the edges. Shading gets lighter as it extends inward. This darker outside area may look like a halo.
  • The lesion is slow-growing and usually appears on only one hand or foot.

Your doctor will perform a physical exam and ask about your medical history as well as your recent travels.

Tinea nigra can look like more serious skin conditions, such as malignant melanoma, a deadly form of skin cancer that can present as dark patches. Because of this, your doctor may want to scrape a sample of the lesion and send it to a lab for testing. In some cases, the lesion can be scraped entirely away and not require any further treatment.

Tinea nigra affects the uppermost layers of skin. Because of this, it responds well to topical ointments and creams. These medications are applied directly to the skin.

Your doctor may recommend medications like salicylic acid, urea, or benzoic acid. These speed cell turnover and cause skin to shed. Antifungal creams used for two to four weeks are also effective. In some cases, drying agents such as aluminum chloride are prescribed.

Since the fungus that causes tinea nigra is found in soil, sewage, and rotting vegetation, the best way to prevent infection is to protect your skin. Wear shoes if you’ll be walking in the hot, humid regions where the fungus is found. If there’s any risk you’ll be touching vegetation — for instance, if you’ll be hiking, gardening, or planting — be sure to also wear gloves.

Tinea nigra is a rare and harmless skin condition. With treatment, it usually resolves in a few weeks. It has no lasting side effects and is unlikely to recur unless you’re reexposed to the fungus.