Topic Overview

There are four major types of fungal nail infection. They are named for the area where the fungal infection starts. Dermatophytes cause almost all fungal nail infections.

• Distal subungual onychomycosis (DSO) is the most common type of fungal nail infection. It is caused by the same fungi that cause most cases of athlete's foot (dermatophytes). DSO infects the skin under the end of the nail (nail bed) and in the nail. The infection starts at the end of the nail bed, and part of the nail often turns yellow or white. Pieces of skin and nail fragments (debris) build up under the nail. As the condition gets worse, the nail may crumble and split, and it may separate from the skin. A thickened nail and a large amount of debris under the nail may cause discomfort when wearing shoes. DSO can be a lifelong infection and hard to treat. Shoes that fit poorly may make the infection worse or, in some cases, even cause the infection.
• White superficial onychomycosis (WSO), the second most common type of fungal nail infection, accounts for about 7% of all fungal nail infections.¹ It can be easily treated. WSO affects the top layer of the nail, first forming white spots on the nail surface. Eventually the entire surface of the nail becomes covered with a crumbly, chalky powder. The nail does not thicken and does not separate from the skin.
• Candida onychomycosis, or yeast infection of the nail, is uncommon but can affect the nail and the skin bordering the nail (nail folds). This type of fungal infection is more common in fingernails than toenails. It may involve all of the nails at the same time and can cause the nail to separate from the nail bed. It invades weakened areas of the nail, which may become discolored white, green, or brownish, with an odd shape. The nail may look thicker than normal, and there may be signs of infection (reddened, swollen, tender, or warm) in the skin next to the nail (nail fold). Unlike the other types of fungal nail infections, the infection may be painful.
• Proximal subungual onychomycosis (PSO) is caused by dermatophytes and accounts for about 3% of all fungal nail infections.¹ But it is more common in people infected with the human immunodeficiency virus (HIV). PSO infects the base of the nail (skin at the nail fold), often thickening the skin, which can separate from the nail. The base of the nail may appear white, and the nail opaque. The skin on top of the foot may become infected.