Types of Nail Fungus

Fungal infection of toe nails and fingernails (onychomycosis) is very common and seems to be increasing, but many people do not know that there are a number of different types of nail fungus infection. Not only are there different species of fungus that can infect nails, fungi also affect the nail in different ways. This article describes the different manifestations of nail fungus infection.

The most common of the various types of onychomycosis is distal subungual onychomycosis. In this condition, the infection typically begins at the hyponychium, the area at the toe or fingertip under the edge of the nail. It is usually caused by the dermatophyte Trichophyton rubrum, a fungus adapted for living in hair, skin, and nails. From the site of infection, the fungus spreads through the nail matrix backwards toward the cuticle and along the sides of the nail. There is some evidence that certain people are more susceptible to distal subungual onychomycosis, a susceptibility that is passed genetically. Unlike some of the other types of nail fungus, dermatophytes can spread to the surrounding skin, and often do.

More uncommon among the types of onychomycosis is white superficial onychomycosis, an infection that typically affects the toenails but not the fingernails. Usually caused by Trichophyton mentagrophytes, another dermatophyte, the fungus begins to grow in the surface superficial layers of nails and subsequently spreads to involve the whole surface of the nail. As with all types of onychomycosis, the fungus breaks down the nail itself to derive nutrients, and in doing so, makes the nail soft, crumbly and deformed.

The most unusual of the types of nail fungus infection is proximal subungual onychomycosis, so uncommon in healthy people that its appearance may signal a abnormality with the immune system. Trichophyton rubrum is usually the culprit, gaining access to the nail bed through the proximal nail fold, the flap of skin that covers the nail just behind the cuticle. From its starting point, the fungus invades the root, where nail is built, and then spreads outward toward the tip of the nail. In a susceptible individual, this type of onychomycosis can affect both fingernails and toe nails equally.

Candida spp., yeasts, are also known to infect nails and the surrounding tissue, and cause some of the types of onychomycosis familiar to dermatologists. Candidal infection of the nails themselves sometimes occurs after injury to the nail, and is sometimes seen in patients who have candidal infections in other locations such as the mouth or vagina. A type of nail fungus infection known as paronychia involves the tissue adjacent to the nail and is generally a rapidly advancing and uncomfortable condition that brings the patient swiftly to the doctor's office or emergency department. Paronychia tends to strike individuals who have their hands in water for long periods.

Finally, any of the types of onychomycosis described above can culminate in total dystrophic onychomycosis, a condition where the entire nail is destroyed by fungal growth.