Nail fungus: what do we know about it?

Onychomycosis, the medical term for fungal nail infection, is the most common nail disorder worldwide.

Although its overall prevalence is estimated at 5.5%, it is strongly correlated with age. While it affects children only rarely, it occurs in more than 20% of adults over 60 years old. This infection is not just a cosmetic inconvenience; it can cause local pain, discomfort while walking, and a significant social impact. Nail fungus usually develops slowly and insidiously. Unlike trauma, which causes a sudden hematoma, onychomycosis gradually alters the structure of the nail. The most common symptoms include:

• Color change : The nail loses its natural transparency and takes on a whitish, yellow, or brownish hue. In some rarer cases, green or black spots may appear.

• Thickening, that is, hyperkeratosis : The nail bed accumulates keratin debris, making the nail plate abnormally thick and difficult to cut.

• Fragility : The nail becomes brittle, crumbles at the edges, or splits into layers.

• Onycholysis (nail lifting) : The nail gradually separates from its nail bed, creating an empty space where debris accumulates.

To reliably distinguish a fungal infection from another condition, such as eczema, psoriasis, or repetitive trauma, which is common in athletes, dermatologists now use dermoscopy, also called dermatoscopy. For this, they use a dermatoscope, an illuminated magnifying device that enlarges the image 10-fold and allows for better visualization of the nail. This non-invasive examination makes it possible to observe highly specific signs, such as a “saw-tooth” proximal edge in the area where the nail is detached, or colored longitudinal streaks. A particularly suggestive sign is the dermatophytoma, which appears as a band or opaque spot inside the nail itself, indicating a dense cluster of fungi.

In some cases, a nail sample may be taken for analysis in order to rule out other conditions that mimic fungal infection.

Contrary to some common misconceptions, onychomycosis is not the result of poor hygiene.

This condition is caused by the invasion of the nail plate by opportunistic microorganisms. These pathogens have specific enzymes, such as proteases and keratinases, that can break down keratin—the rigid structural protein that makes up the nails—allowing them to feed on it and spread through the tissues. Three major groups of fungi are responsible for fungal infections of the fingernails and toenails:

• Dermatophytes : These are the main causative agents. The most common by far is Trichophyton rubrum, responsible for more than half of all infections, followed by Trichophyton mentagrophytes. These fungi show a preference for the toes, often developing after an untreated athlete’s foot.

• Non-dermatophyte molds : Species such as Aspergillus or Fusarium can colonize the nail, often following a prior traumatic injury.

• Yeasts : The genus Candida, particularly Candida albicans, is often involved. Yeasts more commonly affect the fingernails, frequently in individuals whose hands are regularly immersed in water.

A major scientific discovery has recently shed light on infection resistance: the formation of biofilms. Instead of remaining isolated, fungi organize themselves into tightly connected, structured communities, enclosed within a protective matrix. This process is extremely rapid. In just 72 hours, the biofilm is fully formed and firmly anchored to the nail plate. This biological shield acts as a true fortress that prevents antifungal molecules from penetrating effectively and allows the fungi to evade the immune system. It is this structure that explains why the fungal infection is so persistent and why recurrences are common if treatment is not continued until the nail has completely grown back.

Onychomycosis is a highly contagious condition whose spread is promoted by genetic, medical, and environmental factors. Once the infection has managed to establish itself in this favorable setting, the risk of contaminating other family members is estimated to be nearly 45%, through the sharing of towels or bath mats.

Onychomycosis does not always present in the same way. Dermatologists classify the infection into several subtypes according to the part of the nail that is first affected and the way it progresses.

• Distal and lateral subungual onychomycosis : This is the most commonly encountered form. In this condition, the fungus penetrates under the nail, starting at the free edge or from the sides. The infection then progresses toward the nail root, causing gradual separation of the nail plate and an accumulation of yellowish or brownish debris. This is the form that gives the nail its typical appearance of a thickened, brittle horn-like structure.

• Superficial white onychomycosis : This form is characterized by a direct attack on the upper surface of the nail, without spreading from underneath. It manifests as small white, opaque, chalky spots that can eventually cover the entire nail surface. Unlike other forms, the fungus remains on the surface. The spots can in fact be easily scraped or buffed off during a pedicure. It occurs almost exclusively on toenails.

• Proximal subungual onychomycosis : This condition is rarer and, paradoxically, begins at the nail root, under the cuticle, then progresses toward the tip. It presents as a white spot that appears at the lunula, the light, crescent-shaped area located at the base of the nail.

• Endonyx onychomycosis : This is a specific variant in which the fungus directly invades the inner portion of the nail plate, without infecting the underlying nail bed. Visually, the nail develops a milky discoloration and may show cracking or layered splitting. Notably, in this form the nail remains firmly attached to the skin and does not exhibit the typical thickening seen in more classic types of nail fungal infections.

• Total dystrophic onychomycosis : This represents the end stage of the infection when it has not been treated in time. At this point, the entire nail unit is colonized and destroyed by the fungus. The nail becomes extremely thick, completely opaque, and breaks up into crumbly fragments until it disintegrates entirely. At this stage, the nail matrix may be permanently damaged, making the regrowth of a healthy nail much more uncertain.

Treating a fungal infection is a long-distance process that directly depends on how fast the nail grows back. Since the nail on a hand grows about twice as fast as the one on a foot, both the duration and the treatment strategy vary depending on the area affected. Before starting any treatment, it is strongly recommended to consult a healthcare professional so they can confirm the diagnosis and, if needed, take a mycological sample to send to a laboratory. This makes it possible to confirm the presence of a fungus and identify its exact type, as some treatments are more effective against specific strains. Once the diagnosis has been made, several options are generally available.

For mild to moderate infections, particularly when less than 50% of the nail is affected and the matrix is spared, topical treatments are preferred. These consist of medicated nail lacquers containing amorolfine or ciclopirox olamine, applied on a regular basis. These products penetrate the nail plate to eliminate the fungus at the site of infection. For superficial white onychomycosis, simply scraping the nail surface combined with the lacquer may be sufficient. Patience is essential: treatment must be continued without interruption until a completely healthy nail has grown back, which takes about 6 months for fingernails and 9 to 12 months for toenails.

When the fungal infection is deep, reaches the nail matrix, or affects several nails, topical treatments are no longer sufficient because they do not penetrate far enough. The physician then prescribes oral antifungal medications, most often terbinafine. These tablets travel through the bloodstream to reach the nail matrix and become directly incorporated into the newly forming keratin. This treatment is considered the standard of care for severe cases, with high cure rates. However, it requires medical follow-up to monitor for potential side effects, particularly those affecting the liver.

In certain cases, laser sessions are recommended. Although costly and not covered by insurance, laser therapy is an effective treatment for nail fungal infections, and works by heating the fungus to destroy it. It is often used as an alternative for people who cannot take oral medication.

Advice : Additionally, it is strongly recommended to treat your shoes with antifungal powders. This helps prevent relapses, as fungal spores can survive for a very long time in closed environments.

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