Almirall
Onychomycosis

Onychomycosis triggering factors

What causes onychomycosis?

Onychomycosis is a fungal infection affecting the structures that make up the nail unit, including the nail plate, nail bed, and nail matrix. It appears when microscopic fungi adhere to the nail and gradually invade the deeper keratin layers. Both fingernails and toenails can be affected, but toenails are far more commonly involved. This is largely due to repeated minor trauma, pressure from footwear, and the consistently warm, humid environment inside shoes, all of which favour fungal growth. 

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Most cases are caused by dermatophytes, a type of fungi that tend to infect skin, hair, and nails. Across Europe, these organisms account for the majority of confirmed infections, although exact proportions vary between studies. Within this group, Trichophyton species dominate. Trichophyton rubrum is consistently identified as the single most frequent cause worldwide, followed by species within the Trichophyton mentagrophytes complex. Their ability to digest keratin allows them to persist in nails for long periods if untreated. Less commonly, Epidermophyton floccosum and certain Microsporum species can also be responsible.

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A smaller but clinically important proportion of cases is linked to non-dermatophyte fungi. These include:

  • Non-dermatophyte moulds such as Aspergillus, Scopulariopsis, Fusarium, Acremonium and Alternaria. These organisms are widespread in the environment and may be found in nail samples as contaminants, so confirmed infection requires strict diagnostic criteria. When they do cause disease, it often follows prior damage to the nail, previous onychomycosis, psoriasis or trauma.
  • Yeasts, mainly Candida species. These are more often seen in fingernails, particularly in individuals with frequent exposure to water, those working in wet environments or those with underlying medical conditions.
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Recent research has emphasized that mixed infections—where dermatophytes coexist with moulds or yeasts in the same nail—are more common than once thought. These cases can be more challenging to diagnose and treat, underscoring the value of proper mycological testing.

Regardless of the organism involved, the disease typically progresses in a similar way. Fungi attach to the nail surface or underside, spread through and beneath the nail plate, and gradually break down keratin. This leads to the classic signs of onychomycosis: thickened nails, crumbling edges, discolouration ranging from white to yellow or brown and detachment of the nail plate from the nail bed. 

What are the main risk factors?

A variety of predisposing factors render certain individuals more vulnerable to onychomycosis. The most significant include: 

Advancing age

 Infection is far more common in people over 60, due to slower nail growth, reduced circulation, cumulative exposure to fungi and a gradual decrease in immune responsiveness.

Pre‐existing fungal skin infection

Athlete’s foot (tinea pedis) is a frequent precursor, as it provides a direct source of fungi that can spread to the nails.

Nail or skin trauma

Even minor injuries, pressure from tight shoes, or repeated friction can damage the nail barrier and enable an infection to develop.

Walking barefoot in wet public places

Swimming pools, communal showers, and changing rooms expose individuals to fungi on shared surfaces.

Occlusive or poorly ventilated footwear

Shoes that trap heat and moisture create ideal conditions for fungi to thrive.

Diabetes mellitus and peripheral vascular disease

Reduced circulation and impaired healing increase the likelihood of infection and hinder recovery.

Obesity

Several obesity-related factors contribute to higher risk, including increased perspiration, reduced circulation, and additional mechanical stress on the feet caused by excess weight, all contribute to higher risk.

Immunosuppression or other comorbidities

Conditions such as HIV, chronic venous insufficiency, and other health issues reduce the body’s ability to defend against infection.

Nail and skin conditions such as psoriasis

These may alter the nail structure and increase vulnerability to fungal invasion.

Genetic predisposition and household transmission

Shared bathing areas, footwear, or nail care tools can facilitate transmission within families.

 

These risk factors often act together. For example, an older adult with diabetes who uses communal showers and wears tight, non-breathable shoes is at particularly high risk. 

How to prevent onychomycosis

The best ways to prevent onychomycosis are to maintain good foot and nail hygiene, reduce exposure to fungi, and avoid behaviours that promote infection. Helpful practices include: 

Good hygiene

Wash hands and feet daily, especially after touching infected nails. Dry thoroughly and apply moisturizer to prevent cracking.

Careful nail trimming

Cut nails straight across and smooth the edges. Reduce thickened areas if present. Clean or disinfect nail tools after every use.

Moisture control

Wear socks that absorb perspiration and change them during the day if they become damp.

Breathable footwear

Choose shoes made from materials that allow ventilation, such as leather or mesh.

Addressing old footwear

Replace worn shoes or treat them with antifungal products, as spores can persist and cause reinfection.

Manage old footwear

Use flip-flops or shower shoes in places such as pools, gyms, and shared bathrooms.

Safe nail salon practices

Ensure instruments are sterilized between clients, or bring your own clean tools.

Avoid nail polish and artificial nails with active or recurrent infections

These can trap moisture and make it more difficult to monitor your nails.

Consistently following these measures can significantly reduce the risk of developing onychomycosis or experiencing a recurrence after successful treatment. 

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