Fungal Nail Infection: Signs, Causes, Treatment & Prevention

What Is a Fungal Nail Infection?

A fungal nail infection — medically called onychomycosis or tinea unguium — is an infection of the nail plate (and often the nail bed beneath it) by pathogenic fungi. It is the single most common nail disorder worldwide, accounting for more than 50% of all nail disease presenting to dermatologists, with a global prevalence estimated at around 5.5%.

Toenails are affected far more often than fingernails, in a ratio of roughly 4:1, because footwear creates a warm, dark, humid environment in which fungi thrive. The infection is not merely cosmetic: severe onychomycosis can cause pain during walking, impair balance in older adults, and serve as a portal of entry for secondary bacterial infection — a particularly serious concern in people with diabetes or peripheral vascular disease.

What Causes Fungal Nail Infections?

Three groups of fungi cause onychomycosis:

1. Dermatophytes (~70% of cases) The most frequent culprit is Trichophyton rubrum, a dermatophyte that also causes athlete's foot (tinea pedis). T. mentagrophytes and Epidermophyton floccosum are other common dermatophyte species. Dermatophytes are uniquely adapted to digest keratin — the protein that makes up the nail plate — allowing them to invade and colonize nail tissue.

2. Non-dermatophyte moulds (~20% of cases) Moulds such as Scopulariopsis brevicaulis, Fusarium species, and Aspergillus species can also infect nails, particularly in people with prior nail damage. Identifying these organisms matters because some are resistant to terbinafine and require different treatment.

3. Yeasts (~10–20% of cases) Candida species more commonly infect fingernails than toenails, especially in people who have frequent hand-water exposure (dishwashers, healthcare workers, domestic workers) or immunosuppression. Candida nail infection often co-occurs with paronychia — inflammation of the surrounding nail fold.

How Does Infection Spread?

Fungal spores survive for months in warm, moist environments — shower floors, pool surrounds, locker rooms, yoga mats, and the insides of old shoes. Most toenail infections begin as athlete's foot on the surrounding skin; the fungus then invades the nail from its free edge or lateral margins. Sharing nail clippers or nail files with an infected person can also transmit infection.

Risk Factors

Not everyone who walks barefoot in a gym locker room develops onychomycosis. Several factors increase susceptibility:

• Older age — prevalence rises sharply with age; those over 60 have a significantly higher risk than younger adults
• Male sex — men are affected roughly twice as often as women
• Tinea pedis — athlete's foot is the single strongest predisposing factor for toenail onychomycosis
• Diabetes mellitus — impaired immunity and circulation
• Peripheral vascular disease — poor blood flow to the feet
• Immunosuppression — HIV, organ transplant medications, chemotherapy
• Psoriasis — structural changes in the nail plate create points of entry
• Hyperhidrosis (excessive sweating) — a persistently moist nail environment
• Nail trauma — from tight shoes, sport, or repetitive microtrauma

Symptoms: What Does a Fungal Nail Infection Look Like?

Onychomycosis has a gradual onset — most people notice the nail changing over months before seeking help. The classic presentation includes:

Distal and Lateral Subungual Onychomycosis (DLSO) — most common type

• Yellow, brown, or white discoloration starting at the nail's free edge or sides
• Subungual hyperkeratosis — a build-up of crumbly debris under the nail plate, causing the nail to lift away from the nail bed (onycholysis)
• Nail thickening and distortion
• Nail brittleness — the nail crumbles or breaks easily

Superficial White Onychomycosis (SWO)

• Flaky, chalky white patches on the surface of the nail plate
• Caused most often by T. mentagrophytes or non-dermatophyte moulds
• Can superficially resemble nail cosmetic damage

Proximal Subungual Onychomycosis (PSO)

• Infection begins at the nail fold near the cuticle and spreads distally under the nail
• Uncommon in immunocompetent individuals; strongly associated with HIV infection

Total Dystrophic Onychomycosis

• End-stage disease: the entire nail plate is destroyed and replaced by crumbling, thickened debris
• May result from any of the above forms if left untreated

What Onychomycosis Is NOT

Several conditions cause nail changes that look like fungal infection — including nail psoriasis, lichen planus of the nail, and simple nail trauma. This is precisely why laboratory confirmation matters before starting oral antifungal drugs.

Diagnosis: Why You Should Confirm Before Treating

Before starting oral antifungals — which require liver monitoring and have drug interaction potential — a clinician should confirm the diagnosis with:

• KOH (potassium hydroxide) microscopy: A nail scraping is dissolved in KOH and examined under a microscope for fungal hyphae. Rapid and inexpensive, but can miss infection if sampling is inadequate.
• Fungal culture: Identifies the specific organism and its antifungal sensitivities. Takes 2–4 weeks. Critical if a non-dermatophyte mould or Candida is suspected.
• Nail plate histopathology (PAS stain): Examines nail clippings embedded in wax; high sensitivity.
• PCR: Rapid molecular test with the highest sensitivity; increasingly available in specialist centers.

Up to 50% of abnormal-looking nails sent for culture in some series do not have confirmed fungal infection — underlining why laboratory testing before systemic treatment is the clinical standard.

Treatment Options

Oral Antifungals — First-Line for Moderate-to-Severe Disease

Terbinafine (250 mg daily) The gold-standard oral therapy. It works by inhibiting squalene epoxidase — a fungal enzyme essential for ergosterol synthesis — and reaches fungicidal concentrations in the nail plate that persist long after the course ends. Standard course: 12 weeks for toenails, 6 weeks for fingernails. Mycological cure rates for dermatophyte infections: approximately 70–80%. Adverse effects are generally mild (gastrointestinal upset, headache, taste disturbance) but rare serious hepatotoxicity means baseline and on-treatment liver function tests are advisable.

Itraconazole An azole antifungal used as an alternative when terbinafine is contraindicated or for non-dermatophyte mould infections. Can be given as continuous dosing (200 mg daily for 12 weeks) or pulse dosing (400 mg daily for one week per month for three months). More drug interactions than terbinafine.

Fluconazole Less commonly used and less effective; sometimes used for Candida nail infections.

Topical Antifungals — Suitable for Mild-to-Moderate or Limited Disease

Three FDA-approved topical options exist:

Complete cure rates for topical therapies appear modest because "complete cure" requires both mycological and clinical clearance simultaneously; mycological (lab) cure rates are considerably higher. Topical treatment requires patient commitment — typically 48–52 weeks of daily application — and is most appropriate when fewer than three nails are affected and involvement does not extend to the nail matrix (lunula).

Combination Therapy

Evidence supports combining oral terbinafine with a topical agent (particularly efinaconazole) to improve complete cure rates, especially in patients with difficult-to-treat infections, thick nails, or dermatophytomas (fungal balls embedded in the nail).

Nail Debridement

Regular mechanical or chemical nail debridement (thinning the nail plate) by a podiatrist or clinician reduces the fungal burden and improves penetration of topical treatments. It is an adjunct, not a standalone cure.

Device-Based Therapies (Evidence Limited)

Laser devices and photodynamic therapy have been studied but current evidence does not support them as standard-of-care treatments. The AAD does not recommend them as first-line options.

How Long Does Treatment Take?

Treatment is slow because antifungal drugs do not "push out" the infected nail — they prevent new nail growing in from the matrix from becoming infected. The visible nail itself must grow out and be clipped away. Toenails grow approximately 1–1.5 mm per month; a big toenail may take 12–18 months to fully clear even after the course of antifungals is finished. It is normal for the nail to look unchanged — or even worse — during the first 1–2 months of treatment.

Prevention: Stopping Reinfection

Recurrence rates for onychomycosis are high — reported at 20–25% within one year — largely because the source of infection (athlete's foot on the skin, contaminated footwear) is not addressed. To prevent reinfection:

• Treat any concurrent athlete's foot with a topical antifungal (terbinafine cream, clotrimazole) until fully clear
• Disinfect or replace old footwear — fungi can survive in shoes for months
• Use antifungal foot powder inside shoes daily
• Wear moisture-wicking socks and change them if damp
• Avoid walking barefoot in communal showers, pool surrounds, and locker rooms — wear flip-flops
• Keep nails trimmed short and dry
• Use separate nail clippers for infected and uninfected nails; disinfect clippers with alcohol between uses
• Dry feet carefully after bathing, especially between the toes

Onychomycosis in People with Diabetes

People with diabetes face a threefold higher prevalence of onychomycosis and are at elevated risk of serious complications, including secondary bacterial cellulitis from nail-related skin breaks. Oral antifungal therapy is generally recommended rather than topical therapy alone for diabetic patients with onychomycosis, but drug interactions with diabetes medications and the need for liver monitoring require close prescriber oversight. A podiatrist familiar with diabetic foot care should be part of the management team.

When to See a Professional

See your GP, dermatologist, or podiatrist if:

• You notice nail thickening, discoloration, crumbling, or lifting — especially affecting multiple nails
• A nail has become painful or is causing difficulty walking
• You have diabetes, peripheral vascular disease, or immunosuppression — do not delay
• OTC antifungal treatments (nail lacquers) have not improved the nail after 3–6 months
• You are unsure whether your nail change is fungal or another condition

Do not start oral antifungal tablets without a clinician's assessment and, ideally, laboratory confirmation of infection.