Toenail fungus (Onychomycosis) - Symptoms, Causes, Treatment & Prevention

Overview

Onychomycosis, commonly called toenail fungus, is a chronic infection of the nail plate, nail bed, or surrounding tissue caused by various types of fungi (dermatophytes, yeasts, and non‑dermatophyte molds). It typically presents as thickened, discolored, and brittle nails that may crumble or separate from the nail bed.

Who is affected? While anyone can develop onychomycosis, it is most common in adults over 40, men (approximately 2‑3 times more than women), and people with certain medical conditions (e.g., diabetes, peripheral vascular disease). An estimated 10‑14 % of the global population has some form of nail fungal infection, making it one of the most prevalent dermatologic conditions worldwide.

Symptoms

Symptoms develop slowly and may vary depending on the fungus type and the extent of infection.

• Discoloration: Nails may turn white, yellow, brown, or green.
• Thickening: The nail plate becomes noticeably thicker, making it difficult to trim.
• Texture changes: The surface becomes rough, crumbly, or ragged.
• Shape distortion: Nails may become distorted or develop a “spoon” shape.
• Separation from nail bed (onycholysis): A white or yellow space may appear under the nail.
• Odor: A foul smell can develop as the infection progresses.
• Pain or tenderness: Usually mild, but can become more uncomfortable if the nail is thick or if secondary bacterial infection occurs.
• Spread to other nails: In many cases, more than one toe is involved, and infection can spread to the fingernails.

Causes and Risk Factors

Onychomycosis is caused by fungal organisms that invade the nail plate and matrix. The most common agents are:

• Dermatophytes: Trichophyton rubrum and Trichophyton mentagrophytes (≈ 80 % of cases).
• Yeasts: Candida albicans (more common in people with moist environments or immune compromise).
• Non‑dermatophyte molds: Scopulariopsis, Fusarium (less common but can be stubborn).

Key Risk Factors

• Age > 40 years (reduced nail growth rate).
• Male gender.
• History of athlete’s foot or other fungal skin infections.
• Living in warm, humid climates.
• Occupations requiring prolonged foot moisture (e.g., swimmers, nurses).
• Diabetes or peripheral arterial disease (impaired circulation).
• Immunosuppression (HIV, chemotherapy, steroids).
• Trauma to the nail (tight shoes, stubbing).
• Use of public showers, locker rooms, or communal pools without proper foot protection.

Diagnosis

Because nail discoloration can also be caused by psoriasis, trauma, or lichen planus, a definitive diagnosis is essential before starting treatment.

Clinical Examination

• Visual inspection of the nail(s) and surrounding skin.
• Evaluation of nail thickness, color, and pattern of involvement.

Laboratory Tests

1. KOH (potassium hydroxide) preparation: A scrapings of the nail surface are placed on a slide with KOH; the solution dissolves keratin, allowing fungal hyphae to be visualized under a microscope. Sensitivity ≈ 70 %.
2. Fungal culture: Nail clippings are placed on Sabouraud agar; growth takes 2‑6 weeks and identifies the specific organism, guiding therapy.
3. PCR (polymerase chain reaction) testing: Molecular methods detect fungal DNA quickly (1‑3 days) and have higher sensitivity than culture (≥ 90 %).
4. Histopathology (PAS staining): A nail biopsy stained with periodic acid‑Schiff highlights fungal elements; used when other tests are inconclusive.

Professional guidelines (American Academy of Dermatology) recommend confirming the diagnosis with at least one laboratory test before initiating systemic therapy because oral antifungals have potential side effects.

Treatment Options

Therapy is chosen based on severity, type of fungus, patient health, and preference. Treatment may last months because nails grow slowly (≈ 1 mm/month).

Topical Antifungals

• Efinaconazole 10 % solution (Jublia): Daily application for 48 weeks; cure rates 15‑18 %.
• Tavaborole 5 % solution (Kerydin): Daily for 48 weeks; similar efficacy.
• Ciclopirox 8 % lacquer (Penlac): Daily for 48 weeks; modest cure rates (5‑12 %).

Topicals are well‑tolerated but work best for mild disease (< 50 % nail involvement) and when the nail matrix is not severely damaged.

Oral Systemic Antifungals

Patients must have baseline liver function tests (LFTs) and periodic monitoring (every 4‑6 weeks). Pregnancy, severe liver disease, or drug interactions are contraindications.

Procedural Options

• Laser therapy: Nd:YAG or diode lasers claim to destroy fungal DNA; evidence is mixed, and insurance coverage is limited.
• Photodynamic therapy (PDT): Application of a photosensitizer followed by light exposure; still investigational.
• Surgical nail removal: Indicated for severe cases, intolerable pain, or when a rapid cosmetic result is needed. The nail matrix may be removed chemically (e.g., phenol) to prevent regrowth.

Adjunct Lifestyle Measures

• Keep feet clean and dry; change socks at least once daily.
• Trim nails straight across and file down thickened areas.
• Apply a drying powder or antifungal spray to shoes.
• Avoid tight footwear that traumatizes the nail.

Living with Toenail Fungus (Onychomycosis)

Even after successful treatment, recurrence is common (up to 30 % within a year). The following tips help maintain healthy nails:

• Foot hygiene: Wash feet with antimicrobial soap, dry especially between toes.
• Socken rotation: Use breathable, moisture‑wicking socks (e.g., wool or synthetic blends) and rotate shoes to allow them to air out.
• Footwear: Choose shoes with ventilation; avoid shared sandals or flip‑flops.
• Regular nail care: Trim nails short and file rough edges weekly.
• Monitor for changes: Look for new discoloration or thickening; early detection shortens treatment.
• Maintain healthy circulation: Exercise, elevate legs, and avoid smoking.

Prevention

Preventive measures focus on minimizing fungal exposure and maintaining nail integrity.

• Wear shower shoes in public pools, gyms, and locker rooms.
• Disinfect pedicure tools or have them sterilized at reputable salons.
• Avoid walking barefoot on damp floors.
• Use antifungal powders or sprays in shoes if you have a history of athlete’s foot.
• Keep fingernails short and avoid biting or picking at toenails.
• Properly treat athlete’s foot promptly; it often precedes onychomycosis.

Complications

If left untreated, onychomycosis can lead to several problems, especially in high‑risk individuals.

• Secondary bacterial infection: Cracks in the nail or surrounding skin can become portals for bacteria, leading to cellulitis.
• Pain and limited mobility: Thickened nails can cause discomfort in shoes, leading to gait changes.
• Diabetic foot complications: In diabetics, infected nails increase the risk of ulceration and, rarely, osteomyelitis.
• Permanent nail deformity: Chronic infection may cause irreversible changes to nail shape.
• Spread to other nails or skin: The infection can disseminate, making eradication more difficult.

When to Seek Emergency Care

For non‑emergent concerns—persistent nail changes, mild discomfort, or questions about treatment—schedule an appointment with a primary‑care physician or dermatologist.

Sources: Mayo Clinic, CDC, NIH (National Institute of Arthritis and Musculoskeletal and Skin Diseases), WHO, Cleveland Clinic, American Academy of Dermatology, JAMA Dermatology, *Clinical Microbiology Reviews* (2022).