What causes onychomycosis (nail fungus)?

How Nail Fungus (Onychomycosis) Develops

Nail fungus (onychomycosis) is primarily caused by fungal invasion of the nail apparatus, with dermatophytes being responsible for over 90% of cases, particularly Trichophyton rubrum, which typically enters through the distal or lateral nail edges and spreads proximally. 1

Causative Organisms

The main pathogens causing onychomycosis include:

1. Dermatophytes (>90% of cases):

   • Trichophyton rubrum (most common)
   • Trichophyton mentagrophytes/interdigitale
   • Epidermophyton floccosum

2. Yeasts (5-10% of cases):

   • Candida species (primarily affecting fingernails)

3. Non-dermatophyte molds:

   • Less common but increasing in prevalence, especially in warmer climates 2

Infection Mechanisms and Clinical Patterns

Onychomycosis develops through several distinct patterns of nail invasion:

1. Distal and Lateral Subungual Onychomycosis (DLSO)

• Most common pattern (80% of cases)
• Fungus invades the hyponychium at the distal or lateral edges
• Spreads proximally along the nail bed
• Results in subungual hyperkeratosis and onycholysis
• Nail becomes thickened, discolored, and friable
• Almost always accompanied by tinea pedis (athlete's foot) 1

2. Superficial White Onychomycosis (SWO)

• Infection begins at the superficial layer of the nail plate
• Creates white, crumbly patches on the nail surface
• Commonly caused by T. mentagrophytes/interdigitale
• More common in children 1

3. Proximal Subungual Onychomycosis (PSO)

• Fungus invades through the proximal nail fold or cuticle
• Penetrates the newly forming nail plate
• Distal portion remains normal until late stages
• Often associated with immunosuppression (particularly HIV) 1

4. Total Dystrophic Onychomycosis (TDO)

• End-stage of any pattern where the entire nail plate is destroyed
• Primary TDO is rare and usually caused by Candida in immunocompromised patients 1

Risk Factors for Developing Onychomycosis

Several factors increase susceptibility to nail fungus:

• Age: Incidence increases with age 3
• Immunodeficiency: HIV infection, immunosuppressive treatments
• Diabetes mellitus: Impaired circulation and neuropathy
• Peripheral vascular disease
• Repeated trauma to nails
• Wet occupations: Especially for Candida infections
• Genetic predisposition: Family history increases risk 1
• Public exposure: Walking barefoot in public places like swimming pools, gyms 4

Transmission and Spread

Onychomycosis is contagious and spreads through:

1. Direct contact with infected individuals
2. Fomites (contaminated objects like shoes, socks, nail clippers)
3. Autoinoculation from existing fungal infections elsewhere on the body
4. Environmental exposure to fungi in soil or public spaces 1

The primary infectious particles are arthroconidia (fungal spores), which are resistant to adverse conditions and can survive for extended periods in the environment 1.

Important Clinical Considerations

• Diagnostic confirmation is essential: Prior to treatment, appropriate nail specimens should be obtained for laboratory testing (KOH preparation, fungal culture, or nail biopsy) to confirm the diagnosis 5

• Not merely cosmetic: Onychomycosis can cause pain, discomfort, and physical impairment, negatively impacting quality of life 4

• Reservoir for infection: Infected nails serve as a reservoir for fungi that can spread to other body sites or other individuals 1

• Complications in at-risk populations: Can lead to serious complications in diabetic patients, including foot ulcers, cellulitis, and even osteomyelitis 1

By understanding the mechanisms of infection and risk factors, appropriate preventive measures can be taken, including avoiding walking barefoot in public places and disinfecting shoes and socks to reduce the risk of infection and recurrence 4.