What are the different types of foot fungus?

Types of Foot Fungus

Foot fungus is classified into five distinct clinical types based on the location and pattern of infection: distal and lateral subungual onychomycosis (DLSO), superficial white onychomycosis (SWO), proximal subungual onychomycosis (PSO), candidal onychomycosis, and total dystrophic onychomycosis when affecting the nails, plus three main patterns of tinea pedis affecting the skin. 1

Nail Fungus (Onychomycosis) - Five Clinical Types

1. Distal and Lateral Subungual Onychomycosis (DLSO)

• Most common type, accounting for the majority of all nail fungal infections 1
• Almost always caused by dermatophyte infection, predominantly Trichophyton rubrum 1
• Affects the hyponychium at the lateral edges initially, spreading proximally along the nail bed 1
• Results in subungual hyperkeratosis and onycholysis, with eventual nail plate destruction 1
• Nearly always accompanied by evidence of tinea pedis on surrounding skin 1

2. Superficial White Onychomycosis (SWO)

• Much less common than DLSO 1
• Most commonly caused by T. mentagrophytes 1
• Affects the surface of the nail plate rather than the nail bed 1
• Characterized by white (not cream) discoloration with noticeably flaky nail surface 1
• Onycholysis is not a common feature 1

3. Proximal Subungual Onychomycosis (PSO)

• Uncommon variety often related to intercurrent disease 1
• Strongly associated with immunosuppression, particularly HIV infection 1
• Also seen in patients with peripheral vascular disease and diabetes 1
• T. rubrum is the causative fungus in most cases 1
• Should prompt evaluation for underlying immunodeficiency or systemic disease 1

4. Candidal Onychomycosis

• Caused by Candida yeasts, presenting in four distinct ways 1
• Chronic paronychia with secondary nail dystrophy: occurs in patients with wet occupations, with swelling of posterior nail fold and detached cuticle 1
• Distal nail infection: less common presentation 1
• Chronic mucocutaneous candidiasis: systemic condition 1
• Secondary candidiasis: occurs as secondary invader 1

5. Total Dystrophic Onychomycosis

• End-stage presentation where entire nail apparatus is destroyed 1
• Can result from progression of any of the above types 1

Skin Fungus (Tinea Pedis) - Three Main Clinical Types

1. Interdigital Type (Most Common)

• Most frequent presentation of tinea pedis 2, 3
• Characterized by peeling, maceration, and fissuring affecting lateral toe clefts 2
• Involves ecological interplay between dermatophytes and bacteria 3
• Severe cases show bacterial overgrowth with production of methanethiol and sulfur compounds 3

2. Vesiculobullous Type

• Rarer form characterized by vesicles and blisters 2
• Often located on the soles of feet 2
• Associated with episodes of intense inflammation, particularly with T. mentagrophytes infection 3
• Inflammation results from immune contact allergic response to fungal antigens 3

3. Squamous Hyperkeratotic Type (Moccasin Type)

• Affects soles, heels, and lateral sites 2, 3
• Widespread moccasin-type infection typically caused by T. rubrum 3
• Particularly associated with atopic background 3
• Requires both topical and systemic antifungal treatment due to thick stratum corneum 2

Causative Organisms

Dermatophytes cause over 90% of cases, with Trichophyton rubrum being the predominant species 1, 4

• Primary dermatophyte species: T. rubrum, T. mentagrophytes, Epidermophyton floccosum 4, 5
• Yeast infections: Mainly Candida species 1
• Non-dermatophyte moulds: Less common secondary pathogens 1

Critical Clinical Distinction

The key distinction is between dermatophyte infections (which have keratolytic properties and are primary pathogens) versus yeast infections (which lack keratolytic properties and are generally secondary invaders). 6 This distinction is essential because treatment efficacy varies significantly between fungal types, and mycological confirmation should always be obtained before initiating long-term therapy 1.