Common Cause of Toenail Fungal Infection
Dermatophyte fungi, specifically Trichophyton rubrum, cause over 90% of toenail fungal infections (onychomycosis). 1
Primary Causative Organisms
Dermatophytes (Most Common)
- Dermatophytes account for 70-80% of all onychomycosis cases, with toenails affected in 80% of all onychomycosis presentations 1, 2
- Trichophyton rubrum is the single most common pathogen, responsible for the vast majority of dermatophyte nail infections 1, 3
- Trichophyton mentagrophytes (also called T. interdigitale) is the second most common dermatophyte, particularly in superficial white onychomycosis 1, 3
- Other dermatophytes include Epidermophyton floccosum and T. tonsurans, though these are far less frequent 3, 4
Yeasts (Less Common)
- Candida species cause a minority of cases, primarily affecting fingernails rather than toenails 1
- Candida parapsilosis and Candida guilliermondii are more common than Candida albicans in nail infections 3, 5
- Candida typically causes chronic paronychia with secondary nail dystrophy in patients with wet occupations, not the typical toenail presentation 1
Nondermatophyte Molds (Rare)
- Molds such as Fusarium species, Scopulariopsis brevicaulis, Aspergillus spp, and Acremonium spp are emerging pathogens but remain uncommon 2, 5
- These are increasingly detected but still represent a small fraction of cases 5
Clinical Presentation Pattern
Distal and lateral subungual onychomycosis (DLSO) is the most common clinical presentation of dermatophyte toenail infection 1:
- The fungus invades by penetrating the distal or lateral nail margins 1
- Affected nails become thickened and discolored with varying degrees of onycholysis (nail plate separation) 1
- Toenails are more commonly affected than fingernails 1
- Nearly all patients with DLSO have concurrent tinea pedis (athlete's foot) 1
Important Clinical Caveats
- Mycological confirmation is essential before treatment, as only 50% of nail dystrophy is actually fungal in origin 1
- Bacterial infections (particularly Pseudomonas aeruginosa) can mimic fungal infection but cause green or black discoloration 5
- Proximal subungual onychomycosis (PSO), while uncommon in the general population, should raise suspicion for HIV/AIDS or other immunosuppression when present 1