What Causes Ringworm
Ringworm is caused by dermatophyte fungi—specifically species from the genera Trichophyton, Microsporum, and Epidermophyton—that invade keratinized tissues including skin, hair, and nails. 1, 2
Causative Organisms
The dermatophytes responsible for ringworm infections are classified into three main genera:
- Trichophyton species (e.g., T. tonsurans, T. rubrum, T. mentagrophytes, T. verrucosum, T. violaceum) 1
- Microsporum species (e.g., M. canis, M. gypseum, M. audouinii) 1
- Epidermophyton species 2, 3
These are filamentous fungi that specifically target and invade keratinized tissue, producing the characteristic "ringworm" infection pattern. 2, 3
Sources and Transmission Categories
Dermatophytes are categorized by their primary habitat, which determines how humans acquire infection:
Anthropophilic Dermatophytes
- Parasitic organisms that primarily infect humans and spread person-to-person 2
- Examples include T. tonsurans (now the predominant cause of tinea capitis in North America) and M. audouinii 3
Zoophilic Dermatophytes
- Parasitic organisms that primarily infect animals but can transmit to humans, making them agents of zoonosis 2
- M. canis (from cats and dogs) is currently the prevalent agent of tinea capitis in many regions including Spain and is associated with close human-pet contact 1, 3
- T. mentagrophytes and T. verrucosum are more common in rural areas with animal exposure 1, 3
- Multiple documented outbreaks have occurred from contact with infected animals in public settings, including 15 cases of "club lamb fungus" from Trichophyton species and M. gypseum among lamb exhibitors in Georgia 1
- A M. canis outbreak affecting 23 persons was traced to a hand-reared zoo tiger cub 1
Geophilic Dermatophytes
- Saprobic fungi associated with keratinous materials in soil 2
- M. gypseum is an example of a geophilic dermatophyte 1
- Infectious structures can persist for years in the environment, in hair, or in skin scales 2
Transmission Routes
Ringworm is a communicable disease acquired through:
- Direct skin contact with infected humans or animals in public settings 1
- Contact with contaminated fomites (objects or surfaces harboring fungal elements) 2
- Contact with contaminated soil or environmental surfaces where fungal spores persist 2
Important Clinical Context
The severity and inflammatory response of ringworm infections varies by the source organism:
- Anthropophilic fungi (e.g., M. audouinii, T. tonsurans) typically produce minimal inflammatory response 1
- Zoophilic or geophilic species (e.g., M. canis, M. gypseum, T. mentagrophytes, T. verrucosum) typically demonstrate more intense inflammatory reactions, including kerion formation 1
Emerging concern: Trichophyton indotineae, a new species within the T. mentagrophytes complex, has emerged globally since at least 2017 in North America and is notable for causing severe infections with high rates of antifungal resistance (18.6% terbinafine resistance documented). 4