Causes of Ringworm
Ringworm (tinea corporis) is caused by dermatophyte fungi, primarily species from three genera: Trichophyton, Microsporum, and Epidermophyton 1, 2.
Fungal Organisms Responsible
The causative dermatophytes fall into three ecological categories based on their natural reservoir:
Anthropophilic (Human-to-Human Transmission)
- Trichophyton tonsurans - most common in urban areas and competitive wrestling environments 3
- Trichophyton rubrum - widespread anthropophilic species
- Microsporum audouinii and M. ferrugineum - typically cause minimal inflammatory response 1
These organisms are transmitted through direct person-to-person contact or indirectly via contaminated fomites (shared towels, clothing, sports equipment) 2.
Zoophilic (Animal-to-Human Transmission)
- Microsporum canis - most commonly acquired from cats and dogs
- Trichophyton verrucosum - from cattle and farm animals 4
- Trichophyton equinum - from horses 5
- Trichophyton mentagrophytes - from various animals
Zoophilic species typically produce more intense inflammatory responses than anthropophilic fungi 1. Patients with animal contact (farm workers, riders, pet owners) are at higher risk 4, 5.
Geophilic (Soil-to-Human Transmission)
- Microsporum gypseum - causes inflammatory infections 1
- Microsporum praecox - rare geophilic species 6
These are acquired through direct contact with contaminated soil 2.
Transmission Routes
The infection spreads through:
- Direct skin-to-skin contact with infected individuals (most common in wrestlers and contact sports athletes) 3
- Contact with infected animals (pets, livestock, horses) 4, 5
- Contaminated fomites including clothing, towels, sports equipment, and surfaces 2
- Soil exposure for geophilic species 6
Important Clinical Caveat
The intensity of inflammation depends on the fungal source: Zoophilic and geophilic dermatophytes produce significantly more inflammatory, pustular lesions compared to the milder presentations seen with anthropophilic species 1. This distinction is clinically important because highly inflammatory tinea corporis from animal sources may be misdiagnosed as bacterial infection, leading to inappropriate antibiotic use rather than antifungal therapy 1.
Recent epidemiologic data shows tinea corporis affects approximately 79 per 10,000 person-years in Medicaid populations, with highest rates in young children and Black patients 7. The concerning trend is that less than 10% of cases receive proper diagnostic testing, and 7-10% are inappropriately treated with combination antifungal-corticosteroid products 7, which can worsen outcomes and contribute to emerging antifungal resistance 8.