Why People Develop Ringworm
People develop ringworm through direct contact with dermatophyte fungi—primarily Trichophyton, Microsporum, and Epidermophyton species—that invade the keratinized outer layer of skin, hair, or nails. 1
Causative Organisms and Their Sources
The dermatophytes responsible for ringworm infections come from three distinct ecological sources 2:
Anthropophilic dermatophytes (human-adapted): These fungi primarily infect humans and spread person-to-person. Trichophyton tonsurans now accounts for 50-90% of scalp ringworm cases in the U.K., attributed to immigration and travel patterns 1
Zoophilic dermatophytes (animal-adapted): These fungi naturally infect animals but can transmit to humans as zoonotic infections. Microsporum canis remains the most common organism across Europe, typically acquired from infected cats, dogs, or other animals 1, 3
Geophilic dermatophytes (soil-associated): These fungi live in soil containing keratinous materials and can infect humans through environmental contact. Species like Microsporum gypseum fall into this category 3, 2
Mechanism of Infection
Dermatophytes possess specific characteristics that enable infection 2:
Keratinase production: These fungi produce enzymes (acid proteinases, elastase, keratinases) that digest keratin, allowing them to invade the nonliving cornified layer of skin, hair, and nails 2
Limited penetration: The fungi typically cannot penetrate viable tissue in immunocompetent hosts, restricting infection to the outer keratinized layers 2
Inflammatory response variation: The degree of inflammation depends on the fungal species—zoophilic and geophilic species typically provoke more intense inflammatory responses than anthropophilic species 1
Transmission Routes
Ringworm is a communicable disease acquired through 3, 2:
- Direct contact with infected individuals or animals
- Indirect contact with contaminated fomites (clothing, bedding, grooming tools, surfaces)
- Environmental exposure to fungal spores that can persist for years in hair, skin scales, or soil 3
Risk Factors and Susceptible Populations
Certain groups face higher infection risk 1:
Prepubertal children: Ringworm predominantly affects healthy children, particularly in inner-city cosmopolitan communities, with no reduction in incidence 1
Immunocompromised individuals: Adults with cellular immune defects show increased susceptibility and may present with atypical manifestations 1
Geographic and demographic factors: Living in areas with high population density, specific immigration patterns, and close contact with infected individuals or animals increases transmission risk 1
Host Immune Response
The development and severity of infection depends on immune function 2:
Cell-mediated immunity: Development of delayed hypersensitivity and inflammatory response correlates with clinical cure 2
Defective immunity: Lack of adequate cell-mediated immunity predisposes to chronic or recurrent infection, particularly with Trichophyton rubrum, which may produce mannan that suppresses inflammatory responses 2
Common Pitfall
Do not assume all scaling skin lesions are ringworm—the clinical signs can be subtle and highly variable depending on the causative organism and host response. Laboratory confirmation through microscopy and culture is advisable to isolate the organism and direct appropriate therapy 1. Misdiagnosis is particularly common with inflammatory variants like kerions, which may be mistaken for bacterial abscesses 1.