Dermatophytoses of the Scalp: Etiology and Pathogenesis

Overview of Scalp Ringworm

Tinea capitis, a dermatophyte infection affecting the scalp and hair shafts, primarily occurs in children but can affect individuals of all ages. Its presentation varies widely, ranging from mild scaling to severe inflammatory reactions.

Causative Agents: Dermatophyte Species

Various dermatophytes, a group of fungi that thrive on keratin, can induce scalp infections. The specific species involved vary depending on geographic location and host factors.

• Trichophyton species: Common examples include Trichophyton tonsurans, a frequent cause of endothrix infections (where the fungus grows inside the hair shaft), particularly in North America. Other Trichophyton species implicated include T. violaceum and T. schoenleinii.
• Microsporum species: While less prevalent than Trichophyton in some regions, Microsporum canis (often acquired from pets) and M. audouinii are also notable pathogens. Microsporum infections often result in ectothrix infections, where fungal spores are present on the outside of the hair shaft.

Mechanisms of Infection

Dermatophytes invade the stratum corneum (outer layer of skin) and hair follicles. The fungal hyphae secrete enzymes, such as keratinases, that degrade keratin, facilitating their spread within the affected tissues. Infection can occur through direct contact with infected individuals or animals, or through indirect contact with contaminated objects (fomites), such as combs, hats, or furniture.

Risk Factors and Predisposing Conditions

• Age: Tinea capitis is most common in children, particularly those of school age.
• Contact with infected individuals or animals: Close contact increases the likelihood of transmission.
• Poor hygiene: While not always a primary driver, inadequate hygiene practices can contribute to the spread of fungal spores.
• Immunocompromised states: Individuals with weakened immune systems are more susceptible to fungal infections.
• Pre-existing skin conditions: Scalp conditions like eczema may disrupt the skin barrier, increasing susceptibility.

Types of Infection Based on Fungal Invasion

• Ectothrix: Fungal spores surround the outside of the hair shaft. Often associated with Microsporum species.
• Endothrix: Fungal elements are contained within the hair shaft. Commonly seen with Trichophyton tonsurans.
• Favus: A severe form characterized by yellowish, cup-shaped crusts (scutula) around the hair follicles, frequently involving Trichophyton schoenleinii.

Inflammatory Response

The host's immune response plays a crucial role in the clinical presentation of scalp ringworm. Inflammation can range from minimal scaling to severe kerion formation, which involves a boggy, inflammatory mass on the scalp. The intensity of the inflammatory response depends on factors such as the fungal species, the individual's immune status, and genetic predisposition.