Trichophyton is a genus of fungi belonging to a group known as dermatophytes, which are the most common cause of superficial fungal infections in humans and animals. Trichophyton species are responsible for a wide range of conditions collectively termed dermatophytoses, affecting the skin, hair, and nails globally. Understanding this pathogen involves recognizing its biological characteristics, clinical manifestations, transmission pathways, and effective management.
Defining the Fungal Pathogen
The genus Trichophyton is characterized as a keratinophilic fungus, meaning its metabolism relies entirely on keratin, the structural protein found in the outermost layer of skin, hair, and nails. These fungi possess specialized enzymes, called keratinases, which break down and consume this protein as a nutrient source. This specific dietary requirement confines the infection to the keratinized tissues, preventing the fungus from invading deeper, living layers of the body in healthy individuals.
Dermatophytes are categorized into three ecological groups based on their primary habitat.
Anthropophilic Species
Anthropophilic species, such as T. rubrum and T. tonsurans, are adapted to human hosts and transmit primarily from person to person.
Zoophilic Species
Zoophilic species, including T. mentagrophytes and T. verrucosum, commonly reside on animals like pets and livestock. They often cause more inflammatory reactions when transmitted to humans.
Geophilic Species
Geophilic species are adapted to living in the soil, feeding on degraded keratinous material. They cause the least frequent but often most intense human infections.
The Diseases Caused by Trichophyton
Infections caused by Trichophyton are named using the prefix tinea followed by the Latin term for the body area involved.
Tinea Corporis (Body Ringworm)
Tinea corporis, commonly known as ringworm of the body, typically appears as a ring-shaped plaque on the torso or limbs. These lesions feature a distinct, raised, scaly border that expands outward while the center often begins to clear. Active fungal growth occurs primarily at the leading edge of the lesion, which may also contain small papules or vesicles.
Tinea Pedis (Athlete’s Foot)
Tinea pedis is the most frequently encountered dermatophytosis, presenting in several forms. The interdigital type is characterized by scaling, maceration, and fissures between the toes, often affecting the space between the fourth and fifth digits. The hyperkeratotic, or moccasin-type, presents as chronic, diffuse scaling and thickening of the skin across the sole and sides of the foot. A less common form, the vesiculobullous type, involves the sudden appearance of fluid-filled blisters on the sole or arch.
Tinea Capitis (Scalp Ringworm)
Infections involving hair follicles are challenging to manage because the fungus penetrates the hair shaft itself. Tinea capitis primarily affects prepubescent children and presents in distinct ways. The “black dot” pattern occurs when hair shafts break off right at the scalp surface, leaving small, dark stubs in the follicles. A highly inflammatory variant called a kerion involves a painful, pus-filled mass that can lead to scarring and permanent hair loss if not treated promptly.
Tinea Unguium (Nail Infection)
Tinea unguium, or onychomycosis, refers to fungal infection of the fingernails or toenails. The most common presentation begins with discoloration, typically white, yellow, or brown, at the tip of the nail. As the fungus consumes the keratin, the nail plate thickens, becomes brittle, and accumulates debris underneath. This chronic infection can eventually lead to the complete destruction of the nail plate.
Sources and Transmission Routes
The transmission of Trichophyton relies on the shedding of microscopic fungal spores and skin scales from an infected host or environment. Anthropophilic species are spread through direct skin-to-skin contact, such as during close-contact sports or within households. Indirect transmission occurs through contaminated items like shared towels, bedding, or clothing, as fungal spores are resilient and survive for long periods.
Zoophilic species are acquired through contact with infected animals, commonly cats, dogs, or cattle. This animal-to-human transfer often results in a more pronounced inflammatory reaction because the fungus is not fully adapted to the human immune system. Geophilic species are occasionally encountered when individuals have prolonged contact with contaminated soil. Warm, humid conditions and minor skin trauma create the optimal environment for the fungi to establish an infection.
Diagnosis and Management of Infections
Diagnosing a Trichophyton infection typically begins with a physical examination of the characteristic lesions, but confirmation is necessary to guide effective treatment. The most common point-of-care test is the potassium hydroxide (KOH) wet mount. This involves scraping scales or collecting debris, which is then mixed with a KOH solution. The solution dissolves human skin cells but leaves the alkali-resistant fungal structures intact, allowing for microscopic visualization of the characteristic segmented hyphae, the branching filaments of the fungus.
Management of dermatophytosis is divided between topical and systemic antifungal agents, depending on the infection site and severity. Localized skin infections, such as tinea corporis and most tinea pedis cases, are successfully treated with topical creams or sprays. These typically contain allylamines (like terbinafine) or azoles (like clotrimazole), which inhibit fungal cell membrane synthesis directly at the infection site.
Systemic oral antifungals are required for infections involving the hair and nails because topical creams cannot adequately penetrate these dense keratin structures. Oral terbinafine is a common first-line agent, while griseofulvin is often the preferred choice for pediatric tinea capitis due to its long-standing safety profile. These systemic drugs circulate through the bloodstream and are deposited into the hair shaft and nail plate as they grow. Treatment duration for nail infections is lengthy, often requiring several months until the entire infected nail has grown out.