Trichophyton
Trichophyton is a genus of filamentous fungi responsible for various skin infections, particularly affecting the outer layers of the skin, nails, and hair. It includes sixteen identified species, with several commonly associated with human infections, such as Trichophyton rubrum, T. tonsurans, and T. mentagrophytes. These fungi can be transmitted through direct contact, often occurring in settings like daycare centers or among family members, as well as via contaminated surfaces, shared personal items, or animal contact in rural areas.
Infections caused by Trichophyton can manifest in different forms, including tinea corporis (ringworm), tinea pedis (athlete's foot), and onychomycosis (nail infections). The clinical presentation of these infections typically involves red, scaly patches on the skin or discoloration and deformation of nails. While treatment options have evolved, newer antifungal agents like clotrimazole and terbinafine are preferred due to their effectiveness and reduced side effects compared to older medications. However, nail bed infections can be particularly challenging to treat, often requiring a combination of topical and oral therapies. Awareness of Trichophyton and its transmission routes is essential for prevention and effective management of infections.
Trichophyton
- TRANSMISSION ROUTE: Direct contact
Definition
Trichophyton is a genus of filamentous fungi that is a primary cause of infections of the outer layer of the skin, nail beds, and hair.
Natural Habitat and Features
Sixteen species of Trichophyton have been identified. Most are distributed worldwide, although some are restricted to specific geographic regions. Among species (anthropophilic species) whose natural habitat is human beings, those most commonly associated with human infection are rubrum, schoenleinii, tonsurans, and violaceum. Among species (zoophilic species) whose natural habitat is animals, verrucosum (cattle and horses) and mentagrophytes (rodents and rabbits) are most commonly associated with human infection.
Animal-to-human transmission occurs most frequently among rural populations in less developed regions. Direct human-to-human transmission occurs most frequently within families or among children in daycare or school settings. The sexual transmission of certain Trichophyton species has been documented in specific populations. Object-to-human transmission involves wet floors in gym showers or locker rooms; shared towels; and hair, shredded skin, nails, and contaminated tools in barber shops and beauty and nail salons. Self-inoculation occurs when a person first touches an infected area of their own body and then touches a noninfected, vulnerable area elsewhere on the body.
Colonies of Trichophyton are incubated on Sabouraud’s agar at between 77° and 86° Fahrenheit (25° and 30° Celsius) for from seven to fourteen days, depending on the species. Color and physical features vary depending on the species. In general, fronts are white to bright yellowish beige or red violet. The reverse may be pale yellow, brown, or reddish brown. The surface is waxy, glabrous (smooth and hairless), or cottony.
Microscopic examination of colonies of Trichophyton reveals septate, hyaline hyphae (partitioned and transparent tubelike filaments). Additional features vary by species. Macroconidia (large, multicelled spores) are not seen in great numbers. Trichophyton species are smooth, either thin- or thick-walled and are either cylindrical, shaped like a clavate (club), or shaped like a fusiform (spindle). Microconidia (small, single-celled spores) are numerous. They are solitary or arranged in clusters and are round, pyriform (pear-shaped), clavate, or irregular. Microconidia are usually the predominant type of conidia observed in Trichophyton species. Their presence distinguishes Trichophyton from Epidermophyton, in which microconidia are rarely observed. Arthroconidia (jointed spores) and chlamydospores (round, thick-walled spores) may also be observed, especially in older cultures.
Pathogenicity and Clinical Significance
Trichophyton, along with Epidermophyton and Microsporum, is classified as a dermatophyte, a fungus that causes dermatophytosis, an infection of the outer layer of the skin (stratum corneum) and of the hair and nail beds. Trichophyton species are highly adapted to the nonliving outer tissue of the skin, hair shafts, and nail beds. A minor lesion, such as a paper cut or a blister, allows the fungus to penetrate the outer layer of the skin, where it grows and spreads sideways with sharp, advancing margins. Infection of the hair occurs when hyphae penetrate the hair shaft. An erythematous papule (red pimple) develops, followed by scaling and discoloration. Eventually, the shaft breaks off. Infection progresses to surrounding hair in a ringlike pattern. Nail-bed infection begins with discoloration of tissue under the nail. The nail plate then hardens and discolors and may become misshapen.
Drug Susceptibility
Susceptibility methods for testing agents used to treat dermatophytosis caused by Trichophyton species have not been standardized. Limited testing and comparisons have shown that specific newer agents have lower minimum inhibitory concentrations (MICs) than earlier agents, such as griseofulvin, amphotericin B, and fluconazole, which were once the drugs of choice. These newer agents also carry a lower risk of side effects. Fluconazole, the first azole, has been replaced by newer, broad-spectrum azoles with much lower MICs for treating dermatophytoses caused by Trichophyton species. These newer azoles include clotrimazole, econazole, miconazole, oxiconazole, and sulconazole, all available as topical agents (creams, ointments, and solutions). Along with topical formulations of the allylamine drugs naftifine and terbinafine, these are the drugs of choice in otherwise healthy persons for treating tinea corporis (infection of the trunk, legs, and arms), tinea cruris or jock itch (infection of the groin and pubic area), tinea manuum (infection of one or both hands), and tinea pedis (athlete’s foot).
For persons with spreading, persistent, or recurring skin or hair infections, oral drugs may be required in addition to or in place of topical agents. Oral terbinafine, which has an especially low MIC, is the drug of choice in these cases. Oral formulations of azoles, including itraconazole, ketoconazole, and voriconazole, may also be used. All these drugs are more effective and convenient than (and do not have the risks associated with the use of) griseofulvin. Nail bed infections (tinea unguium) caused by Trichophyton species are difficult to treat in all persons. These infections require long-term treatment with both a topical agent and oral griseofulvin. Antibiotic resistance has become a public health issue in some strains of Trichophyton, notably T. indotineae.
Bibliography
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