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DERMATOPHYTOSIS
Dermatophytes means skin plants, they are group of filamentous
fungi belonging to class deuteromycetes "fungi imperfecti".
Agents
Of Dermatophytosis :
Three genera of fungi, Microsporum, Trichophyton, and Epidermphyton, are etiologic agents of dermatophytosis. Species within these genera are keratinophilic, that is, they are adapted to grow on hair, and cutaneous layers of skin that contain the scleroprotein keratin. Infection of deep tissue by these fungi is rare, but occasionally, extensive inflammation and nail bed involvement may result.
Epidemiology :
Most of the agents of dermatophytosis live freely in the environment but a few have adapted almost exclusively to living on human host tissues and these are very rarely recovered from any other source. Distribution of many dermatophyte species is wolrdwide, whereas others are found only in restricted geographic regions. Approximately 43 species of dermatophytes and dermatophyte-like fungi have been described, and just over 24 of these have been documented to cause human infection. Those dermatophytes that primarily inhibit the soil are termed geophilic. Most geophilic fungi produce large numbers of conidia and therefore are among the most readily identified species. Zoophilic dermatophytes are typically adapted to live on animals and are not commonly found living freely in soil or on dead organic substances. They often cause infections in their animal hosts and may be spread as disease agents to humans. Fewer conidia are produced by zoophilic fungi than by geophilic species. A few dermatophytes have become adapted exclusively to human hosts and termed anthropophilic. Although they are encountered almost always as agents of human disease, the infections are seldom inflammatory. Species identification may be quite difficult because most arthropophilic species produce few candida.
Commonly Encountered Dermatophytes :
i. Microsporum: Includes 14 species, produce both micro and macroconidia. The macroconidia are multiseptate, variable in forms, big sized, having a thick wall and irregular surface. Species of this genus attacking mainly hair and skin. The most important species are: M. audouinii. M. canis. M. gypseum. 
ii. Trichophyton:
Fungi in this genus having smooth-walled microconidia. Microconidia are thick-walled, range from clavate to fusiform in shape. Microconidia may be spherical to pyriform or may be irregular in shape and size. These are 20 species in this genus. The most important species are: T. rubrum. T. violaceum. T. mentagrophytes. 
iii. Epidermophyton:
This genus is monotypic, includes E. floccosum. The macroconidia are clavate, of thin and smooth wall, pyriform or oval in shape, no microconidia. Species within these genera are keratinophilic, so, they are adapted to grow on hair, nails and cutaneous layers of skin that contain the scleroprotein keratin. Infection of deep tissue by these fungi is rare, but occasionally, extensive inflammation may result. 
Clinical Infections :
Various forms of dermatophytosis and the respective affected sites:
Dermatophytoses usually involve a restricted region of the host, and traditionally, these diseases are named with respect to the portion of the body affected. Because the infections were at one time believed to be the result of burrowing worms that formed ring-shaped patterns in the skin, the term tinea was applied to each disease. We continue to describe the various forms of "ringworm" in these terms, as shown in above table. Each ringworm lesion is the results of a local inoculation on the skin with the etiologic agent; many lesions enlarge with time, usually with most inflammation occurring at the growing edge. In some forms of ringworm, there is a persistent allergic reaction, the dermatophytid, which is manifested in the formation of sterile, itching lesions on body sites distant from the point of infection. Symtoms of dermatophyte infections vary from slight to moderate and occasionally severe. Infections Involving Hair and Hair-Follicules: Different body sites manifest different symptoms. Infections in the scalp, where hair follicles are the initiation sites, may be among the most severe and disfiguring forms of the disease. Tinea favosa, or favus, begins as an infection of the hair follicle by Trichophyton schoenleinii and progresses to a crusty lesion made up of dead epithelial cells and fungal mycelia. Crusty, cup shaped flakes called scutula are formed. Hair loss and scar tissue formation commonly follow. 
Two distinct forms of tinea capitis, grey-patch ringworm and black dot
ringworm, are caused by different species of dermatophytes.
Grey-patch ringworm is a common childhood disease easily spread among children. The fungus primarily colonized the outer portion of hair shafts, the so called "ectothrix" hair involvement. The lesions are seldom inflamed, but luster and color of the hair shaft may be lost. Microsporum audouinii and M canis are agents of this form of disease. 
Black-dot ringworm is an endothrix hair involvement. The hair follicle
is the initial site of infection, and fungal growth continues within the
hair shaft, causing it to weaken. The brittle, infected hair shafts
break of beneath the scalp, leaving the "black-dot" stubs. Trichophyton
tonsurans and Trichophyton violaceum are the most common fungi
implicated in this form of ringworm.
Infections Involving the Nail and Nail Bed:
Onychomycosis is the most often caused by dermatophytes but also may be the result of infection by other fungi. These nail and nail bed infections may be among the most difficult dermatophytosis to treat. Long-term, costly therapy with griseofulvin is considered the best but many still result in unsatisfactory resolution of the disease. Some common agents that infect the nail are the Trichophyton species: T. rubrum, T. mentagrophtes, and T. tonsurans. Athlete's Foot: Among the shoe-wearing human population, tinea pedis or ringworm of the foot is a common disease, particularly of men. Various sites on the foot may be involved, but most often, tinea pedis affects the toe webs or toenails. In more severe cases, the sole of the foot may develop extensive scaling with fissuring and erythema. The disease may progress around the sides of the foot from the sole, giving rise to use of the term "moccasin foot", descriptive of the shape of the lesion. Infections of the glabrous skin range from mild with only minimal scaling and erythema to severely inflamed lesions. Causative agents are mainly T. rubrum and T. interdigitale. Suoperadded bacterial infections are usually found (proteus spp. and pseudomonas spp.).
Laboratory Diagnosis :
The laboratory diagnosis of ringworm is best made by direct microscopy. Culture is required to identify the infecting species. Collection of specimens: 1. Cleanse the affected area with 70% v/v ethanol. 2. Collect skin scales, crusts, pieces of nail, or hairs on a clean piece of paper as follows: Skin scales: Collect by scarping the surface of the margin of the lesion using a sterile scalpel blade. Nail pieces: Collect by taking snippings of the infected part of the nail using sterile scissors. Hairs: Collect by removing dull broken hairs from the margin of the lesion using sterile tweezers. Direct microscope: The direct microscopical examination of specimens is the recommended method of diagnosing ringworm. Material from skin, hair, or nails is first softened and cleared with a strong alkali such as 20% potassium hydroxide (KOH). The purpose of the alkali is to digest the keratin surrounding the fungi so that the hyphae and spores can be seen. The method of examining a KOH preparation is as follows: 1. Place a drop of potassium hydroxide solution. 2. Transfer the specimen (small pieces) to the drop of KOH, and cover with a cover glass. Clearing can be hastened by gently heating the preparation over the flame of Bunsen burner. 3. Examine it microscopically using the 10x and 40x objectives. Appearance of skin scales, nails or crusts in KOH preparation: Branching hyphae, chains of arthrospores or a mixture of both should be looked for. All species of ringworm fungi have a similar appearance. Ringworm fungi must be distinguished from epidermal cell outlines, elastic fibres, and artefacts such as intracellular cholesterol and strands of cotton or vegetable fibres. Fungal hyphae can be differentiated from these structures by their branching, uniform width, and cross-walls (septa). Appearance of infected hair in KOH preparation: Look for arthrospores and hyphae, and note whether the infection is on the outside of the hair or within it. When the infection is outside the hair it is referred to as ectothrix. When the infection is in the hair substance, it is referred to as endothrix. Culture: If identification of the infecting dermophyte is required, it is necessary to culture the fungus on a medium that will encourage hyphal growth and spore formation and such a medium should inhibit the growth of bacteria and saprophytic fungi. A medium which is recommended for the identification of dermophytes is SDA containing cycloheximide (Actidione) and an antibiotic such as chloramphenicol. Incubate aerobically at room temperature (25-30°C) for up to 3 weeks, examining every few days for growth. The following features are used in the identification of ringworm fungi species: - Hyphal characteristics. - Absence or presence of species. - Morphology of spores.
Treatment :
Successful treatment of dermatophytic skin infection is usually accomplished with keratinolytic agents, which remove the outer layers of skin along with fungal elements. Local treatment: Application of lotion and ointments like clotrimazole or Miconazole. Systemic oral drugs: The use of griseofulvin, ketoconazole, fluconazole, itraconazole and Tiribinafine. These systemic drugs are very important in the treatment of chronic dermatophytic infection. Recurring infections are common with most types of ringworms, even with best therapy. Nail bed infections are often resistant to therapy. |
DERMATOPHYTOSIS
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