Dematophytes

Dematophytes Dermatophytes (name based on the Greek for 'skin plants') are a common label for a group of three types of fungus that commonly causes skin disease in animals and humans. These anamorphic (asexual or imperfect fungi) genera are: Microsporum, Epidermophyton and Trichophyton. There are about 40 species in these three genera. Species capable of reproducing sexually belong in the teleomorphic genus Arthroderma, of the Ascomycota The organisms are transmitted by either direct contact with infected host (human or animal) or by direct or indirect contact with infected exfoliated skin or hair in combs, hair brushes, clothing, furniture, theatre seats, caps, bed linens, towels, hotel rugs, and locker room floors. Depending on the species the organism may be viable in the environment for up to 15 months. There is an increased susceptibility to infection when there is a preexisting injury to the skin such as scares, burns, marching, excessive temperature and humidity. Dermatophytes are classified as anthropophilic, zoophilic or geophilic according to their normal habitat. Geophilic species species are usually recovered from the soil but occasionally infect humans and animals. They cause a marked inflammatory reaction, which limits the spread of the infection and may lead to a spontaneous cure but may also leave scars. Anthropophilic dermatophytes are restricted to human hosts and produce a mild, chronic inflammation. Zoophilic organisms are found primarily in animals and cause marked inflammatory reactions in humans who have contact with infected cats, dogs, cattle, horses, birds, or other animals. This is followed by a rapid termination of the infection. Dermatophytes cause infections of the skin, hair and nails due to their ability to obtain nutrients from keratinized material. The organisms colonize the keratin tissues and inflammation is caused by host response to metabolic by-products. are usually restricted to the nonliving cornified layer of the epidermis because of their inability to penetrate viable tissue of an immunocompetent host. Invasion does elicit a host response ranging from mild to severe. The development of cell-mediated immunity correlated with delayed hypersensitivity and an inflammatory response is associated with clinical cure. Whereas the lack of or a defective cell-mediated immunity predisposes the host to chronic or recurrent dermatophyte infection. Some of these infections are known as ringworm or tinea. Toe- and fingernail infection are referred to as onychomycosis. Dermatophytes usually do not invade living tissues, but colonize the outer layer of the skin. Occasionally the organisms do invade subcutaneous tissues, resulting in kerion development. Types of Dermatophyte Infections Athlete's foot or tinea pedis. Jock itch or tinea cruris. Ringworm of the body or tinea corpora. Facial ringworm or tinea faciei. Blackdot ringworm or tinea capitis. Ringworm of the hands or tinea manuum. Ringworm of the nail, Onychomycosis, or tinea unguium. Anthropophilic Epidermophyton floccosum Microsporum audouinii Trichophyton mentagrophytes (cottony and velvety) Trichophyton rubrum Trichophyton schoenleinii Trichophyton soudanense Trichophyton tonsurans Trichophyton violaceum Zoophilic Microsporum canis (cats, dogs, etc.) Microsporum equinum (horses) Microsporum nanum Trichophyton mentagrophytes(granular) (rodents, rabbits, hedgehogs, etc.) Trichophyton verrucosum (cattle) Geophilic Microsporum gypseum Clinical Significance Dermatophyte Skin Hair Nails Microsporum X X Trichophyton X X X Epidermophyton X X Conidia, sometimes termed asexual chlamydospores, or chlamydoconidia are asexual, non-motile spores of a fungus, from the Greek word for dust, konis. They are also called mitospores due to the way they are generated through the cellular process of mitosis. The two new haploid cells are genetically identical to the haploid parent, and can develop into new organisms if conditions are favorable, and serve in biological dispersal. The asexual spores may be large (macroconidia, chlamydospores) or small (microconidia, blastospores, arthroconidia) The dermatophytes consist of three genera: Epidermophyton produces only macroconidia, no microconidia and consists of 2 species, one of which is a pathogen. Microsporum Both microconidia and rough-walled macroconidia characterize Microsporum species. There are 19 described species but only 9 are involved in human or animal infections. Trichophyton When produced the macroconidia of Trichophyton species are smooth-walled. There are 22 species, most causing infections in humans or animals. Microscopic morphology of the micro and/or macroconidia is the most reliable identification character, but you need a good slide preparation and you may need to stimulate sporulation in some strains. Culture characteristics such as surface texture, topography and pigmentation are variable and are therefore the least reliable criteria for identification. Clinical information such as the site, appearance of the lesion, geographic location, travel history, animal contacts and race is also important, especially in identifying rare non-sporulation species like M. audouini, T. concentricum and T schoenleinii etc. Tinea capitis Tinea capitis refers to dermatophytosis of the scalp. Three types of in vivo hair invasion are recognised: 1. Ectothrix invasion is characterised by the development of arthroconidia on the outside of the hair shaft. The cuticle of the hair is destroyed and infected hairs usually fluoresce a bright greenish yellow colour under Wood's ultraviolet light. Common agents include M. canis, M. gypseum, T. equinum and T. verrucosum. Laboratory DiagnosisDirect Examination Examine hair for fluorescence ¡ Wood’s lamp ¡ Yellow green fluorescence = positive 2. Endothrix hair invasion is characterised by the development of arthroconidia within the hair shaft only. The cuticle of the hair remains intact and infected hairs do not fluoresce under Wood's ultraviolet light. All endothrix producing agents are anthropophilic eg T. tonsurans and T. violaceum. 3. Favus usually caused by T. schoenleinii, produces favus-like crusts or scutula and corresponding hair loss. Endothrix Ectothrix Blackdot ringworm or tinea capitis Infected hair shafts are broken off just at the base, leaving a black dot just under the surface of the skin. Scraping these residual black dot will yield the best diagnostic scrapings for microscopic exam. Numerous green arthrospores will be seen under the microscope inside the stubbles of broken hair shafts at 400x. Tinea capitis can not be treated topically, and must be treated systemically with antifungals Laboratory Identification of Dermatophytes Specimen Collection: Skin should be scraped from the margin of the lesion. Hair should be plucked, not cut, from the edge of the lesion. Choose hairs that fluoresce under a Wood's lamp or, if none fluoresce, choose broken or scaly ones. Nails scrapings are obtained from the nail bed or from infected areas after the outer layers are discarded. A Wood's lamp is a diagnostic tool used in dermatology by which ultraviolet light is shone (at a wavelength of approximately 365 nanometers) onto the skin of the patient; a technician then observes any subsequent fluorescence. For example, porphyrins — associated with some skin diseases — will fluoresce pink. Direct Examination: A small sample of the specimen is selected for direct microscopic examination and investigated for the presence of fungal elements. The specimen is mounted in a small amount of potassium hydroxide or calcofluor white. The KOH slides are gently heated and allowed to clear for 30 to 60 minutes before examining on a light or phase contrast microscope. Calcofluor white slides are examined on a fluorescent microscope. When present in the direct examination dermatophytes appear as hyaline (non-pigmented), septated elements. Hyphae rounding up into arthroconidia are diagnostic of dermatophyte involvement. Without the presence of arthroconidia the elements could also be due to a non-dermatophyte agent of onychomycosis or a small segment of a contaminating organism. When hair is involved the arthroconidia may be found on the periphery of the hair shaft (ectothrix) or within the shaft (endothrix). Culture Nails are scraped or minced into small pieces Hair is cut into short segments Each specimen is divided between at least two types of culture media The use of antibiotics will inhibit the overgrowth of bacteria and incorporation of cycloheximide will prevent the overgrowth of the rapidly growing saprophytic fungi The cultures are incubated at 30°C and examined frequently for 4 weeks. Potato dextrose agar is a media useful for the production of pigment. Sabouraud dextrose agar (Emmon's modification) is a non-selective media which supports the growth of most fungi. A special media called Dermatophyte Test Medium (DTM) has been formulated to grow and identify dermatophytes. Without having to look at the colony, the hyphae, or macroconidia - one can identify the dermatophyte by a simple color test. The specimen (scraping from skin, nail, or hair) is embedded in the DTM culture medium It is incubated at room temperature for 10 to 14 days. If the fungus is a dermatophyte, the medium will turn bright red. If the fungus is not a dermatophyte, no color change will be noted. If kept beyond 14 days, false positive can result even with non-dermatophytes. Specimen from the DTM can be sent for species identification

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