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The below mentioned article provides a short note on Dermatophytes:- 1. Meaning of Dermatophytes 2. Diseases Due to Dermatophytes.
Meaning of Dermatophytes:
Common (superficial) dermatophyte infections (ringworm or tinea) of skin, hair and nails are caused by the members of a group of thirty to forty related filamentous fungi that can digest keratin. These fungi are called “dermatophytes“.
The saprophytic members of this group— which normally live in the soil and from which the human and animal pathogens have probably been derived—play an important role in breaking down the keratinized tissue of dead animals.
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From 1843 till recently, only the asexual forms of the fungi were known and were placed under fungi imperfecti in three genera—Epidermophyton, Microsporum and Trichophyton—distinguished by the morphology of their large asexual spores or “macro-conidia“. Recently, however, the sexual forms of perfect states of about half the Microsporum and Trichophyton species have been identified.
They belong to genera of the Ascomycetes. The species whose perfect states have been identified are mostly soil organisms but include some common pathogens. The following species are responsible for ninety per cent of the dermatophyte infections of man.
Microsporum audouinii; M. canis; Trichophyton rubrum (Fig. 99.3d); T. mentagrophytes (Fig. 99.3e); T. verrucosum and Epidermophyton floccosum; Drechslera helminthosporidium.
Their morphology consists of hyphae and arthrospores formed by segmentation of a hypha into a row of separate thick-walled cells. In artificial culture, two kinds of asexual spores, micro-conidia and macro-conidia are also formed. The micro-conidia (Fig. 99.3b) are unicellular small (2 to 5 µm), round or pear shaped, borne laterally or terminally singly or in clusters.
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The macro-conidia (Fig. 99.3c, d ,e) are multicellular and their shape, size and surface are used to differentiate the genera. In Microsporum, they are usually numerous, spindle-shaped, with a rough surface and 5 to 15 segments and measure 40 to 150 µm in length. In Epidermophyton, they are pear shaped with a smooth surface and measure about 30-40 µm. Two or three often arise from the same hypha.
In Trichophyton, they are usually scanty and often irregular in shape; characteristic ones are smooth; cylindrical, with 2 to 6 segments and 10 to 50 µm long. At room temperature (26 to 28°C), Sabouraud’s medium is suitable for isolation and is improved by adding cycloheximide which inhibits many common contaminants but does not affect dermatophytes.
Their spores are not killed by the mild wet heat of laundering so their destruction in clothing’s may require boiling or formaldehyde vapour. The benzoic and salicylic acids of Whitfield’s ointment are fungicidal. (Griseofulvin is fungi-static to treatment with it). Cyclopiroxolamine (Batrafan) cream, Miconazole nitrate (zole) dusting powder are recent antifungal drugs.
Diseases Due to Dermatophytes:
The usual name given to these infections is ringworm or tinea and this name is qualified by the name of the site affected, e.g., tinea capitis or tinea pedis, but special names are also used in dermatology for particular manifestation, e.g., kerion, favus.
When the skin is infected, the fungus spreads radially in the dead keratinized layer in the form of branching hyphae with occasional arthrospores. The inflammatory reaction from living tissue below may be very mild and only little dry scaling or hyperkeratosis is seen.
More commonly, here is irritation, erythema, oedema and some vesiculation, especially at the spreading edge and this irregular pink periphery gives rise to the name “ringworm“. The species that commonly attack the skin are Trichophyton sp. E. floccosum (groins and feet) and M. canis.
Infection of the nails renders them irregular, discoloured and friable. The fungus grows deep into the substance of the nail. The species usually responsible are T. rubrum and both human and animal strains of T. mentagrophytes.
When the scalp is infected the fungus grows in the horny layer of the epidermis and down into hair follicles. The hyphae surround and invade the hair shaft and, once within it, they grow outwards. The hair continues to grow and the hyphae break up into long chains of arthrospores, in some species within the shaft (endothrix infections) but more commonly mainly on the outside (ectothrix infections).
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After two or three weeks growth the weakened hair breaks off, leaving either a black dot at the follicle mouth as in endothrix infections. Ectothrix infections are caused by Microsporum sp., and T. mentagrophytes, whereas endothrix infections are due T. violaceum and T. schoenleinii. The last is the normal cause of favus.
Laboratory diagnosis of ringworm is based on direct microscopic demonstration of hyphae arthrospores in the keratinized tissue and on colonial appearance and hyphal and conidial morphology in culture. Genera are distinguished according to the morphology of macro-conidia and species by a combination of microscopic and colonial features.
The specimens of the nails should be collected after cleansing with 70 per cent ethyl alcohol. From skin lesions, scales of 2 to 3 mm in diameter should be scrapped outwards with a blunt scalpel from the active periphery of the lesion and the vesicles should be sniffed off.
Good material from nails can be obtained by scraping from the nail with a scalpel. The dusty stumps of ectothrix infections can be recognised by the naked eye, the hairs when located can be plucked with fine forceps. Specimens should be folded up in black paper.
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In the laboratory, the specimens are “cleared” with 20 per cent KOH and examined. The hyphae appear greenish, branching, running across the outlines of the colourless cells of skin or nail. In nails, arthrospores are easy to see than the hyphae and their position within or outside the hair shaft may allow a presumptive diagnosis of the species.
Culture is carried out an Sabouraud’s medium with added cycloheximide at 28°C (room temperature). Colonies of dermatophytes may be visible in two to three days. Cultures are usually reported as negative until after three weeks.
Oral griseofulvin is the treatment of choice for hair infection and those due to T. rubrum. It may be less effective for infections of the skin and nails, particularly those of the feet and those due to human strains of T. mentagrophytes for which Whitfield’s ointment should be tried first.
