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Microsporum canis: Tinea corporis (ringworm)

A 23-year old man presented to skin clinic with multiple, annular-oval pruritic lesions on the arms and legs. The borders of the lesion were erythematous, raised and had few vesicles. The man reported that he owned a pet cat, which too had some skin lesions.

What is your diagnosis?
It could be ringworm, also known as tinea corporis. Differential diagnoses include cutaneous candidiasis, erythrasma, pityriasis posea, psoriasis, secondary syphilis and tinea versicolor.

What is the specimen collected?
Since dermatophytes undergo radial growth, the centers of infected skin patches may consist of the older and poorly viable material. Before specimen collection, any ointments or other local applications present should first be removed with an alcowipe. Collection is best made by collecting epidermal scales from near the advancing edges of the rings. The sample should be taken from the active border of a lesion because this gives the highest yield of fungal elements. The lesion is lightly disinfected with alcohol in gauze and then scraped from edge to center, crossing the lesion margin, using a sterile scalpel blade. Suppurating lesions may be sampled with a swab when it is impractical to obtain scrapings.

How is the sample transported?
Sample materials are best transported in dry, strong black folded paper or envelope. Alternatively, scrapings can be transported in sterile Petri dishes or between two glass slides. Moisture of any kind has to be avoided. Black paper allows easy visualization of small skin squames.

Which the necessary investigations to be performed?
Skin scrapings are observed for presence of fungal elements by microscopy. Different methods of microscopy include 20% KOH mount, 20% NaOH mixed with 5% glycerol, 20% KOH with 36% dimethyl sulfoxide, warming the slide by passing few times over the flame, heating the mount for one hour at 51-54oC or by fluorescence staining using calcofluor white or Congo red. The KOH helps dissolve the keratin and leaves fungal elements intact. A counterstain for KOH mount, such as chlorazol black E or Parker blue-black ink, may be useful in helping to visualize hyphae under the microscope. A KOH preparation from a vesicular lesion should be made from the roof of the vesicle. Microscopy should be followed by culture on Sabouraud's Dextrose Agar with additives. Dermatophytes may take 1-2 weeks to form colonies on agar.

Describe the culture methods?
Emmons’ modification of  Sabouraud's Dextrose Agar with cycloheximide and chloramphenicol is commonly used. It is commercially available under various names such as Mycobiotic and Mycosel agars. Dermatophyte Test Medium can also be used. Due to the presence of pH indicator Phenol red, the alkalinity produced by the growing dermatophyte changes the colour of the medium to red. Casamino Acids-erythritol-albumin medium is a highly selective medium for isolating dermatophytes from lesions heavily contaminated by bacteria or Candida. Bromocresol purple-casein-yeast extract agar, which grows all dermatophytes is another alternative medium. The cultures are incubated at room temperature until two weeks.

What is your observation?
KOH preparation of skin scrapings reveals numerous septate branching hyphae. SDA culture yields colonies having a white to yellow color with a yellow to orange reverse. A tease mount preparation stained with lactophenol cotton blue (LPCB) shows  abundant macroconidia that are thick-walled and with many septa. Macroconidia are often hooked or curved at ends. Microconidia are small and clavate (club-shaped). If tease mount fails to reveal the arrangement of conidia, a slide culture may be set up.

What is your identification?
The fungal isolate is Microsporum canis.

What are dermatophytes?
Dermatophytes are fungi that invade only the dead, cornified layers of the skin, nails and hair. There are three genera of dermatophytes, namely Trichophyton, Microsporum and Epidermophyton. These are anamorphic members of class Hyphomycetes of the phylum Deuteromycota. The sexually reproducing forms (teleomorphs) are classified in the genus Arthroderma in family Arthrodermataceae of the phylum Ascomycota. Infection of skin by fungi other than dermatophytes is called dermatomycosis. Dermatophytes do not invade living tissue but colonize keratinised stratum corneum of skin.

How are dermatophytes classified?
Dermatophytes are classified ecologically and based on host preference and endemicity into three groups; Anthrophilic, Zoophilic and Geophilic. Members of Anthrophilic group include E. floccosum, M. audouinii, T. concentricum, T. mentagrophytes var. interdigitale, T. rubrum, T. schoenleinii, T. tonsurans, T. violaceum. Members of zoophilic group include M. canis, M. equinum, M. gallinae, T. equinum, T. verrucosum, T. mentagrophytes var. mentagrophytes. Members of geophilic group include M. gypseum, M. amazonicum, M. cookei, T. ajelloi, T. terrestre, T. flavescens.

What are the major differences between these three genera?
Microsporum species produce abundant macroconidia and few microconidia, Tricophyton produce abundant microconidia and few macroconidia whereas Epidermophyton produces only macroconidia. Microsporum infects hair and skin but not nail, Epidermophyton infects nail and skin but not hair whereas Trichophyton can infect hair, nail and skin.

Which group does the isolate from this patient belongs to and what is the source of infection?
Microsporum canis is a zoophilic fungus and the possible source of his infection is his infected pet cat. M. canis is usually acquired from infected cats or dogs, although limited human-to-human transfer leading to outbreaks can occur. Zoophilic and geophilic dermatophytes in general tend to form lesions that are more inflammatory than those formed by anthropophilic dermatophytes but are also more likely to resolve spontaneously and are easier to treat.

Which are the other dermatophytes that can cause tinea corporis?
Trichophyton rubrum is the most common infectious agent in the world. Trichophyton tonsurans, Microsporum canis and Trichophyton mentagrophytes are the other common dermatophytes infecting skin. Tinea imbricata is a form of tinea corporis found mainly in Southeast Asia, the South Pacific, Central America, and South America and is caused by Trichophyton concentricum.

How do you treat this condition?
Topical therapy should be applied to an area at least 2 cm beyond the edge of the identified lesion once or twice a day for at least 2 weeks. The topical azoles include econazole, ketoconazole, clotrimazole, miconazole, oxiconazole, and sulconazole. Systemic therapy may be indicated for cases of tinea corporis that are extensive, those that involve patients who are immunocompromised, or those that are not responsive to topical therapy. Orally administered antifungals are griseofulvin, ketoconazole, fluconazole, itraconazole, and terbinafine.

Which are the other infections caused by dermatophytes?
Besides tinea corporis, dermatophytes can also cause tinea capitis, tinea barbae, tinea mannum, tinea unguim, tinea cruris, and tinea pedis. A hypersensitivity reaction to their antigen called "id reaction" is known to occur elsewhere on the body. For more information on id reaction, refer my notes.

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