Mycology-Superficial Mycoses | Detailed Revision Notes for NEET-PG

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Tinea Versicolor

Superficial fungal infection involving stratum corneum of the skin caused by Malassezia furfur

C/F

Commonly occurs in a humid environment

Flat round scaly patches of hypo to hyperpigmented skin

Non-inflammatory lesions

Lab Dx

Collect skin scrapings——–>treat with 10% KOH——–>budding yeasts with septate hyphae seen

                                                                                                                (spaghetti and meatball appearance)

                                         Incubation period of 5-7 days                                    

Culture on SDA agar—————————————–> fried egg appearance

Positive urease test

Wood’s lamp examination- scaly lesion show golden yellow fluorescence under Wood’s lamp

Tinea Nigra

 

Superficial Fungal infection involving palm and soles caused by hortaea werneckii

C/F

Painless, black, non-scaly lesion on palm and soles

Piedra

Superficial fungal infection of the hair shaft

C/F

While/black nodule attached to the hair shaft

It may be white piedra or black piedra

White piedra

White nodule  loosely attached to the hair shaft

It is caused by Trichosporon beigelii ( yeast-, like a fungus)

Dx

Produces white creamy colonies

Hyaline septate hyphae intervening with arthrospores can be seen under the microscope

Black piedra

Black nodule firmly attached to the hair shaft

It is caused by Piedraia hortae (phaeoid fungus)

Dx

Produces reddish brown colonies

Brown septate hyphae with ascospores can be seen under the microscope

Dermatophytoses/ Ringworm

MC superficial mycoses

Superficial fungal infection of skin, hair, and nails caused by dermatophytes

Classification

Based on habitat

  1. Arthrophilic(infects humans)————–> T. rubrum and T. mentagrophytes
  2. Zoophilic(infects animals and birds)—–>M. canis and M. equinum
  3. Geophilic(infects soil)———————–>M. gypseum  and E. stockdaleae

Dermatophytes include

Trichophyton species—————–>infects skin+ hair+nail———–> T. rubrum and T. mentagrophytes

Microsporum species—————–>infects skin+ hair          ———–> M. canis and M. gypseum

Epidermophyton species————->infects skin           +nail———–> E. Floccosum

Pathogenesis

Direct contact with animal/soil/humans———->scratching of the inoculated site———->spores carried to different parts

  1. Spores carried to skin———->fungus grow in a centrifugal pattern in stratum corneum———->formation of well-demarcated annular/ring shaped pruritic scaly skin lesions with central clearing and raised edges
  2. Dermatophytes invade nails and spread
  3. Spores carried to shaft———->inhlammation in response to fungal antigen———->hair becomes brittle with areas of alopecia

Clinical types of Dermatophytes

  1. Tinea capitis-infection of the scalp with scaly patches on the scalp
    1. Kerion——->painful boggy lesions on the scalp
    2. Favus——.–>crust formation on infected hair
    3. Ectothrix—->arthrospores formed on hair shaft——————————————> caused by M. canis, M. mentagrophytes, etc
    4. Endothrix—>arthrospores formed within the hair shaft(can result in alopecia)——–>caused by T. Tonsurans, etc
  2. Tinea corporis – infection of non-hairy skin of the body
  3. Tinea pedis(athelet foot)- infection of web space between toes which spreads to the sole in a moccasin pattern.
  4. Tinea cruris- infection of the groin area

etc

Lab Dx

  1. Collect skin scrapings/hair plucks/nail clippings etc based on site of infection

                                                                                   10% glycerol to prevent drying

  1. Keratinized tissue specimen + 10% KOH————————————————–> keratin digested and

        ( skin scraping or hair)               (20-40% for nails and biopsy tissue)                                 clear fungal hyphae seen

                                                          4 weeks

Innoculate onto SDA agar————–>observe the rate of growth, texture, pigmentation, and topography———-> do LPCB mount and view under the microscope to demonstrate hyphae and spores———–> macro and microconidia can be seen with thin/spiral/racquet/chandeliers hyphae.

Other methods

  1. Hair preformation test- take a petri dish containing water, yeast extract, and hair ——->innoculaye a colony onto a petri dish——->fungi(Trichophyton mentagrophytes and Microsporum canis) pierce the hair——-> produce wedge-shaped perforations.
  2. Trichophyton mentagrophytes is urease positive
  3. Dermatophyte test medium
  4. Dermatophyte identification medium
  5. PCR
  6. Skin test to detect hypersensitivity RXN due to Trichophyton.

 

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