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Skin infections

Fungal infections

Objectives

  • Be able to identify and manage common dermatophyte and yeast skin infections.
  • Be familiar with the range of appearance of fungal infections.

Key points

  • Dermatophytes (tinea) are Trichophyton, Microsporum and Epidermophyton species.
  • Tinea pedis is most frequently due to Trichophyton rubrum, T. Interdigitale or Epidermophyton floccosum.
  • Fungal nail infections are often confused with psoriatic nail dystrophy.
  • Zoophilic ectothrix hair shaft infections such as Microsporum canis fluoresce with Wood's light examination.
  • Anthropophilic endothrix hair shaft infections such as T. tonsurans and T. violaceum can be contracted from an asymptomatic carrier.
  • Confirm the presence of dermatophyte infection prior to treatment by sending scrapings and nail clippings for KOH microscopy and fungal culture.
  • Predisposing factors for candidiasis include general debility, immune suppression due to disease or drugs, broad-spectrum antibiotics, high oestrogen contraceptive pill or pregnancy, diabetes mellitus and iron deficiency. Local factors include humidity, occlusion and an underlying skin disease.
  • Proliferation of malassezia species results in relatively non-pruritic and flaky skin rashes.
  • Malassezia has characteristic "spaghetti and meatballs" microscopy but is difficult to culture.

Introduction

Fungi are classified according to the appearance of microscopy and in culture, and by the method of reproduction.

Growing fungi have branched filaments called hyphae, which make up the mycelium (like branches are part of a tree). Some fungi are compartmented by cross-walls (called septae). Arthrospores are made up of fragments of the hyphae, breaking off at the septae. Asexual spores (conidia) form on conidiophores. The sexual reproductive phase of many fungi is unknown; these are fungi imperfecta.

Yeasts form a subtype of fungus characterised by clusters of round or oval cells. These bud out similar cells from their surface to divide and propagate.

Superficial fungal infections affect the outer layers of the skin, the nails and hair. The main groups of fungi causing superficial fungal infections are:

Dermatophyte infections (tinea)

Tinea infections are named according to the site affected. The most common presentations are:

Tinea pedis is most frequently due to Trichophyton rubrum, T. Interdigitale or Epidermophyton floccosum. Tinea pedis has various patterns and may affect one or both feet.

Tinea pedis is more common in adults than in children and frequently recurs after initially successful treatment with topical antifungal agents because of reinfection. Fungal spores can persist for months or years in bathrooms, changing rooms and around swimming pools.

Localised hyperkeratosis

Cluster of blisters

Moccasin tinea pedis

Athlete's foot with pseudomonas infection

Athlete's foot due to tinea in a diabetic patient


Tinea unguium is most often due to T rubrum and T. mentagrophytes var interdigitale spread from tinea pedis. Fungal nail infections (onychomycosis) may also be due to yeasts or moulds. It is particularly prevalent in the elderly. Tinea unguium may affect one or more toenails and/or fingernails and most often involves the great toenail or the little toenail. It is often confused with non-infected nail dystrophy due to skin disease, particularly psoriasis.

It can present as several different patterns:

Lateral onychomycosis

Superficial white onychomycosis

Total nail destruction


Tinea capitis most often presents in young children. In New Zealand, the zoophilic dermatophyte Microsporum canis has traditionally been the cause of tinea capitis and is due to contact with an infected kitten or rarely an older cat or dog. However in the last decade, T. tonsurans and T. violaceum have also become common causes of tinea capitis; these are anthropophilic organisms, ie they naturally infect human. Anthropophilic tinea capitis may result in asymptomatic carriage in adults and children and can be transmitted by sharing hairbrushes and clothing.

Tinea capitis is classified according to how the fungus invades the hair shaft.

Wood’s light

A Wood's light is a hand-held fluorescent lamp emitting "long wave" ultraviolet A. In dark conditions, certain infections fluoresce on exposure to the light.

  • Fungal infection due to the cat ringworm Microsporum canis fluoresces green
  • Yeast infection due to certain species of malassezia (pityriasis versicolor) fluoresce yellow-green
  • Bacterial infection due to erythrasma Corynebacterium minutissimum fluoresces coral-pink

As colour contrast increases on exposure to a Wood's light, the extent of pigmentary disorders can be determined.

  • Vitiligo (white patches)
  • Pityriasis versicolor (pale patches)
  • Melasma (brown patches)

Characteristic urinary fluorescence is also helpful in the diagnosis of hepatic porphyrias (porphyria cutanea tarda).

Tinea capitis may present in several ways.

Tinea capitis due to M. canis

Tinea corporis due to T. rubrum

Tinea cruris due to E. floccosum

Kerion due to T. verrucosum in cattle farmer


Tinea corporis presents as one or more round or oval erythematous scaly plaques that slowly enlarge. The term "ringworm" refers to the tendency for an annular raised edge and central healing. T. rubrum is the most common cause in New Zealand. Infection often originates as tinea pedis or tinea unguium.

M. canis from cats and T. verrucosum, from cattle tend to cause more acute tinea corporis including kerion (inflammatory abscess).

Tinea cruris refers to groin infection and is most often seen in adult men. In New Zealand, T. rubrum and E. floccosum are the usual causes, spread from the feet or nails. The rash has a scaly raised red border that spreads asymmetrically down the inner thighs from the inguinal folds or scrotum and is frequently very itchy. Tinea cruris may form ring-like patterns on the buttocks. It is not often seen on the penis or vulva or around the anus.

Diagnosis of dermatophyte infections

To establish or confirm the diagnosis of a fungal infection prior to treatment, skin, hair and nail tissue is collected for microscopy and culture (mycology).

In the mycology laboratory, the material is examined using potassium hydroxide (KOH) to dissolve keratinocytes, then staining the preparation with blue or black ink to identify KOH-resistant mycelium and arthrospores by direct microscopy. Fungal elements are sometimes difficult to find, especially if the tissue is very inflamed, so a negative result does not rule out fungal infection.

Fungal culture may take several weeks, incubated at 25-30C. The specimen is inoculated into a medium such as Sabouraud's dextrose agar containing cycloheximide and chloramphenicol. A negative culture may arise because:

Microscopy of fungal infection (M. canis)

Culture of M. canis

Wood's light on M. canis infection of scalp


Treatment of dermatophyte infections

General measures:

Deep fungal infections

Subcutaneous fungal infections are generally due to soil fungi that normally live on rotting vegetation. They can get pricked into the skin as a result of an injury but usually stay localised at the site of implantation. In New Zealand Pacific Islanders occasionally present with mycetoma due to eumycetes (fungi) or actinomycetes such as Nocardia species, or chromoblastomycosis caused by various dematiaceous (brown) fungi.

Mycetoma

Chromoblastomycosis


Candida infections

Candida yeasts depend on a living host for survival. C. albicans is a normal inhabitant of the human digestive tract from early infancy, where it lives without causing any disease most of the time. However, if the host's defences are lowered, C. albicans and other Candida species can cause infection of the mucosa (the lining of the mouth, anus and genitals), the skin, and rarely, deep-seated infection.

Candidal skin infections include:

Characteristically candida causes red and white patches on mucosal surfaces. In skin folds it results in moist fissuring with satellite superficial papulopustules.

Refer to DermNet. for further information about each of these presentations.

Predisposing factors for infection include:

Microscopy and culture of skin swabs and scrapings id in the diagnosis of candidiasis. However, candida can live on a mucosal surface quite harmlessly. It may also secondarily infect an underlying skin disorder such as psoriasis; thus positive culture does not always mean a dermatosis is candidiasis.

Malassezia infections

Malassezia species may be identified in apparently normal skin. There is some controversy as to whether specific species cause different skin diseases.

Several skin conditions are associated with proliferation of malassezia, especially with predisposing factors humidity, sweating and seborrhoea. They include:

The diagnosis of malassezia infections is made from skin scrapings. Microscopy of potassium hydroxide (KOH) preparations shows clusters of yeast cells and long hyphae. The appearance is sid to be like "spaghetti and meatballs". Malassezia species are difficult to grow in the laboratory so scrapings may be reported as culture negative. The yeast grows best if olive oil is added to the culture medium.

Culture of C. albicans

Microscopy of malassezia