Fungal Infections of the Skin and Nails

Raimo E. Suhonen, Rodney P.R. Dawber, David H. Ellis

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9781000148961
• Edition: 1
• Topic: Medicine
• Subtopic: Medical Theory, Practice & Reference

This concise, comprehensive guide is divided into two sections; nails and the skin. Each section includes information on the types of infections, aetiology, diagnostic procedures, such as sampling techniques, and therapy, including topical, systemic and adjunctive.

1 AETIOLOGY AND LABORATORY DIAGNOSIS

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The cutaneous mycoses are superficial fungal infections of the skin, hair or nails. Essentially no living tissue is invaded. However, a variety of pathological changes occur in the host because of the presence of the infectious agent and/or its metabolic products. The principal aetiological agents are as follows: dermatophytic moulds belonging to the genera Microsporum, Trichophyton and Epidermophyton which cause ringworm or tinea of the scalp, glabrous skin and nails; Malassezia (Pityrosporon) furfur, a lipophilic yeast responsible for pityriasis versicolor, follicular pityriasis, seborrhoeic dermatitis and dandruff; and Candida albicans and related species, which cause candidiasis of skin, mucous membrane and nails; the latter may also colonise many moist skin eruptions without being causative.

In world terms, fungal infections of the skin, hair and nails are some of the commonest infections in humanity. The development of very active and successful antifungal drugs in recent years has greatly increased clinical interest in these particular infective agents. The advent of very potent immunosuppressive drugs for a variety of diseases (including HIV infection) has also enabled a variety of fungi to penetrate and cause severe superficial and systemic spread that was previously extremely rare.

Aetiological agents

Dermatophytosis (tinea or ringworm) of the scalp, skin and nails

Dermatophytosis of the scalp, glabrous skin and nails is caused by a closely related group of fungi known as dermatophytes which have the ability to utilise keratin as a nutrient source, i.e. they have a unique enzymatic capacity (keratinase). The disease process in dermatophytosis is unique for two reasons: first, no living tissue is invaded — the keratinised stratum corneum, hair or nail is simply colonised. However, the presence of the fungus and its metabolic products typically induces an inflammatory response in the host. The type and severity of the host response are often related to the species and strain of dermatophyte causing the infection. Second, the dermatophytes are the only fungi that have evolved a dependency on human or animal infection for the survival and dissemination of their species. In fact, the common anthropophilic species (Table 1.1) are primarily parasitic on humans. They are unable to colonise other animals and have no other environmental sources. Geophilic species normally inhabit the soil where they are believed to decompose keratinaceous debris. Some species may cause infections in animals and humans following contact with soil. Zoophilic species are primarily parasitic on animals and infections may be transmitted to humans following contact with the animal host (Table 1.1). Zoophilic infections usually evoke a strong host response on the skin where contact with the infective animal has occurred, i.e. arms, legs, body or face.

Table 1.1 Ecology of common dermatophyte species

Species	Natural habitat	Incidence
Epidermophyton floccosum	Humans	Common
Trichophyton rvbrum	Humans	Very common
T. mentagrophytes var. interdigitale	Humans	Common
Trichophyton tonsurans	Humans	Common
Trichophyton violaceum	Humans	Less common
Trichophyton concentricum	Humans	Rare*
Trichophyton schoenleinii	Humans	Rare*
Trichophyton soudanense	Humans	Rare*
Microsponjm audouinii	Humans	Less common*
Microsporum ferrugineum	Humans	Less common*
T. mentagrophytes var. mentagrophytes	Mice, rodents	Common
Trichophyton equinum	Horses	Rare
Trichophyton verrucosum	Cattle	Rare
T. mentagrophytes var. quinckeanum	Mice, hedgehogs	Rare*
T. mentagrophytes var. erinacei	Cats	Common
Microsporum canis	Soil	Common
Microsponjm gypseum	Soil/pigs	Rare
Microsporum nanum	Soil	Rare
Microsponjm cookei

*Geographically restricted.

Infections by anthropophilic dermatophytes are usually caused by the shedding of skin scales containing viable infectious hyphal elements (arthroconidia) of the fungus. Desquamated skin scales may remain infectious in the environment for months or years. Therefore transmission may take place by indirect contact long after the infective debris has been shed. Substrates, such as carpet and matting that hold skin scales, make excellent vectors. Thus, transmission of such dermatophytes as Trichophyton rubrum, T. mentagrophytes var. interdigitale and Epidermophyton Jloccosum is usually via the feet. In this site infections are often chronic and may remain subclinical for many years only to become apparent when spread to another site, usually the groin or skin. It is important to recognise that the toe web spaces are the major reservoir on the human body for these fungi and therefore it is not practical to treat infections at other sites without concomitant treatment of the toe web spaces. This is essential if ‘cure’ is to be achieved. It should also be recognised that individuals with chronic or subclinical toe web infections are carriers and are constantly shedding infectious skin scales.

Onychomycosis

Onychomycosis or fungal infection of the nails may be caused by either dermatophyte fungi (tinea unguium) or non-dermatophyte fungi and yeasts. Dermatophytes are the principal pathogens, accounting for 90% of toenail infections and at least 50% of fingernail infections. Trichophyton rubrum and T. mentagrophytes var. interdigitale are the dominant dermatophyte species involved. Candida is mainly associated with paronychia that initially primarily affects the nail folds. The main non-dermatophyte moulds involved in onychomycosis are Scopulariopsis and Scytalidium, and such infections account for between 1.5 and 6% of nail infections. Many incidental non-dermatophytes and to a lesser extent yeasts may be isolated from non-sterile nail samples; these may become secondary colonisers.

In such countries as Australia, the UK and the USA, the incidence of onychomycosis has been estimated to be about 3% of the population, increasing to 5% in the elderly; some subgroups (such as miners, service personnel and sportspersons) have an incidence of up to 20% due to the use of communal showers and changing rooms. It is important to stress that fewer than 50% of dystrophic nails are of fungal aetiology and that it is therefore essential to establish a correct laboratory diagnosis by microscopy and/or culture before treating a patient with a systemic antifungal agent.

Identification of common dermatophytes

The identification of dermatophytes (Table 1.2) is primarily based on the microscopic morphology of the fungus. A good slide preparation is required and in some strains sporulation stimulation may be required. 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-sporulating species such as M. audouinii, T. concentricum, T. schoenleinii and many others.

Epidermophyton group

Epidermophyton floccosum is an anthropophilic dermatophyte with a worldwide distribution that often causes tinea pedis, tinea cruris and tinea corporis. Key features include characteristic greenish-brown or khaki-coloured cultures, the production of smooth, thin-walle...

Table of contents

• Cover
• Half Title
• Dedication
• Title Page
• Copyright Page
• Table of Contents
• Preface
• 1. Aetiology and laboratory diagnosis
• 2. Dermatomycoses
• 3. Nails (onychomycosis): clinical aspects
• 4. Therapy for skin, hair and nail fungal infections
• Index