Superficial and Cutaneous Mycoses

Superficial and cutaneous mycoses are fungal infections that remain in the keratinised, largely non-living tissues of the body — the outer skin, hair, and nails. They are the most common fungal infections of humans, and although they rarely threaten life, they cause considerable discomfort and are a frequent reason for seeking care. The dermatophytoses, or tinea infections, are the central example.

Definition

Fungal infections confined to keratinised tissue — the stratum corneum of the epidermis, hair, and nails — without invasion of living tissue; the cutaneous mycoses are typified by the dermatophytoses (tinea), and the superficial mycoses by minimally inflammatory infections of the outermost layers.

Scope

The topic covers infections limited to the stratum corneum, hair shafts, and nail plate: the superficial mycoses such as pityriasis (tinea) versicolor and tinea nigra, and the cutaneous mycoses caused by dermatophytes (Trichophyton, Microsporum, Epidermophyton) that invade keratin. It treats these as a reference category and does not give individualised diagnostic or treatment advice.

Key concepts

• Keratinophilic and keratinolytic fungi
• Dermatophytes (Trichophyton, Microsporum, Epidermophyton)
• Tinea nomenclature by body site (capitis, corporis, pedis, unguium)
• Anthropophilic, zoophilic, and geophilic sources
• Pityriasis versicolor (Malassezia)
• Restriction to non-living keratin
• Direct microscopy and culture for diagnosis

Mechanisms

Dermatophytes produce keratinases and related enzymes that allow them to use keratin as a nutrient, colonising the stratum corneum, hair, and nail while generally not penetrating living tissue; the inflammatory response reflects the host's reaction to the fungus rather than deep invasion (Seebacher, 2008; Rippon, 1988). Dermatophytes are classified by their natural reservoir as anthropophilic (human-adapted), zoophilic (animal), or geophilic (soil), and this source influences how readily they transmit and how much inflammation they provoke. Superficial yeasts such as Malassezia exploit the lipid-rich skin surface. The depth of these infections — restricted to keratin — is what places them at the shallow end of the depth-and-distribution scheme.

Clinical relevance

Because they are so common, superficial and cutaneous mycoses are a routine part of primary care and dermatology, and recognising the tinea pattern and confirming it with simple microscopy or culture is a core diagnostic skill. This entry describes the category and its microbiology for reference and is not a guide to treating any individual patient.

Epidemiology

Dermatophytoses are estimated to affect a very large fraction of the world's population at some point, and superficial fungal infections collectively number in the hundreds of millions of cases, making them among the most prevalent of all human infections; their distribution is ubiquitous, with the specific causative species varying by region, climate, and patterns of human and animal contact (Bongomin, 2017; Seebacher, 2008).

History

The dermatophytoses were among the first human infections shown to be caused by a microbe: in the late 1830s the fungal cause of favus was identified, predating much of bacteriology, and the tinea infections became a foundational subject of medical mycology as the field was consolidated in texts such as Rippon's Medical Mycology (Rippon, 1988). The classification of dermatophytes by anthropophilic, zoophilic, and geophilic source emerged from this tradition (Seebacher, 2008).

Seminal works

• seebacher-2008
• bongomin-2017
• rippon-1988

Frequently asked questions

What is the difference between superficial and cutaneous mycoses?

Both stay in keratinised tissue, but superficial mycoses involve only the outermost layers with little or no inflammation, while cutaneous mycoses — the dermatophytoses or tinea — invade the keratin of skin, hair, and nail and typically provoke an inflammatory skin reaction.

Why is ringworm not caused by a worm?

"Ringworm" is the common name for tinea, a dermatophyte infection; the name comes from its ring-shaped, advancing skin lesion, not from any worm.