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Fungal Infections of the Skin and Skin structures


Introduction

The clinical nomenclature for the fungal infections of the hair, nail, skin, and subcutaneous tissues is often confusing upon first encounter. The terms overlap, some of the terms sound like names of fungi (but aren't), and different definitions may be encountered in different texts. A general overview to this area is found below, and it provides one consistent scheme for thinking about this area. As you read it, it is helpful to understand these terms:

1. Phaeohyphomycosis: Used for infections due to melanin-producing, or dematiaceous, species of moulds. Melanin is a black pigment, and fungi that produce melanin are visible black or brown. The term applies to fungal structures in tissue or clinical specimens whether or not the melanin is visible without special stains. The Fontana-Masson stain is used to identify melanin in the cell walls of the fungus. The actual morphology in the tissue may consist of any combination of hyphae, pseudohyphae, or yeast cells of the infection causing fungus. A detailed discussion on Phaeohyphomycosis and its subtypes is also available.

2. Hyalohyphomycosis: This term parallels phaeohyphomycosis, but is used for infections due to NON-melanin-producing, or non-dematiaceous, species of moulds.

3. Dermatophyte: This term refers to three specific genera of fungi: Epidermophyton, Trichophyton, and Microsporum. These three genera have an especially strong association with fungal infections of the skin, hair, and nails. Indeed, the association has historically been so strong that diseases due to these three genera have often been given their own names, even though clinically identical diseases might be caused by fungi of other genera.

Forms of the disease

The syndromes discussed here are:
Dermatophytosis

The classic skin and hair infections that characterize the focus of much of medical dermatology are grouped here. There are three genera of moulds that contain the dermatophytosis-causing species. These are Epidermophyton, Trichophyton, and Microsporum. The characteristic features of a dermatophyte mould are:

  1. It belongs to one of the three genera listed above,
  2. It is keratinophilic (which is to say that it can grow on keratin), and
  3. It can do so on a living host.
There are fungi which are keratinophilic, but which don't grown on the living host. For example, Chrysosporium indicum or C. keratinophilum will grow on keratin, but only if the keratin is completely separate from the host. Feathers, animal hooves, hair, and animal skin all can be used as substrates for the keratinophilic fungi. In fact, these types of materials are used to isolate keratin from soil and other environments where this group of fungi grow.

The dermatophytes as a group are special because of their history and medical importance. They are the agents that are associated with the "tineas," a series of named diseases that use Latin binomials for their naming structure. These binomials can sound like names of fungi, but aren't. For example, Tinea capitis is the general term for dermatophytic infection of the scalp. These diseases are as shown in the list and are discussed in detail on their respective pages:
Dermatomycosis

The organisms that cause dermatomycosis produce infections of the skin and hair that in many ways parallel those caused by the dermatophytes. A wide variety of yeasts and moulds can produce infection. For example, one might see:
  1. A Tinea pedis-like infection due to Scytalidium dimidiatum
  2. A Tinea barbae-like infection due to Geotrichum candidum
Note that we do not usually use the word Tinea to describe these infections, as those terms are generally restricted to disease produced by the dermatophytes. There are, of course, exceptions to this rule.

The first exception is Tinea versicolor. Correctly known as Pityriasis versicolor, this is a distinctive syndrome in which yellow-to-brown lesions appear on the chest, trunk, or abdomen. The name versicolor comes from the range of colors that are seen. This infection is due to Malassezia furfur.

Another exception is Tinea nigra. This refers to a dark-colored superficial infection of the stratum corneum by a dematiaceous fungus called Hortaea werneckii. The palms are the most commonly involved site, but any glabrous region may be involved. Thus, this syndrome could be said to be Tinea manuum- or Tinea corporis-like. This infection occurs in people who contact salt and salty water. The fungus has a high tolerance for salt.
Onychomycosis

Similar to the idea of dermatomycosis as a parallel to the dermatophytosis, infections of the nail are so important that they get their own name. The term Onychomycosis is used both to refer to non-dermatophyte nail infections or to any fungal nail infection caused by any fungus. The term Tinea unguium can be applied only if the infection is due to a dermatophyte. However, the distinction is narrow, technical, and with not much clinical value. The leading non-dermatophyte mould cause of onychomycosis is Scopulariopsis brevicaulis. The leading yeast cause of onychomycosis is Candida albicans. It is not uncommon to have more than one fungus species jointly causing the infection.
Piedra

Piedra refers to colonization of the hair shaft that results in firm, irregular nodules. If the nodule is dark, the infection is Black Piedra and is due to Piedraia hortae. These nodules will be firmly adherent to the shaft and cannot be readily detached. The nodule is the ascomycete fruiting body of the fungus, know as an ascostroma. If the nodule is white, the infection is White Piedra and is due to Trichosporon beigelii. These nodules are a loose aggregate of hyphae and arthroconidia. They are easily detached from the hair shaft by rubbing along its length.
Mycetoma

Mycetoma and chromoblastomycosis are loosely related syndromes along a spectrum of invasion of the subcutaneous tissue. They thus differ fundamentally from all of the above forms of fungal infection due to their greater depth of tissue invasion.

Mycetoma is characterized by tissue swelling, draining sinuses, and the release of sclerotia (also known as granules or grains) from these draining sinuses. Dematiaceous or non-dematiaceous fungi may cause the infection. This results in different colors of the sclerotia and can be helpful in the identification of the causal agent. This disease is typically chronic and may result in amputation of the infected body part.

On pathological examination, one sees masses of aggregated hyphae and associated host tissue response. The architecture or pattern of the aggregated hyphae in the sclerotium can be used to help identify the causal agent.
Chromoblastomycosis

Chromoblastomycosis is an infection involving the cutaneous and subcutaneous tissue. A key pathological requirement is for the production of muriform cells, or cells that contain both horizontal and vertical dividing walls. Such infections are always due to melanin producing, dematiaceous fungi such as Fonsecaea pedrosoi. This disease may also be chronic, but amputation is never necessary.

If one sees subcutaneous infection by a mould, but without the formation of muriform cells, the general terms phaeohyphomycosis or hyalohyphomycosis are employed depending on the melanin-producing character of the fungus. Note the subtle difference here in terms of causative agents: chromoblastomycosis is always caused by melanin producing fungi.
Lobomycosis

Lobomycosis is an uncommon, except in some highly geographically restricted areas, cutaneous and subcutaneous infection caused by an organism that is called Lacazia loboi. The fungus is unique in that it has never been cultured. Based upon molecular based information, the fungus is an ascomycete.
Other

Basidiobolus, Conidiobolus, and Sporothrix can also cause indolent skin structure infections. Likewise, Candida causes skin structure infections.

Besides, several systemic fungal infections can invade the skin and cause more or less characteristic skin lesions as part of their clinical pictures. They include endemic mycosis (histoplasmosis, coccidioidomycosis, and blastomycosis) and opportunistic fungal infections (invasive candidiasis, aspergillosis, and zygomycosis).





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