Mycosis

The Term mycosis (plural: mycoses) refers to conditions in which fungi pass the resistance barriers of the human or animal body and establish infections.

Classification

Mycoses are classified according to the tissue levels initially colonized:

1. Superficial mycoses - limited to the outermost layers of the skin and hair.
2. Cutaneous mycoses - extend deeper into the epidermis, as well as invasive hair and nail diseases. These diseases are restricted to the keratinized layers of the skin, hair, and nails. Unlike the superficial mycoses, host immune responses may be evoked, resulting in pathologic changes expressed in the deeper layers of the skin. The organisms that cause these diseases are called dermatophytes. The resulting diseases are often called ringworm (even though there is no worm involved) or tinea. Cutaneous mycoses are caused by Microsporum, Trichophyton, and Epidermophyton fungi, which together comprise 41 species.
3. Subcutaneous mycoses - involve the dermis, subcutaneous tissues, muscle, and fascia. These infections are chronic and can be initiated by piercing trauma to the skin, which allows the fungi to enter. These infections are difficult to treat and may require surgical interventions such as debridement.
4. Systemic mycoses due to primary pathogens - originate primarily in the lungs and may spread to many organ systems. Organisms that cause systemic mycoses are inherently virulent. Generally, primary pathogens that cause systemic mycoses are dimorphic.
5. Systemic mycoses due to opportunistic pathogens - infections of patients with immune deficiencies who would otherwise not be infected. Examples of immunocompromised conditions include AIDS, alteration of normal flora by antibiotics, immunosuppressive therapy, and metastatic cancer. Examples of opportunistic mycoses include Candidiasis, Cryptococcosis and Aspergillosis.

Another example of a fungal infection is Tinea versicolor: Tinea versicolor is a fungus infection that commonly affects the skin of young people, especially the chest, back, and upper arms and legs. Tinea versicolor is caused by a fungus that lives in the skin of almost all adults. It doesn't usually affect the face. This fungus produces spots that are either lighter than the skin or a reddish-brown. There may be just a few spots or lots of spots. Tinea versicolor is caused by a fungus that lives in the skin of almost all adults. This fungus exists in two forms, one causes visible spots. Factors that can cause the fungus to become more visible include high humidity and immune or hormone abnormalities. But almost all people with this very common condition are healthy.

Causes

As the epidermal layer continually replaces itself, cells are pushed outward where they eventually die and flake off. In most people, these flakes of skin are too small to be visible. However, certain conditions cause cell turnover to be unusually rapid, especially in the scalp. For people with dandruff, skin cells may mature and be shed in 2 - 7 days, as opposed to around a month in people without dandruff. The result is that dead skin cells are shed in large, oily clumps, which appear as white or grayish patches on the scalp skin and clothes.

Dandruff has been shown to be the result of three required factors:^[1]

1. Skin oil commonly referred to as sebum or sebaceous secretions^[2]
2. The metabolic by-products of skin micro-organisms (most specifically Malassezia yeasts)^{[3][4][5][6][7]}
3. Individual susceptibility

Common older literature cites the fungus Malassezia furfur (previously known as Pityrosporum ovale) as the cause of dandruff. While this fungus is found naturally on the skin surface of both healthy people and those with dandruff, it was discovered that a scalp specific fungus, Malassezia globosa, is the responsible agent.^[8] This fungus metabolizes triglycerides present in sebum by the expression of lipase, resulting in a lipid byproduct oleic acid (OA). Penetration by OA of the top layer of the epidermis, the stratum corneum, results in an inflammatory response in susceptible persons which disturbs homeostasis and results in erratic cleavage of stratum corneum cells.^[5]

Rarely, dandruff can be a manifestation of an allergic reaction to chemicals in hair gels/sprays, hair oils, or sometimes even dandruff medications like ketoconazole.

There is no convincing evidence that food (such as sugar or yeast), excessive perspiration, or climate have any role in the pathogenesis of dandruff.

Dandruff

Dandruff (also called scurf and historically termed Pityriasis capitis) is due to the excessive shedding of dead skin cells from the scalp. As it is normal for skin cells to die and flake off, a small amount of flaking is normal and in fact quite common. Some people, however, either chronically or as a result of certain triggers, experience an unusually large amount of flaking, which can also be accompanied by redness and irritation. Most cases of dandruff can be easily treated with specialized shampoos. Dandruff is not an organism like lice; it is just dead skin that accumulates in the scalp. Dandruff is unlikely to be the cause of hair loss.

Excessive flaking can also be a symptom of seborrhoeic dermatitis, psoriasis, fungal infection or excoriation associated with infestation of head lice.

Dandruff is a global phenomenon and many people find that dandruff can cause social or self-esteem problems. Treatment may be important purely for psychological reasons.