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An immune-mediated cause of patchy hair loss. Historical “tinea decalvans” cannot be assumed to denote a proven fungal infection.

The historical name pityriasis grouped finely scaling eruptions; its varieties were not all one fungal infection. A useful period list is simple or scalp pityriasis, pityriasis rubra, pityriasis versicolor and pityriasis nigra. The names describe distribution or colour, and classifications differed. In particular, “nigra” was sometimes a very dark form of “versicolor,” sometimes a pigmentation disorder with little true scaling. It cannot safely be translated into one present species of fungus.^[ref]

Simple pityriasis, often called pityriasis capitis on the scalp, gives loose, whitish, bran-like scales with little or variable redness. Microscopy of a scale principally shows flattened keratinised epithelial cells and debris. An oily or inflamed scalp can also contain organisms normally living there; this does not make every case identical to pityriasis versicolor. Present descriptions of dandruff and seborrhoeic dermatitis explain many such appearances, but the historical label cannot retrospectively specify the cause of each case.^[ref]

Pityriasis rubra denoted a conspicuously red, scaling eruption, sometimes extensive. Detached scales are again principally epidermal material. A modern skin biopsy may show an inflammatory pattern, but there is no single microscopical organism that defines the historical category. It must not automatically be renamed pityriasis rubra pilaris, a distinct follicular disorder. An extensive red and peeling skin is a pattern requiring causal diagnosis, rather than proof of one infection.^[ref]

Pityriasis versicolor produces irregular pale, yellowish-brown or brown patches, commonly on the upper anatomical trunk, with a fine surface scale. Here the microscopy is especially discriminating: a superficial scraping, cleared with potassium hydroxide, shows clusters of round yeast forms and short curved hyphae within the stratum corneum. Modern classification places the organisms in Malassezia. The older “Microsporon furfur” terminology should not be read as identification of a modern Microsporum dermatophyte. On a histological section the organisms occupy the horny surface layer and are made conspicuous by suitable fungal stains.^[ref]

Pityriasis nigra calls first for verification of what is actually being described. If it is dark pityriasis versicolor, the same superficial yeast and hyphae may be found; if the lesion is pigmentation without a fungal eruption, this finding cannot be assumed. A contemporary discussion explicitly recorded the overlap of nigra and versicolor and disputes over the significance of the supposed parasite.^[ref] Thus epidermal scales are common to the description, but demonstrable fungal structures particularly distinguish pityriasis versicolor. Examine actual scrapings with a optical microscope, compare several fields and use appropriate controls before attributing a different scaling disorder to a fungus.

The historical common principle was to identify and destroy a superficial fungal parasite, remove material harbouring it and prevent reinfection. Modern diagnosis preserves this principle for genuine fungal infection, but rejects its indiscriminate application to “tinea decalvans”. The three labels must therefore be discussed separately.

In pityriasis versicolor, look for finely scaling patches of altered skin pigmentation, commonly on the trunk. Examine superficial skin scrapings by microscopy when uncertain. Historically, washing, removal of loose scale and a topical “parasiticide” aimed to reach the organism in the outer epidermis. Today pityriasis versicolor is associated with overgrowth of Malassezia yeasts already present on the skin; it is not ordinary contagious ringworm and is not evidence of poor cleanliness.^[ref] Topical ketoconazole or selenium sulfide can control it. Extensive or recurrent disease sometimes needs other treatment. Loss of scale and control of the yeast need not immediately restore skin pigmentation; residual colour can persist for weeks or months. A Wood's lamp can assist examination, but is not a substitute for the whole clinical assessment.^[ref]

“Tinea decalvans” is the critical historical trap. Some nineteenth-century writers interpreted patchy baldness as parasitic and advocated extraction of affected hairs followed by prolonged topical treatment. A January–June 1867 digest explicitly calls the condition contagious and reports a mercury(II) chloride preparation. That is evidence of a contemporary belief, not evidence that the proposed organism caused every case of patchy baldness.^[ref] The label often covered what is now called alopecia areata, an immune-mediated form of hair loss. Smooth bald patches require differentiation from scaly patches with broken hairs and demonstrated fungal infection. Confirm the diagnosis before treating. In established alopecia areata, a period “parasiticide” is inappropriate: observation or diagnosis-specific treatment, including local corticosteroids in suitable cases, is considered instead. Neither mercury(II) chloride nor destructive epilation is a modern treatment for alopecia areata.^[ref]

For contagious ringworm, establish the site and extent. An enlarging scaly ring on bare skin suggests tinea corporis; scaly bald areas with broken hairs suggest tinea capitis. Examine scrapings or hairs by microscopy and, where appropriate, fungal culture. Clean clothing, towels, combs and brushes; avoid sharing them and investigate relevant human or animal sources. These measures address transmission, while an antifungal drug treats the affected tissue.^[ref] Limited tinea corporis can often be treated topically. Tinea capitis needs an oral antifungal drug, such as griseofulvin or terbinafine chosen for the circumstances, because a cream alone does not clear the infected hair-bearing site. An adjunctive shampoo can reduce spread but does not replace systemic treatment. Unsupervised corticosteroid cream can mask or worsen ringworm.^[ref][ref]

Thus the useful unifying idea is cause-directed treatment that reaches the affected tissue, with transmission control when infection is transmissible. It does not follow that all three nineteenth-century names denote the same class of disease, or that every bald patch requires killing a parasite.

A dermatophyte fungal infection affecting keratin-containing tissue; the name does not imply a worm.