Favus

Favus, also termed tinea favosa, is a chronic inflammatory dermatophytic infection usually caused by Trichophyton schoenleinii.[1, 2, 3] Rarely, favus is caused by Trichophyton violaceum, Trichophyton mentagrophytes var quinckeanum, or Microsporum gypseum. Favus typically affects scalp hair but also may infect glabrous skin and nails. The causative agent of mouse favus is T mentagrophytes var quinckeanum, also termed Trichophyton quinckeanum, which can cause favus in humans, although rarely.[4, 5]

Favus is a superficial dermatophyte infection usually caused by T schoenleinii and characterized by scutula. In most patients, favus is a severe form of tinea capitis; however, it may occur, although rarely, as onychomycosis, tinea barbae, or tinea corporis. Rarely, Tviolaceum, Trichophytonverrucosum, Microsporum audouinii, Microsporum gallinae, M gypseum, and Microsporum canis have been linked with favus.[6]  Nannizzia incurvata (formerly Microsporum incurvatum) is also a rare cause of favus.[7]

Favus is one of three primary patterns of hair infection (ectothrix, endothrix, favus). Typically, hair is not as heavily infected as in trichophytosis caused by Trichophyton tonsurans. Hair is able to grow, and frequently, long hairs are observed in the disease state. The most characteristic feature is the formation of air spaces between hyphae within the infected hair. These air spaces (air tunnels) form as a result of autolysis of the hyphae. Arthroconidia rarely are seen within the hair. Such infected hair commonly is termed favus-type hair. In the sera of patients, antibodies to causative fungi are found by charcoal agglutination and immunodiffusion assay; however, the exact role of antibodies is not clear.[8]

A survey of patients with tinea capitis in Karachi, Pakistan aged 1-14 years showed the following clinical types: gray patch in 71 (35.1%), black dot in 63 (31.2%), kerion in 50 (24.8%), favus in 10 (5%), diffuse pustular in 6 (3%), and diffuse scale in 2 (1%) patients.[9]

Primarily caused by T schoenleinii, favus rarely may be caused by T violaceum, T mentagrophytes var quinckeanum, or M gypseum. It has also been described with T rubrum.[10] Although vertical transmission may occur, family attack rates are highly variable. Most evidence suggests that favus is not a highly contagious disease. Comprehensive genome and transcriptome analysis of T schoenleinii showed candidate pathogenic genes.[11]

Frequency

United States

Favus is uncommon in the United States, although foci have been described in past decades in rural areas of West Virginia, New York, Kentucky,[12] and Arkansas. Favus often is seen in geographic regions where lifestyles are associated with malnutrition, neglect, and poverty.

International

Foci of favus have been seen worldwide, including Poland,[13] Southern and Northern Africa,[14] Pakistan,[15] the United Kingdom, Australia, South America (Brazil),[16, 17] Canada (Quebec),[18, 19] and the Middle East.[9]  Favus had been the predominate form of tinea capitis in the Nanchang region of southern China, with a 1965 prevalence of 3.4% of the population. However, it has been successfully controlled, decreasing its prevalence to 0.01% by 1977, with favus now replaced by the black dot form of tinea capitis.[20]

Race

Favus shows no racial or ethnic preference.

Sex

Both females and males may be affected equally; however, some report a slight predominance of female patients with favus.

Age

Favus appears in both children and adults. Favus usually is acquired during childhood or adolescence and typically persists into adulthood.

Favus shows no tendency to resolve spontaneously. Cicatricial alopecia with skin atrophy is a common feature of long-lasting disease. In these patients, both scalp and glabrous skin often are affected. Permanent alopecia with scarring often follows favus, which is a chronic disfiguring infection.

Favus usually begins on the scalp, often in childhood, and persists for many years as unsightly, crusted plaques. According to the severity of the disease, the following three main stages are described:

• First stage: Only erythema of the scalp is seen, primarily around follicles. Hairs are not loose or broken.

• Second stage: Formation of scutula is seen with the beginning of hair loss.

• Third stage: The most severe stage involves large areas of the scalp (at least one third); extensive hair loss, atrophy, and scarring result. Formation of new scutula at the periphery of plaques is common.

It may also be seen elsewhere, including on the scrotum.[10]

The scutulum, a yellow cup-shaped crust that surrounds a hair and pierces its center, is characteristic. Scutula form a dense plaque, each composed of mycelia and epidermal debris. Often, a secondary bacterial infection occurs in the plaque. Plaque removal leaves an erythematous moist base. The dense masses of yellow crusts may be solitary or numerous, and in patients who are severely affected, involve the entire scalp. A mousy odor typically is present. Glabrous skin may show similar yellow crusting.[21]

On glabrous skin, favus is a papulovesicular and papulosquamous eruption in which typical scutula may be evident. As an onychomycosis, tinea favosa resembles other forms of tinea unguium.

In addition to typical scutular favus on the scalp, several atypical manifestations of favus have been described.

Favus pityroides mimics dandruff or seborrheic dermatitis. Numerous small-to-large scales are present. On the surface, scales are loose; however, deeper layers are attached strongly to the base. Removal of scales uncovers reddish, moist, and scarring areas of skin.

Favus psoriasiformis is a psoriasis-imitating favus, both on the scalp and on glabrous skin. Instead of yellowish scutula, patients present with whitish scales mimicking the typical lesions of psoriasis.

Favus follicularis is characterized by cone-shaped wax-colored papules around the follicles. Hair shows the typical features of favus.

Favus impetigoides is characterized by yellowish (honey-colored) crusts imitating impetigo that are located on the scalp. Frequently, they form larger plaques that do not improve with antibacterial treatment. The characteristic mousy odor and dull hair suggest the diagnosis of favus.

Favus papyroides is characterized by small loci on the scalp that are covered by a brittle substance similar to parchment. Beneath, typical small scutula may be present.

Favus herpetiformis is an atypical variant of favus corporis. Round, erythematous, scaling plaques with small papules, vesicles, pustules, and/or crusts on the border are located on the trunk and extremities. This form of favus shows an annular shape and resembles the lesions typical of tinea corporis.

See the images below.

Base the diagnosis on mycologic examination via direct microscopy and culture. Sporadically, the diagnosis can be confirmed by histologic evidence of fungal invasion of the hair. Wood lamp examination may demonstrate a dull green fluorescence. See the image below.

Perform routine direct microscopic examination in 10-20% potassium hydroxide (KOH) solution, with or without dimethyl sulfoxide. This solution helps visualize fungal elements. With favus, hair must be examined immediately after adding KOH solution to observe the bubbling of KOH through the air spaces between hyphae elements. Examination of scutulum fragments shows segmental filaments and spores.

The causative organism is identified on culture, which usually is performed on Sabouraud agar with the addition of cycloheximide and chloramphenicol. These 2 substances inhibit growth of bacteria and other fungi; thus, pure dermatophyte colonies are obtained. In patients who are chronically affected, positive culture results may be difficult to obtain. See the images below.

Grocott staining may detect fungal elements in the crust and hair bulbs, with T schoenleinii diagnosed by polymerase chain reaction–based DNA sequencing using the formalin-fixed, paraffin-embedded tissue samples.[26]

Hematoxylin and eosin staining may visualize fungal elements insufficiently; therefore, periodic acid-Schiff staining is recommended. Biopsy sections reveal mycelium in the scutulum and horny layer of the epidermis, which is atrophic. Scutulum borders show well-preserved hyphae; however, the center of the lesion contains degenerating mycelium and granular debris. A moderately severe inflammatory infiltrate is present in the dermis.

Although favus is not highly contagious, several family members may be affected, and all should be treated simultaneously. Treatment outcome depends on the stage at which the disease is arrested. Severe long-lasting disease can cause irreversible scarring alopecia. A Chinese traditional plant-derived medicine for favus may be of value and merit development as a modern drug.[27]

In most patients, favus involves hair; therefore, the disease requires systemic treatment. Additional topical agents, such as shampoo (2% ketoconazole, 2.5% selenium sulfide), lotion, and cream may be helpful. X-ray epilation no longer is used. General hygiene of the scalp must be improved, and debris and crusts must be removed.

Favus usually is controlled by griseofulvin, the standard treatment of tinea capitis; however, a longer treatment course than usual for tinea capitis may be advisable. Currently, favus is uncommon; therefore, no clinical trials with newer antifungals are available. In vitro studies indicate that T schoenleinii is sensitive to newer antifungal drugs, similar to other dermatophytes. Terbinafine, itraconazole, and fluconazole in a similar dosage schedule to tinea capitis may eradicate the fungus and cure the disease.[28] In young children, the terbinafine tablet may be split and hidden in food.[29] A 2017 study favored the use of terbinafine and ketoconazole.[30]

Breaking the lifestyle chain associated with malnutrition, neglect, and poverty can prevent this infection and possibly deter recurrence.

Monitor patients and their families for favus in regions where lifestyles are associated with malnutrition, neglect, and poverty.

The goals of pharmacotherapy are to eradicate the infection, to reduce morbidity, and to prevent complications.

Class Summary

The mechanism of action usually involves inhibiting pathways (enzymes, substrates, transport) necessary for sterol/cell membrane synthesis or altering the permeability of the cell membrane (polyenes) of the fungal cell.

Griseofulvin (Fulvicin P/G, Grifulvin V)

Griseofulvin has fungistatic activity. Dermatophytes are sensitive, but yeastlike fungi and molds are resistant. Fungal cell division is impaired by interfering with microtubule. Griseofulvin binds to keratin precursor cells. Keratin is gradually replaced by noninfected tissue, which is highly resistant to fungal invasions.

Terbinafine (Lamisil)

Terbinafine is a fungicidal agent and member of the allylamine family. It inhibits squalene epoxidase, which decreases ergosterol synthesis, causing fungal-cell death. Use until symptoms significantly improve.

In young children, the tablet may be split and hidden in food.

Itraconazole (Sporanox)

Itraconazole has fungistatic activity. It is a synthetic triazole antifungal agent that slows fungal cell growth by inhibiting cytochrome P-450-dependent synthesis of ergosterol, a vital component of fungal cell membranes.

Itraconazole is not registered for administration in children in most countries.

Fluconazole (Diflucan)

Fluconazole has fungistatic activity. It is a synthetic oral antifungal (broad-spectrum bistriazole) that selectively inhibits fungal cytochrome P-450 and sterol C-14 alpha-demethylation, which prevents conversion of lanosterol to ergosterol, thereby disrupting cellular membranes.