Favus. Trichophyton Schoenleinii infection. Latin: favus, honeycomb (note that kerion, a different form of tinea capitis/barbae, also means honeycomb but in Greek).
Hair invasion results in favus endothrix pattern. The fungus is entirely confined within the hair shaft but does NOT fragment into arthroconidia (thus the infected hair commonly grows to normal lengths). Actually the relatively few hyphae run intact through the hair, forming tunnels within its structure. When first immersed in KOH, air is initially trapped around the hyphae forming the characteristic, long air spaces. These rapidly fill in with KOH, when the hyphae themselves become visible.
The classical picture of tinea capitis caused by this organism is characterized by the presence of yellowish cup-shaped crusts known as scutula. Adjacent crusts enlarge to become confluent and form a mass of yellow crusting (with unpleasant odour). Having said so, there might be less distinctive changes in the early cases (perifollicular redness and some matting of the hair). Seborrhoeic dermatitis with or without alopecia areata, pityriasis amiantacea, psoriasis and impetigo are considered in the differential diagnosis. Scalp demodicidosis mimicking favus has been reported*.
Trichophyton schoenleinii causes a pale green fluorescence of infected hair under Wood’s light. Because fungi growing in culture or on hair in vitro do not fluoresce in this way, the phenomenon must be attributed to some substance (? a pteridine) produced by the interaction of the fungus and the growing hair.
Extensive, patchy hair loss with cicatricial alopecia and atrophy among patches of normal hair may be found in long-standing cases, where much of the hair loss is irreversible. Discoid lupus erythematosus, lichen planus and other causes of cicatricial alopecia are considered in the differential diagnosis.
Favus appears chiefly on the scalp but may affect the glabrous skin and nails. When infected the glabrous skin develops similar yellowish crusts. T. schoenleinii nail invasion (up to 3% of infections) is indistinguishable from other causes of onychomycosis. T. violaceum and M. gypseum infection can produce a favus-like clinical picture. It is interesting to note that favus among the Bantus in South Africa is called witkop (in Afrikaans it means “the white head”). Tinea capitis in general is predominantly an infection of children, although adult cases are occasionally seen. Favus in particular, shows little if any tendency to clear spontaneously at puberty. Tinea capitis may also be seen in adults with AIDS.
Ketoconazole shampoo or selenium sulphide shampoo is helpful to prevent spread in the early phases of therapy, when used in combination with an oral treatment [higher dosage of griseofulvin (20 mg/kg/day) for much longer course may be needed].
*García-Vargas A, Mayorga-Rodríguez JA, Sandoval-Tress C. Scalp demodicidosis mimicking favus in a 6-year-old boy. J Am Acad Dermato. 2007; 57, suppl. 2: S19-S21.