Favus



Favus (Latin for "honeycomb") is a disease of the scalp, but occurring occasionally on any part of the skin, and even at times on mucous membranes. The uncomplicated appearance is that of a number of yellowish, circular, cup-shaped crusts (scutula) grouped in patches like a piece of honeycomb, each about the size of a split pea, with a hair projecting in the center. These increase in size and become crusted over, so that the characteristic lesion can only be seen round the edge of the scab. Growth continues to take place for several months, when scab and scutulum come away, leaving a shining bare patch destitute of hair. The disease is essentially chronic, lasting from ten to twenty years. It is caused by the growth of a fungus, and pathologically is the reaction of the tissues to the growth. It was the first disease in which a fungus was discovered by J. L. Schönlein in 1839; the discovery was published in a brief note of twenty lines in Millers Archive for that year (p. 82), the fungus having been subsequently named by Robert Remak; Achorion schoenleinii after its discoverer. The fungus was named after a microscopic structure termed "achorion" (a term not used in modern science), seen in scrapings of infected skin, which consists of slender, mycelial threads matted together, bearing oval, nucleated fungal substrate-arthroconidia either free or jointed. This structure is currently called "scutula." The fungus itself is now called Trichophyton schoenleinii.

During initial infection, the fungal spores would appear to enter through the unbroken cutaneous surface, and to germinate mostly in and around the hair follicle and sometimes in the shaft of the hair. In 1892, two additional "species" of the fungus were described by Paul Gerson Unna, the Favus griseus, giving rise to greyish-yellow scutula, and the Favus sulphureus celerior, causing sulfur-yellow scutula of a rapid growth. This was in the days before scientists learned to rigorously distinguish microorganism identities from disease identities, and these antique, ambiguous disease-based names no longer have status either in mycology or in dermatology.

Up until the advent of modern therapies, favus was widespread worldwide; prior to Schönlein's recognition of it as a fungal disease, it was frequently confused with Hansen's disease, better known as leprosy, and European sufferers were sometimes committed to leprosaria. Today, due to this species' high susceptibility to the antifungal drug griseofulvin, it has been eliminated from most parts of the world except rural central Asia and scattered rural areas of Africa. It is mainly a disease connected to demographic poverty and isolation, but is so readily treatable that it is among the diseases most likely to be completely eliminated by modern medicine.

Similar looking infections, sometimes diagnosed as favus but more often as atypical inflammatory tinea, may rarely be produced by agents of more common dermatophyte fungal infections, in particular Microsporum gypseum, the most common soil-borne dermatophyte fungus, and Trichophyton mentagrophytes (name used in post-1999 sense for a phylogenetic species formerly referred to as Trichophyton mentagrophytes var. quinckeanum), the agent of favus infection of the mouse.