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I know ringworm is caused by a fungus on the skin, nails or scalp but what causes the rash to form as a ring instead of like a normal spread out rash?

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    $\begingroup$ Maybe the fungal spore infects 1 point in the skin, and the hyphae spread out through the skin in a circular pattern. The ring would be where the edge of the hyphae is. $\endgroup$ – user137 Apr 20 '15 at 14:39
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dermatophyte fungi

Ringworm is caused by a several species of Trichophyton fungi (T. rubrum, T. tonsurans, T. interdigitale, and/or T. mentagrophytes); and a couple others are well known (Microsporum canis, and Epidermophyton floccosum).

The ring pattern is not unique to ringworm causing fungi. (Fairy) Rings are totally common with fungi because mycelium grows outward from the center; and as the mycelium grows, the inner mycelium often dies (from starvation, age, attack by immune system?, or it is cannibalized) leaving only the outer ring of mycelium, which may continue expanding.

enter image description here

Unfortunately the medical world doesn't spend much time on the problems that they can already solve (like with an antifungal ointment or pill), so there hasn't been much effort devoted (to my knowledge) toward answering the mechanistic questions of ring formation of dermatophytic fungi. But it is known that some Trichophyton species sexually reproduce and Trichophyton rubrum also probably does reproduce sexually (ie., plasmogamy, karyogamy, and gametangia). I don't have any grounds for mentioning it (besides logic), but sexual reproduction is probably "age" related in fungi as is growth- so, a ring may be more pronounced in an area where fungal hyphae conjugate... much like the expression of fruiting bodies of other mushrooms.

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  • $\begingroup$ Hmm.. I too find the question interesting and the fairy rings came to my mind. The dermatophytes are keratophilic and feed on keratin. Fairy rings spread circular coz the centre is now devoid of nutrients. It is ok to relate that way but the host immune reaction should not be left unconsidered... There must be some references hidden deep in litereature :) Hope someone digs it out! $\endgroup$ – Polisetty Sep 13 '16 at 18:23
  • $\begingroup$ the fairy ring phenomenon may not be a good analogy. Apart from host responses, nutrients are unlikely to run out on the skin which constantly makes keratin that dermatophytes live on. Incidentally, there are a number of infective and non-infective conditions which have a ring-like or annular morphology. Lupus vulgaris, a type of tuberculosis, mid-borderline leprosy, pityriasis rosea, granuloma annulare and erythema annulare centrifugum, idiopathic inflammatory disorders and annular variants of lichen planus and psoriasis are some of them. So there must be more to it $\endgroup$ – Polisetty Jan 25 '17 at 17:45

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