The process that causes ringworm in cats (and why symptoms are circular)

There is a distinct process, which causes the ringworm symptoms that we see on cats and ourselves. It starts with the presence of one of three species of parasitic fungi: Microsporum canis (the most common), Trichophyton mentagrophytes and Microsporum gypseum. These are dermatophytes – ‘pathogenic fungi that grow on skin, mucous membranes, hair, nails, feathers, and other body surfaces…’. These fungi are not parasites of living skin but dead keratin.

Ringworm on cat in a typical position
Ringworm on cat in a typical position. Photo in public domain and modified by MikeB.
Two useful tags. Click either to see the articles: Toxic to cats | Dangers to cats

So, for cats they are parasites of keratin, a major structural protein in hair and skin. The fungus invades the hair follicle degrading it into ‘metabolites’ (substance caused through metabolism) which can be assimilated. This weakens the hair follicle causing hair loss. The skin’s keratin layer is affected leading to the keratin being sloughed off from the skin.

Toxic by-products are produced or there is an allergic reaction (anaphylactic reaction). These two processes cause inflammation which is what we see as ringworm symptoms; the redness on the skin and the flaky top layer of the skin.

Why are the symptoms circular? I believe that this is because the fungus moves from the starting point i.e. where it initially is, to outer areas. It moves away from the site of infection uniformly. The original site of infection at the centre heals and the inflammation at the edge is formed (source: Creative Biolabs).

We all know that ringworm is both highly contagious and zoonotic (transmits between animal and human and vice versa). Beware of ringworm as it is hard to eliminate permanently from humans.

Ringworm on person's arm
Ringworm on person’s arm. Pic: MikeB

Here is a little bit more information about ringworm. Cats can carry the fungus without showing any apparent infection and therefore they will present a source of infection to people and cats where they live without the recipients being aware of it. Children are particularly likely to catch the disease. Adult humans seem to be relatively resistant. Mild cases regress spontaneously. In cats with normal immunity a re-occurrence is uncommon. As I’m getting older my immune system is becoming weaker which is why, I believe, my ringworm resurfaces every several months or perhaps after 12 months. It is there but there are no symptoms normally.

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