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Despite the name, ringworm is caused by a fungus, Microsporum canis and less frequently by other species.
Ringworm infections remain limited to skin and superficial structures such as hair and less frequently nails in cats and dogs. The infecting fungi require the keratin in superficial skin layers and nails, horns, etc. for their metabolism and furthermore do not grow well at the warmer temperatures of subcutaneous tissues, hence the superficial distribution.
Note that ringworm agents are obligate parasites — they normally live on the skin, although not in pathogenic numbers.
It can be transmitted between animals by skin abrasion or mild trauma, grooming tools, scabs etc., particularly if the animal’s immune system has been compromised — for example, with steroids.
In a normal, healthy animal, ringworm infections are usually mild and self-limiting, say 1–2 months. A major motivation for getting rid of a ringworm infection is to prevent you yourself from getting it from your dog.
The round, ring-like lesions are suggestive, but not diagnostic. Your pet may have itchy, scaly, crusty, and hairless areas. Fungal culture is probably the best diagnostic method, but many vets are not set up to culture fungi. Microscopic examination of skin scrapings may reveal the actual organism.
If you think your dog or cat has ringworm, a trip to the veterinarian is in order so you can get a proper diagnosis and treatment. If your pet does have ringworm, you can get it too, but prevention is straightforward — treat your pet.
Edwin Barkdoll contributed to this article.