This article is from the Canine Medical Information FAQ, by Cindy Tittle Moore with numerous contributions by others.
(prepared by Edwin Barkdoll)
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 like 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
ringoworm 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, e.g. 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_ the owner from getting it.
If it is a mild infection topical application of lime sulfur is
supposed to be good, although it can be smelly. Chlorhexidine shampoo
is also effective as is also a relatively new 2% miconazole shampoo
($$$). If the infection is severe, oral griseofulvin is effective but
The round, ring-like lesions are suggestive but _not_ diagnostic and
are not even the typical lestion in cats and dogs. The animal 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. A Wood's lamp can be used but not all ringworm agents will
fluouresce so absence of fluourescence does not mean no ringworm,
furthermore other things besides ringworm also fluoresce. In other
words Wood's lamp is not a great test. Microscopic examination of skin
scrapings may reveal the actual organism.
Finally, if you think your dog/cat has ringworm take it to the vet for
diagnosis and treatment. If it does have ringworm, _you_ can get it,
but prevention is straightforward - treat your animal.