Ringworm is caused by several types of fungi which live on the skin, like athlete's foot in people. The medical term for ringworm is dermatophytosis. The lesions caused by ringworm can vary a lot in appearance.
Ringworm is diagnosed by culturing some hairs plucked from the animal.
This means the hairs are placed in a tube that contains the ingredients the fungi need to grow. Generally it takes the fungi 10-14 days to grow to a number large enough for us to identify. Some ringworm infections will cause the hair to fluoresce under a black light, and this can help in the diagnosis. The fungi can infect dogs and cats, usually when they are young. Ringworm is contagious to people, so it is very important to handle infected animals with care.