Ringworm (feline dermatophytosis) is among the most frequently occurring skin disorders affecting the worldwide cat population. Despite its name, it is a fungal infection having nothing at all to do with worms. And the only thing it has to do with rings is the circular area of itchy rash that typically—but not necessarily—will appear on the skin of an infected animal.

The fungi responsible for the condition are called dermatophytes, microscopic organisms that originate in soil but can flourish as parasites beyond their home environment. Once entrenched in a host’s body, the fungi thrive by digesting keratin, a protein substance that is the main structural component of hair and nails. As they consume the keratin, the microscopically small fungi reproduce rapidly, creating millions of single-cell reproductive bodies (spores) that are capable of developing into new microorganisms.

When the dermatophytes come into contact with healthy feline tissue, several different phenomena may occur: they may be brushed off by a meticulously self-grooming cat; they may lose out in competition with more robust microorganisms and eventually disappear; they may establish residence on the skin without causing any adverse reactions; or, worst case, they may settle in droves on the animal’s skin and cause dermatitis—an inflammatory disease that can manifest itself in a variety of unpleasant ways.

If untreated, says William Miller Jr., a professor of dermatology at Cornell University’s College of Veterinary Medicine, ringworm in an otherwise healthy cat will probably self-resolve over time. Nevertheless, he points out, “Feline ringworm is not something that you want to take lightly. If it does go away without treatment, the process will typically take anywhere from nine months to a year, during which time the animal’s hair will keep falling out and its bare skin will be exposed. This can increase the animal’s risk of skin wounds and subsequent infection.”

Moreover, he points out, ringworm is a zoonotic disease—it can be passed from an infected cat to a human who comes in contact with it. Thus, the condition can pose a significant threat to people as well as to cats.

The clearest and most common clinical signs of feline ringworm include the following: circular areas of hair loss, broken and stubbly hair, scaling or crusty skin, alterations in hair or skin color, inflamed areas of skin, excessive grooming and scratching, infected claws or nail beds, and dandruff.

If ringworm is suspected, a veterinarian will first attempt to exclude the presence of other dermatologic conditions that the infection may resemble, such as flea allergy. Toward this goal, says Dr. Miller, the veterinarian may examine the cat’s coat with a Wood’s lamp—an ultraviolet light under which a fungus-coated hair will glow with a yellowish-green color. The hairs that fluoresce in this manner are then examined microscopically for specific spores and other fungal characteristics.

If ringworm is diagnosed, treatment will typically involve the application of topical antifungal medications to all infected areas and, in most cases, a systemic medication as well. If the lesions are present in many areas of a cat’s skin, a full-body rinse or dip may be used. It will take at least six weeks or so of repeated treatments to completely cure a feline ringworm infection, he notes, “and maybe a whole lot longer in some cases.”