The correct term for ringworm is "dermatophytosis". This is the skin disease caused by certain types of fungus called "dermatophytes"(meaning skin, lover-of). These are types of fungus that "love" the skin, more specifically in animals, the hair. The term "ringworm" is an old term that is still with us today, though on the positive side, it is much easier to say than "dermatophytosis".
Dermatophytes are fungi that can be found in the soil "geophilic" or can be found on animals "zoophilic" or humans "anthropophilic". All types can infect the skin of dogs and cats, though the zoophilic and geophilic do so much more often than anthropophilic types. Infection is thought to occur after the skin surface is broken and the microscopic spore form of the dermatophyte attaches to the skin. Whether an infection occurs probably depends on the susceptibility of the individual's immune system and the virulence of the dermatophyte. Microsporum canis is the most common species of dermatophyte in dogs and cats. Microsporum gypseum or Trichophyton mentagrophytes occasionally infect dogs. Dermatophytosis can occur in any animal with skin including humans and is considered contagious to other animals, including humans.
For most dogs, dermatophytosis is a short-lived infection that only involves a small area of skin usually on the head or front legs. It can be treated with topical antifungal creams or ointments and assuming the dog's overall health is good, the infection can resolve within 2 months. However, one species of dermatophyte "Trichophyton" can cause widespread lasting infections that require more aggressive therapy. Trichophyton infections can be so severe that the lesions can be mistaken for those of autoimmune skin disease. However, this is the exception and for most dogs and their owners, dermatophytosis is a temporary inconvenience. Dermatophytosis in cats is an entirely different situation.
Feline dermatophytosis can be one of the most frustrating diseases a cat owner can face. Unlike some diseases in which the cause is not clear, the cause of dermatophytosis is quite plain. The difficulty is the resilience of the fungus and the difficulty in eradicating the fungus from a severely infected cat. Unlike the case in dogs, cats often develop generalized dermatophytosis (over most or all of the skin). These large areas of infection usually produce large numbers of infectious spores that can infect humans and other animals and spread the infection to other areas of the infected cat.
Avoidance is the best prevention. Fortunately, most indoor cats will not be exposed to dermatophyte spores unless spores are brought inside the house by a human, by a dog or by a cat that spends time outdoors. A common way for cats to be exposed is when an adopted cat, infectected with dermatophytosis, is brought home. Understandably dermatophytosis is a major problem in animal shelters, though most shelters now have programs designed to reduce the spread of dermatophytosis among shelter animals.
If you are considering adopting a cat from a shelter, be suspicious of any areas of hair loss, especially in kittens. Ask to have the kitten checked for ringworm by dermatophyte culture before adoption if possible. Use of lights or lamps to check for dermatophytosis (some dermatophtye infected hairs glow when exposed to a black light) is not reliable enough of a test. If you are a "softie" and a particular cat is too sweet to pass up, take it to your veterinarian immediately to have hairs collected for dermatophyte culture, better yet have it cultured twice. Keep the adopted cat in a separate room, separate litter pan, separate feeding until the culture results are negative. If positive, you at least have reduced exposure of your other cats while you treat the adopted cat.
Studies shows that immunocompetent cats (those with a normal functioning immune system) can recover from the infection on their own in about 12-16 weeks.
Systemic (oral) antifungal medications appear to speed the recovery for most infected cats. There are antifungal drugs, approved for use in dogs and cats, which are often effective, though there can be significant side effects. Human antifungal drugs are used for often in cats and may have lower incidence of side effects than the approved drug therapies. The human drugs require special compounding for the appropriate dose for cats.
Intensive topical antifungal therapy using "lime-sulfur" (an unpleasant-smelling, staining solution) on the entire cat can speed the process greatly and may speed recovery of dermatophytosis.