A fungus, not a worm
Ringworm, or dermatophytosis, is a skin infection caused by fungi, most often Microsporum canis. It spreads through direct contact and through spores that come off infected hairs and can survive in the environment for many months, often more than a year, on brushes, bedding, carriers and show cages. Kittens, longhaired breeds and cats with weakened immunity are most at risk. Ringworm also infects people, especially children, who can get itchy, round patches on the skin, so everyone in the household should know about it and see a doctor if they get symptoms.
Signs and diagnosis
Typical signs are round, hairless patches with scaling or crusts, often on the face, ears and front legs, but ringworm can look like almost any skin problem, and some cats, especially longhairs, carry spores without any visible lesions. Under a Wood's lamp about half of Microsporum canis infections glow apple-green, but a negative result does not rule ringworm out. The vet confirms the diagnosis with a fungal culture or PCR from hairs brushed out with a new toothbrush. Because PCR also detects dead fungal material, a culture is used to show when a cat is cured.
Treatment
Treatment combines medicine for the cat with cleaning of the environment. The vet usually prescribes an oral antifungal such as itraconazole, together with treatment of the whole coat, for example lime sulphur dips or a shampoo with miconazole and chlorhexidine twice a week. Clipping a longhaired cat can help, but it should be done gently since it can spread spores. Treat every cat that has been in contact, not only the ones with lesions, and continue until the vet confirms they are cured, usually with two negative cultures in a row. Stopping too early is the most common reason ringworm comes back.
Clearing the cattery
Spores are removed mechanically first: vacuum and wipe all surfaces, wash with detergent, and then disinfect with a product proven to work against dermatophyte spores, such as diluted household bleach or accelerated hydrogen peroxide, following the label. Wash textiles twice at the highest temperature they tolerate, and throw away scratching posts and things that cannot be cleaned. Keep affected cats in a room that is easy to clean. Pause matings, sales and shows until the cattery is clear, tell buyers and other breeders honestly if they may have been exposed, and check new cats before they meet your own.