Ringworm in cats is not a worm at all. It’s a fungal infection of the skin and hair caused most commonly by a fungus called Microsporum canis. The fungus feeds on keratin, the protein that makes up hair and the outer layer of skin, and it can spread easily between cats, other pets, and people. It’s one of the most common skin conditions in cats, especially in kittens and cats living in shelters or multi-cat households.
What Causes Ringworm in Cats
Cats pick up ringworm by coming into contact with fungal spores, either directly from an infected animal or from a contaminated environment. The spores are microscopic and remarkably hardy. They can survive on surfaces like furniture, bedding, brushes, food bowls, and carpet for up to 18 months.
For an infection to take hold, three things generally need to happen: the cat must be exposed to enough spores, there needs to be some minor damage or irritation to the skin, and the skin needs to be moist enough for the spores to germinate. Under ideal conditions, the fungal spores can begin invading skin and hair shafts within 6 to 8 hours of exposure. The fungus grows along the outside of the hair shaft, forming a cuff of spores that weakens and breaks the hair.
Kittens, elderly cats, and cats with weakened immune systems are most vulnerable. But healthy adult cats can carry the fungus too. Estimates suggest that anywhere from 17% to 80% of cats may be asymptomatic carriers, meaning they harbor the fungus and shed spores without ever showing visible signs of infection. One study of 160 cats in multi-cat households found that about 17% were carrying the fungus with no symptoms at all. This silent carriage is one reason ringworm spreads so efficiently in environments with many cats.
What Ringworm Looks Like
The classic sign is a roughly circular patch of hair loss, often with broken, stubbly hair at the edges. But ringworm doesn’t always look textbook. According to Cornell University’s College of Veterinary Medicine, the most common signs include:
- Circular areas of hair loss
- Broken or stubbly hair
- Scaling or crusty skin
- Changes in hair or skin color
- Inflamed, reddened skin
- Excessive grooming or scratching
- Infected claws or nail beds
- Dandruff
Lesions often appear on the face, ears, and paws first, but they can show up anywhere on the body. Some cats develop only mild flaking that’s easy to miss, while others get widespread, crusty patches. The infection isn’t always itchy, so a cat that isn’t scratching can still have ringworm. In rare cases, cats develop raised, nodular lumps rather than the typical flat patches.
How Vets Diagnose Ringworm
A vet can’t reliably diagnose ringworm just by looking at it, because several other skin conditions cause similar symptoms. The first screening tool is often a Wood’s lamp, which is a type of ultraviolet light. Under this light, some strains of the ringworm fungus will glow a characteristic apple-green color along the hair shaft. It’s a useful quick check, but it’s not definitive. Skin flakes, lint, and certain medications can create false positives, and not all ringworm strains fluoresce. Results from a Wood’s lamp typically need to be confirmed with additional testing.
The gold standard for diagnosis is a fungal culture, where hair and skin samples are placed on a special growth medium and monitored over one to three weeks to see if the fungus grows. It’s slow but reliable. PCR testing is a faster alternative that detects fungal DNA and is highly accurate for confirming an active infection. However, PCR has a notable limitation: it can pick up dead fungal material or spores that are just sitting on the coat without causing infection, which means it sometimes returns false positives. On the flip side, a negative PCR result in a cat being treated is a strong indicator that the infection has been eliminated.
Treatment: What to Expect
Treating ringworm in cats typically involves both oral antifungal medication and topical treatment applied to the skin and coat. Most cats need both approaches simultaneously to clear the infection effectively.
Oral antifungals work from the inside to stop the fungus from reproducing. Treatment courses usually run several weeks, and the medication continues until testing confirms the infection is gone, not just until the skin looks better. Stopping too early is one of the most common reasons for relapse.
Topical treatment serves a different purpose: it kills spores on the surface of the coat to reduce shedding into the environment. Lime sulfur dip, applied twice weekly with applications spaced 3 to 4 days apart, is the most effective topical option available. It sterilizes the coat and provides a residual protective effect for 3 to 4 days after each application, preventing new spores from being shed. It smells unpleasant and can temporarily stain light-colored fur, but it works. Antifungal shampoos containing miconazole combined with chlorhexidine can also be effective, though chlorhexidine alone is not. For stubborn spots in tricky locations like the face or ears, antifungal ointments can be applied once daily as a supplement to the broader treatment.
The total treatment timeline varies but commonly takes 6 to 12 weeks. Your vet will retest periodically using fungal cultures to confirm the infection has truly cleared. Fur regrowth in the bald patches usually begins before treatment ends, which can be misleading. The cat may look healed while still carrying and shedding the fungus.
Cleaning Your Home During Treatment
Environmental decontamination is just as important as treating the cat. Because fungal spores can persist for up to 18 months on household surfaces, skipping this step makes reinfection likely.
Vacuum thoroughly and frequently in every area your cat uses. Wash bedding, blankets, and any fabric the cat contacts in hot water. For hard surfaces, diluted chlorine bleach (one quarter cup per gallon of water) is effective at killing ringworm spores. Strong detergents can also work. Discard items that can’t be adequately cleaned, like cardboard scratchers or heavily contaminated fabric toys. Grooming tools like brushes and combs should be soaked in a disinfecting solution or replaced entirely.
If you have multiple cats, any cat that tests positive should be isolated in a room that’s easy to clean (think tile or hardwood floors rather than carpet). Other cats in the home should be tested even if they look fine, given the high rates of asymptomatic carriage.
Can You Catch It From Your Cat
Yes. Ringworm passes readily from cats to people. In humans, it typically appears as red, itchy, ring-shaped patches on the skin. Children, elderly individuals, and anyone with a compromised immune system are at higher risk, but anyone can get it.
To protect yourself during your cat’s treatment, wear gloves and long sleeves when handling the cat. Wash your hands thoroughly with soap and water after every interaction. Keep the cat out of bedrooms and off upholstered furniture when possible. If you develop suspicious skin lesions during this time, see a doctor, as human ringworm is straightforward to treat with antifungal creams or oral medication.