Ringworm is a fungal skin infection, not a worm, and in cats it is usually Microsporum canis. Three things go wrong in how it is handled at home: a dark Wood's lamp is taken as proof the cat is clear, a cream is used on an infection that lives inside the hair shaft, and treatment stops the week the skin looks normal. Each of those ends with the same result, a cat that relapses in 3 weeks and an owner who has been infected by then.
Who gets it, and how it spreads
Microsporum canis accounts for the large majority of feline cases. The fungus lives in the hair shaft and the dead outer layer of skin, and it spreads through shed hairs and scale, which carry spores that stay viable in a home for many months, with reports of up to 18 months in dark, humid corners. Incubation runs about 1 to 3 weeks, so a cat that never went outside can still develop lesions after a new kitten, a groomer visit, or a carrier brought in on clothing.
Kittens under a year, long-haired cats, senior cats, and cats on steroids or with feline immunodeficiency virus are over-represented. Long hair matters practically: a Persian or Maine Coon can carry the fungus with almost no visible lesion, which is why screening before a cattery introduction is routine. Parasite control is a separate issue but often checked at the same visit; see fleas and worms.
What the lesions look like
Classic lesions are circular, 1 to 3 cm across, with a scaly rim, broken hairs, and a clearing center. They appear most often on the head, ears, muzzle, and front legs. In cats the picture is often less tidy: patchy hair loss on the ears, crusty nail beds, a motheaten coat on the back, or nothing visible at all beyond a slightly dull coat and dandruff. Any cat with patchy alopecia and scale should be cultured rather than assumed to have allergies.
Why the Wood's lamp is not a test
Under ultraviolet light, some Microsporum canis strains fluoresce apple green. Only a share of them do, commonly cited as around half, so a dark lamp is meaningless. A negative lamp cannot exclude ringworm, and this is the single most common reason treatment is delayed in a cat that clearly has it.
False positives are the other problem. Petroleum jelly, some topical ointments, antiseptics, and even skin scale can glow, and the colour is easy to misread without practice. A lamp is worth using as a quick screen that may reveal infected hairs to pluck, and nothing more.
Culture and PCR: the actual diagnosis
Definitive diagnosis means growing the fungus or detecting its DNA. Both start with a sample taken from the active edge of a lesion: plucked hairs and scale, or a toothbrush combed through the coat for a widespread or silent case.
- Fungal culture on dermatophyte test medium (DTM) usually turns positive in 7 to 14 days, sometimes 21. The medium shifts from amber to red as the colony grows, and the colony is then examined microscopically to confirm the species.
- PCR detects dermatophyte DNA in 1 to 3 days and identifies the species, which matters because the source and the human risk differ. It can be negative early, so a suspicious case may still need culture.
- A positive culture does not prove the cat is infectious forever; the same test is used to prove the cat is clear at the end.
Treatment: oral drugs do the work
Topicals alone cannot reach spores inside the hair follicle, which is why creams and shampoos used alone fail. The standard approach pairs an oral antifungal with topical therapy and environmental cleaning.
Itraconazole is the usual first choice, at roughly 5 mg per kg once daily, frequently on a pulse schedule of 7 days on and 7 days off to reduce liver exposure, for a total of 3 to 6 weeks. Terbinafine is an alternative, often at 20 to 30 mg per kg once daily. Both need a baseline liver panel in older cats or cats on other drugs, and both are prescription-only. Griseofulvin is still used in some countries but has more side effects, including bone marrow suppression.
Topical therapy is adjunct, not optional: lime sulfur dips at a 1:16 dilution once or twice weekly, or an enilconazole or miconazole-chlorhexidine shampoo, whole body, including the ears and between the toes. Clip long coats before starting, since hair holds spores and blocks the dip. Bathing technique matters more than the product; see bathing a cat.
Cleaning the house is half the treatment
Spores in the environment reinfect the cat and infect the people. The practical protocol:
| Surface | Method | Frequency |
|---|---|---|
| Hard floors, crates, carriers | 1:10 household bleach, 10 minutes contact, then rinse | Twice weekly |
| Carpets, cat trees, soft furniture | Vacuum with a HEPA bag, then discard the bag outside | Every 2 to 3 days |
| Bedding, blankets, clothing | Wash at 60 degrees Celsius or hotter, or dry-clean | Weekly |
| Grooming tools, bowls, litter scoop | Bleach solution or discard | After each use |
| Porous items that cannot be cleaned | Seal in a bag and discard | Once |
Bleach is corrosive to some plastics and fabrics, so test a hidden spot first. General household hygiene during an outbreak is covered in cat proofing your home. Quarantine the affected cat in one room if there are kittens, seniors, or immunosuppressed people in the house, and handle it with gloves and a dedicated towel.
It is zoonotic, and children matter
People catch ringworm from cats, most often on the forearms, neck, and face, appearing as itchy red rings 1 to 4 weeks after contact. Children, older adults, and anyone on chemotherapy or immunosuppressants are at higher risk of more widespread infection. A human with a persistent ring-shaped rash who lives with a cat should mention the cat to their doctor, since treating only the person guarantees reinfection.
This is not a reason to rehome the cat. It is a reason to treat properly, clean properly, and keep the cat off beds and kitchen counters until two negative cultures are in.
Knowing when to stop
Visible healing comes first, usually in 2 to 4 weeks, but the fungus can still be viable for weeks after. The accepted end point is a negative culture taken at least 2 to 4 weeks after starting treatment, and a second negative culture 2 to 4 weeks after that for multi-cat homes or long-haired cats. Hair regrowth lags behind cure and is not a useful marker either way. Stopping at the first sign of new fuzz is the most common cause of relapse, and relapse means starting the whole 3 to 6 week course again.
Frequently asked
My cat's Wood's lamp showed nothing. Does that mean no ringworm?
No. Only a share of Microsporum canis strains fluoresce, commonly cited as around half, so a dark lamp proves nothing. If the cat has patchy hair loss with scale, ask for a fungal culture or PCR from the edge of a lesion rather than accepting a negative lamp.
How long does ringworm treatment take?
Plan on 3 to 6 weeks of oral antifungal treatment, usually itraconazole on a 7 days on, 7 days off schedule, plus twice-weekly topical dips and environmental cleaning. The end point is a negative culture, not the moment new hair appears, and many vets confirm with a second culture.
Can I catch ringworm from my cat?
Yes. Ringworm is zoonotic and shows up on people as itchy red rings on the forearms, neck, or face, usually 1 to 4 weeks after contact. Children and immunosuppressed people are at higher risk, so treat the cat properly and clean the environment instead of relying on avoiding contact.