A bare patch that shows up over a weekend is rarely an emergency, and almost never nothing. Ringworm can take 10 to 21 days to grow on a culture, a flea-allergy patch can double in 7 days, and a stress-driven overgroom can copy both. The 2-week window below is how you tell them apart without treating blind.
Two weeks, three numbers
Photograph the patch next to a ruler on day 0, day 7, and day 14. Write 3 numbers: the longest dimension in mm, how many separate patches you can find, and whether the border is sharp or blurred. A patch that holds the same size for 14 days is more often a scar or a healed bite. One that grows 5 mm a week is active. One that goes from 1 patch to 4 in 14 days points to infection or parasites rather than stress.
Read the edge before you read the internet
Ringworm lesions usually carry a scaly rim, broken hairs at the border, and a clearing center, and they favor the head, ears, and front legs. Flea allergy clusters at the tail base and the back of the thighs, with 3 to 10 mm crusts. Overgrooming from stress or pain leaves even, symmetric thinning on the belly and inner thighs, with no crust and hair tips that are still intact. The shape decides which test runs first, so draw it before you search; a wider map of patch patterns sits at /symptom/cat-hair-loss-patchy/.
Group 1: fungal and yeast
Microsporum canis is the usual ringworm in cats, and roughly 1 in 3 infected cats carries it with no visible lesion at all. A Wood's lamp fluoresces in only 30 to 50% of M. canis strains, so a glow supports the guess and a dark lamp proves nothing. The real test is a fungal culture: 10 to 21 days on dermatophyte test medium, which turns red as the colony grows. Two patches, little itch, and broken hairs at the rim send you to culture, not to a medicated shampoo bought online.
Group 2: parasites and scratch damage
Flea allergy is the most common itchy cause. One bite can set off 24 hours of scratching in a sensitized cat, and a 5-minute comb may find nothing at all. Notoedres and Cheyletiella mites live in surface scale, so a superficial scrape catches them; Demodex needs a deeper scrape and turns up more often in cats on steroid treatment. Clues: coffee-ground debris at the tail base, 2 to 3 mm papules on the neck, or grooming that comes in bursts of 30 seconds or longer.
Group 3: allergy, hormones, and pain
Food allergy in cats tends to show on the face and neck, and a proper trial diet runs 8 to 12 weeks on one novel protein, not 10 days. Endocrine disease is rarer but real: a cat with a thin coat, a round belly, and 15% weight gain over 3 months needs thyroid and adrenal testing, and one losing weight on a good appetite needs blood work for diabetes or hyperthyroidism. Pain drives licking too: arthritis in the hips or spine makes a cat chew one flank for hours, and dental pain makes it paw the same side of the face every evening.
What a scrape finds, and what it misses
A skin scrape collects surface cells and mites from a 2 to 4 cm area and reads in about 10 minutes under oil. It is strong on mites and on confirming inflammation, weak on ringworm, which needs culture, and useless for allergy, which is a clinical diagnosis built from history. Most vets run 3 to 5 scrapes in one visit because mites cluster. If scrapes come back clean and the patch is still bigger at day 14, the next step is a culture plus a trichogram, a hair pluck that shows broken ends against tapered ones.
| Patch size in mm | Edge and site | Most likely group | First test | Change per week |
|---|---|---|---|---|
| 5 to 15 | scaly ring, broken hairs, head or front legs | fungal | culture, 10 to 21 days | plus 5 mm |
| 10 to 40 | crusts 3 to 10 mm, tail base, itchy in 24 h | flea allergy | comb plus flea trial | 0 mm once treated |
| 20 to 80 | even thinning, belly and thighs, no crust | overgrooming | pain exam, 14-day photos | 0 mm |
| 3 to 10 | papules 2 to 3 mm, neck, intense itch | mites | 3 to 5 scrapes | 2 new spots every 4 days |
| 15 to 50 | greasy scale, chin or back, no bald ring | yeast or bacteria | cytology in 10 minutes | 50% more area |
Four things to record for 14 days
Patch size in mm on day 0, 7, and 14, always with a ruler in frame. New patches per week, with the body map written down. Itch count: how many scratching or grooming bursts in a 10-minute window, twice a day. Household exposure: a new kitten, a foster cat, a garden visit, or a person with a new rash. Ringworm is zoonotic, so a human rash on the forearms is a data point rather than a coincidence.
Red lines
Five or more patches in 14 days. Skin that is broken, oozing, or smells wrong. A cat that stops eating or hides (see /symptom/cat-sudden-appetite-loss/). A person in the house with a spreading ring-shaped rash. Hair loss with weight change, or with more drinking. A cat that will not let one side be touched. Any of these turns watching into booking. In a multi-cat home, ringworm means a plan for every cat, not only the one with the bald spot.
Next steps
- Take the day-0 photo with a ruler today, before anything is washed or creamed.
- Comb the tail base and back for 5 minutes over white paper and bag what falls out.
- Do not start a steroid cream: it can turn a fungal patch into a spreading one in 7 days.
- Wash bedding at 60 C and vacuum twice a week while the patch is active.
- Book the scrape and culture if the patch grows at day 7 or a second one appears.
Take the ruler photos, the combed debris, and the itch counts to the visit. That packet often shortens the workup from 3 visits to 1, and it tells the vet whether the whole house needs treating or only the cat.
Terms mentioned in this article
Frequently asked
How fast does ringworm spread on a cat?
A lesion typically widens about 5 mm a week, and a culture needs 10 to 21 days before it turns positive, which is why a single photo proves little. Two patches with broken hairs at the rim and almost no itch should go to culture at the first visit.
Does a Wood's lamp diagnose ringworm?
No. Only 30 to 50% of Microsporum canis strains fluoresce, so a glow supports the suspicion while a dark lamp rules nothing out. Culture on dermatophyte test medium remains the confirmation, and roughly 1 in 3 infected cats shows no lesion at all.
Should I put an over-the-counter cream on the bald patch?
Not before a diagnosis. Steroid creams can make a fungal lesion spread faster within 7 days and thin the skin, and antifungal products bought without a culture often miss the actual cause. Photograph and measure for 7 days, then let a scrape and culture decide.