Patchy hair loss in a cat is sorted by pattern before anything else: symmetric loss on both flanks, ring-shaped patches 1 to 4 cm wide, or scattered patches where a paw or a flea reached. The pattern decides the first test, and 14 days decides whether the plan is working. Color, crust and itch matter, but the shape of the loss is what keeps you from treating ringworm for 6 weeks when the cause was hormones.
| Pattern | Where it sits | Patch size | Itch, per 10 min | First test | Review at |
|---|---|---|---|---|---|
| Symmetric | 2 flanks, belly, perineum, inner thighs | 5 to 15 cm areas | 0 to 2 | Total T4, glucose, urine | 90 days |
| Ring-shaped | Face, ears, forelegs, anywhere | 1 to 4 cm circles | 2 to 10 | Culture read at 7, 14, 21 days | 21 days |
| Flea map | Tail base, lower back, hind legs, chin | 20 to 50 scabs | 20 to 60 | 5-minute comb, wet test | 14 days |
| Injection site | Between the shoulder blades | 2 to 3 cm | 0 to 1 | Photo with a ruler | 12 weeks |
| Overgroom bar | Belly, inner thighs, forelegs | 1 straight strip | 5 to 15 | 4-angle photo series | 14 days |
Symmetric loss: the 60-second mirror test
Stand behind the cat and compare left with right. Symmetric loss covers both flanks, the belly, the perineum and the inside of the hind legs at once, and the skin underneath is thin, smooth and not crusted. That pattern points away from parasites and toward hormones, or toward a breed pattern seen in Siamese, Burmese and Bengal lines. In cats over 10 years, hyperthyroidism is the most common endocrine cause and turns up in roughly 1 in 10 of that age group; a total T4 above 4.0 ug/dL supports it, while about 1 in 3 early cases reads normal and needs a free T4. Diabetes shows as weight loss with a big appetite, glucose over 250 mg/dL and sugar in the urine. Regrowth after the trigger is fixed takes 3 to 6 months, so 14 days tells you nothing here.
Ring-shaped: 1 to 4 cm circles and a 21-day culture
Ringworm in cats produces circular patches 1 to 4 cm across with a scaly rim, broken hairs at the edge, and little itch compared with the size of the lesion. Kittens under 12 weeks, seniors, and cats on steroids are the usual cases, and spores survive 18 months in dust and on brushes. A dermatophyte culture is read at 7, 14 and 21 days, which is why the 21-day mark, not the 7-day one, is the answer. Treating a suspected ringworm for 2 weeks and stopping when the patch looks better is how households end up with a 4-month outbreak.
Scattered patches: 5 places to check first
Flea allergic dermatitis clusters at the tail base, lower back, back of the hind legs and under the chin, and produces 20 to 50 tiny scabs you can feel but barely see. Comb for 5 minutes over white paper, then wet the debris: flea dirt turns red-brown, ordinary dirt stays grey. Injection sites on the shoulder blades can lose a 2 to 3 cm patch that lasts 4 to 12 weeks. Overgrooming leaves a bar of short hair on the belly or inner thighs with no broken skin. Stud tail shows as a greasy strip 2 cm wide on the top of the tail base.
Wood's lamp: 5 minutes, 2 hard limits
A Wood's lamp costs $30 to $60, needs 5 minutes in a dark room with 10 minutes of warm-up, and has 2 limits worth knowing before you buy one. Only about half of Microsporum canis strains fluoresce apple green, and the other common feline species never glow at all, so a negative lamp proves nothing. Skin flakes, ointment residue and lint also fluoresce, most of it blue-white rather than apple green. The useful habit is to photograph the glow, mark the patch with a pen, and let a culture confirm it.
2 weeks is the decision point
Fourteen days is enough for a flea-treatment plan to show results, for a food or contact trigger to be removed, and for a stress bar to start filling in. It is not enough for ringworm, endocrine disease or a full seasonal shed. The rule: if the patch has not shrunk and no new hair has appeared at the edges in 14 days, the next step is a vet visit with photos, not a 3rd shampoo. If it has shrunk by 20 percent or more, the plan is right and gets 14 more days.
6 numbers to record for 14 days
Number of patches, largest diameter in millimetres measured with a ruler against the skin, itch count per 10 minutes, hairs released versus broken in a gentle pull of 10, flea-comb debris score after 5 minutes, and body weight to 100 g weekly. Photograph the same 4 angles every 7 days in the same light, because patch edges move 1 to 2 cm a week and memory is a poor ruler. A cat that loses 200 g in 2 weeks alongside a patch has a different problem from one with a stable weight.
When to get help: the hour thresholds
Now, meaning 1 to 2 hours: broken, weeping or bleeding skin over a patch, a patch that doubled in 24 hours, or a cat biting itself hard enough to yelp. Within 12 hours: a patch that appeared overnight with facial swelling, or lesions on the eyelids or nose in a cat that is squinting. Within 24 hours: a person in the house with a new ring-shaped rash, a kitten under 12 weeks with any scaling patch, or a cat that stops eating. Within 48 hours: 3 or more new patches in 7 days, a patch that has not shrunk in 14 days, or itch that keeps the cat awake. Within 7 days: any symmetric loss with no itch in a cat over 8 years, since that needs bloodwork rather than shampoo.
First 14 days: what to do and what to stop
1. Photograph 4 angles on day 1 and again on day 7 and 14, with a ruler in frame. 2. Comb for fleas on day 1 and wet-test the debris; treat every cat and dog in the house for 3 consecutive months if it is positive. 3. Stop new treats, new litter and new cleaning sprays for 14 days, 1 change at a time. 4. Wash bedding at 60 C and vacuum every 2 days for the first 2 weeks if ringworm is on the list. 5. Do not start a medicated shampoo or an antifungal before a culture, because it flattens the test. 6. Book the visit with the photos and the 6-number log. If the cat has also lost weight with a normal appetite, read weight loss with a normal appetite before assuming the coat is the whole story, and check the plant list in the toxicity list if the cat walks a sprayed garden. Patch shapes and their usual causes are compared in patches by pattern.
Terms mentioned in this article
Frequently asked
Why does my cat have bald patches but no itching?
Loss without itch usually means hormones rather than infection. In cats over 10 the first test is a total T4 for hyperthyroidism, which affects roughly 1 in 10 of that age group, then glucose and a urine test. Symmetric loss in Siamese or Burmese lines can occur with normal bloodwork.
Can a Wood's lamp diagnose ringworm in cats?
No. Only about half of Microsporum canis strains fluoresce apple green, and the other common species never glow, so a negative lamp means nothing. A toothbrush culture read at 7, 14 and 21 days is what actually confirms or rules out ringworm.
How long before a bald patch should improve?
Use 14 days as the review point for fleas, contact triggers and stress bars. If the patch has not shrunk and no new hair has appeared at the edges by day 14, book a vet visit with photos and measurements instead of changing shampoo again.