Cat Hair Loss Patches: Symmetry, Wood's Lamp, Fleas

A bald patch on a cat is decided by 2 questions asked in the first 60 seconds: is it the same on both sides, and is the cat itchy there? Symmetric loss with intact, smooth skin points away from parasites and toward hormones or a genetic pattern. One-sided, scabby, or self-barbered loss points at fleas, fungus, mites, or allergy, and those get treated very differently.

Why symmetry is the first question

Endocrine and metabolic hair loss follows hormone gradients, so it appears on both flanks, the belly, the perineum, and the inside of the hind legs at once. Itchy skin disease lands wherever a flea bites, a fungus lands, or a paw can reach, which is rarely a mirror image. Photograph the cat from above, from behind, and from both sides on day 1, because patch edges move by 1 to 2 cm a week and memory is a poor record.

Feature	Symmetric loss (endocrine)	Asymmetric loss (itch, infection)
Distribution	2 flanks, belly, perineum, inner thighs (4 zones)	Tail base, lower back, 1 side, face, 2 ears
Skin surface	Thin, smooth, 0 crust, 0 redness	2 to 5 crusts per cm, redness, scabs, broken hairs
Itching	0 to 1 scratch an hour	5 to 30 scratches or bites an hour
Hair pull test	10 to 20 hairs release at once, 0 break	5 to 15 hairs break, 0 to 2 release
Likely causes	1 of 4: thyroid, diabetes, adrenal, breed pattern	1 of 4: fleas, ringworm, mites, food or airborne allergy

Endocrine causes and the 3-number panel

In cats over 10, 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, though 1 in 3 early cases read normal and need a free T4. Diabetes shows up as weight loss with a ravenous appetite, and a glucose over 250 mg/dL with fructose in the urine. Hyperadrenocorticism is rare, under 1% of cats, and usually follows months of steroid tablets or injections. Feline symmetric alopecia in Siamese, Burmese, and Bengal lines often has no hormone abnormality at all, and regrowth takes 3 to 6 months once the trigger is gone.

Ringworm: what the lamp can and cannot see

A Wood's lamp costs $30 to $60 and takes 5 minutes, but only about half of Microsporum canis strains fluoresce apple green, and the other common species never glow. A negative lamp proves nothing. The reliable test is a dermatophyte culture: press a toothbrush over the lesion for 10 seconds, inoculate the agar, and read it at 7, 14, and 21 days. Treatment runs 4 to 6 weeks with oral terbinafine or itraconazole plus twice-weekly lime sulfur dips, and the room needs vacuuming every 2 days because spores survive 18 months in dust.

Flea allergy has a map

Flea allergic dermatitis in cats clusters at the tail base, the lower back, the back of the hind legs, and the neck under the chin, and it produces miliary dermatitis, meaning 20 to 50 tiny scabs you can feel but barely see. A single flea bite sets off 24 to 48 hours of itching in a sensitized cat, so finding no fleas does not rule it out. Comb for 5 minutes over a white paper towel, wet the debris: flea dirt turns red-brown, ordinary dirt stays grey. Treat every cat in the house for 3 consecutive months with a vet-approved product, and vacuum daily for the first 14 days.

Stress overgrooming versus psychogenic alopecia

Both look like a cat licking itself bald, and the difference is where and why. Overgrooming is usually driven by an identifiable stressor: a new baby, a moved box, 2 cats competing for 1 perch. Psychogenic alopecia is a compulsive disorder that continues after the stressor is removed, and it is diagnosed only after fleas, fungus, mites, and allergy are excluded, which takes 4 to 8 weeks.

Feature	Stress overgrooming	Psychogenic alopecia
Onset	Within 1 to 14 days of a change	Gradual, over 2 to 6 months
Pattern	Belly, inner thighs, forelegs (3 zones)	Belly to flank bar, 1 straight strip, tail tip
Trigger test	Stops 7 to 21 days after the stressor goes	Continues 30 days or more after
Timing	When the house is busy or empty, 2 to 5 bouts a day	Any time, meals included, 8 to 20 bouts a day
Response to 4 weeks of enrichment	Hair regrows by week 6	Little change by week 6, needs 8 weeks of behavior work

Allergy, and the 8-week elimination trial

If fleas, fungus, and mites are negative and the loss is itchy and asymmetric, food allergy is next. A proper trial means 8 weeks on a single novel protein or hydrolyzed diet, no treats, no table food, no flavoured toothpaste, and no outdoor hunting. About 1 in 3 itchy cats improves on the trial, and most of those relapse within 14 days of the old food returning, which is the proof. Airborne allergy is seasonal, worse in spring and late summer, and shows on the face, ears, and paws rather than the flanks. Details of how to run the trial without breaking it are in the elimination diet walkthrough.

What to do in the first 14 days

1. Photograph all 4 sides and note the date. 2. Comb for fleas over white paper and do the wet test. 3. Check both ears and the chin for black specks, which can be feline acne, and compare with the itchy and scabby skin guide before assuming ringworm. 4. Stop all new treats, new litter, and new cleaning sprays for 2 weeks. 5. Book a vet visit if the patch grows more than 1 cm in 7 days, if the skin is broken or weeping, or if the cat is over 10 with symmetric loss. 6. Ask for a skin scrape, a Wood's lamp, and a culture in 1 visit rather than 3, and bring the photos. If the cat has walked through a sprayed garden or chewed a houseplant, run the plant name through the toxicity list before you blame the shampoo.

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