Crusts and scabs on a cat fall into 3 groups: parasites, fungi, and allergy or immune disease. Where the crusts sit sorts most of it before a vet sees the cat. Tail base and neck point to fleas, ear edges and face to mites, a 1 to 3 cm ring on the head or foreleg to ringworm, and a linear scab down the back of a thigh to an eosinophilic granuloma. Score the lesions 0 to 3 on day 0 and log 14 days.
3 groups, sorted by where they sit
| Group | Where | What it looks like | Itch 0-3 | Confirm by |
|---|---|---|---|---|
| Parasites | Tail base, neck, ear edges, 1 to 2 mm scale | Clusters of 10 to 20 small scabs | 2 to 3 | Flea comb for 5 minutes |
| Ringworm | Head, forelegs, 1 to 3 cm rings | Scale at the rim, hair loss, broken hairs | 0 to 1 | Culture, 10 to 14 days |
| Allergy or immune | Face, belly, legs, 2 to 5 cm linear | Thickened skin, flat scab, no pus | 2 to 3 | 8 to 12 week diet trial |
| Secondary bacteria | Under any crust, 0.5 to 2 cm | Pustules, yellow crust, moist | 1 to 2 | Cytology at 400x |
Two numbers do useful work here. Crusts smaller than 2 mm that sit in clusters of 10 or more are parasitic until proven otherwise. A single patch over 3 cm with scale only at the rim is fungal until cultured. Anything that itches at 2 or 3 out of 3 for more than 14 days without responding is allergic, and the answer is a diet trial rather than a stronger cream.
Parasites: fleas, mites, lice
Flea allergy needs few bites: 1 to 3 fleas every 2 to 3 weeks keeps 10 to 20 scabs at the tail base, and the cat grooms the fleas off before you look. Run a fine comb for 5 minutes over the tail base and put the debris on damp white paper; flea dirt turns rust red in 30 seconds. Cheyletiella shows as 1 to 2 mm walking dandruff along the spine, and Notoedres crusts the ear edges and the skin in front of them. All 3 need 3 to 4 weeks, and every cat and dog in the house is treated on the same day: the cycle is 2 to 3 weeks and 1 untreated animal restarts it.
Ringworm: rings, not worms
Microsporum canis gives 1 to 3 cm circular patches with scale at the rim and hairs broken off 1 to 2 mm above the skin. Itch is often 0 or 1 out of 3, which is why ringworm is missed for weeks on a cat that is not scratching. A Wood's lamp is positive in roughly half of cases, so a negative lamp means nothing and a culture runs 10 to 14 days. It spreads to people, so wash hands after handling and vacuum the cat's spots twice a week. Treatment usually runs 4 to 6 weeks past the first negative culture.
Allergy and immune disease
Three patterns matter. Food allergy often starts with itching around the head and neck, sometimes with 2 to 5 mm crusts at the ear bases, and needs an 8 to 12 week trial diet with no treats. Atopic dermatitis usually begins at 6 months to 3 years and targets the face, belly and legs. Eosinophilic granuloma shows as a 2 to 5 cm linear scab on the back of a thigh, or a raised pink plaque on the lip. Steroids calm all 3 within 7 to 14 days, which is useful for diagnosis and useless as a cure if the trigger stays.
The 0 to 3 score, taken every 3 days
| Score | Body area | Crust size | Scratches in 60 seconds | Action |
|---|---|---|---|---|
| 0 | None | 0 mm | 0 to 2 | Record only |
| 1 | Under 10 percent | 1 to 2 mm | 3 to 10 | Topical care, recheck in 7 days |
| 2 | 10 to 30 percent | 2 to 5 mm | 11 to 20 | Vet within 72 hours |
| 3 | Over 30 percent or oozing | Over 5 mm | Over 20 | Vet within 24 hours |
Score the same 4 body zones every time: head, back and tail base, belly, legs. A drop of 1 point in 14 days means the plan works. No change at day 14 with a score of 2 or 3 means the diagnosis is wrong, and the next step is skin scrapings, cytology, and a fungal culture rather than a longer course of the same product.
Hour-level triage: how fast to move
| What you see | Act within | Area involved | Reason |
|---|---|---|---|
| Hives, face swelling, trouble breathing | 1 hour | Any, starts in minutes | Anaphylaxis risk after a sting or drug |
| Fever 39.5 C, pus, off food 24 hours | 6 hours | 10 to 30 percent | Deep bacterial infection |
| Spreading over 24 hours to 30 percent or more | 6 hours | Over 30 percent | Fast spread means infection, not allergy |
| New scabs weekly for 2 weeks | 72 hours | Under 10 percent | Parasite or fungal cycle still running |
| Scabs plus 0.3 kg weight loss in a month | 5 days | Any | Systemic disease behind the skin |
| 1 to 3 stable patches, eating well | 7 days | Under 10 percent | Routine culture and cytology |
The 14-day record
Photograph the same 4 zones in daylight at the same distance every 3 days. Write the 0 to 3 score, the number of new scabs since the last entry, and the flea comb count. Add scratching minutes per day, measured in 60-second samples at 3 fixed times. Two weeks of this beats a description at the visit, because the vet needs the direction of travel, not a snapshot.
Before the visit, do not do these
Do not bathe the cat within 48 hours of the appointment: shampoo strips the scale that culture and cytology need, and a wet coat hides how much hair is broken. Do not apply cream, oil, or coconut fat for 24 hours before, since it makes scrapings useless. Do not start or stop steroids on your own, because 7 to 14 days of steroid can turn a fungal patch into a widespread infection. Do not use dog flea products containing permethrin, which is toxic to cats at any dose. Do not shave a suspected ringworm patch with household clippers, as that spreads spores over the whole coat.
What clears in 2 weeks, and what does not
Flea scabs start to flatten in 10 to 14 days once every animal in the house is treated. Ringworm often looks worse at day 10 before it improves, and 1 negative culture does not end treatment. Allergic cats improve on a trial diet at 4 to 6 weeks, not 2. If a score of 2 or 3 has not moved by day 14, ask for scrapings, a fungal culture, and cytology in the same visit, and expect 3 tests rather than 1.
5 steps for tonight
- Map the scabs by zone and photograph them.
- Score 0 to 3 and count scabs in a 60-second scratch sample.
- Flea comb for 5 minutes and test the dirt on damp paper.
- Book a visit inside 72 hours if the score is 2 or higher.
- Treat every cat and dog in the house on the same day.
Related: skin twitching often comes from the same itch, and a lump under the skin that changes size in 2 weeks needs its own check. If a plant or a human cream is involved, run it past the toxicity list.
Terms mentioned in this article
Frequently asked
How do I know if cat scabs are from fleas or ringworm?
Position and itch separate them more reliably than appearance. Flea scabs cluster in 10 to 20 spots at the tail base and neck, itch 2 or 3 out of 3, and a 5 minute comb turns up grit that goes rust red on damp paper. Ringworm gives 1 to 3 cm rings on the head or forelegs with scale only at the rim, itch 0 or 1, and broken hairs 1 to 2 mm above the skin. A Wood's lamp helps in about half of cases; culture takes 10 to 14 days.
What does a 0 to 3 skin score actually measure?
Four things at once: how much body area is involved, how large the crusts are, how many scratches happen in 60 seconds, and whether the skin oozes. Zero is clear, 1 is under 10 percent with 1 to 2 mm crusts, 2 is 10 to 30 percent with 2 to 5 mm crusts, 3 is over 30 percent or oozing. Score the same head, back, belly and leg zones every 3 days; a 1 point drop in 14 days means the plan works.
Why does the vet want 2 weeks of records before treating?
Because the direction of change matters more than one snapshot. Flea scabs flatten in 10 to 14 days, ringworm often looks worse at day 10, and food allergy needs 4 to 6 weeks on a trial diet. Two weeks of scores also catches the pattern that a single visit misses, such as new scabs every 7 days, which points to a parasite cycle rather than allergy. The sheet also decides whether day 14 needs scrapings, culture and cytology together.