Solar dermatitis in cats is sunburn-pattern inflammation that appears on the nose leather and the ear tips and almost nowhere else. A crusty, thin-coated patch in either place that fades in October and returns in April is the classic picture. The workable plan is a 4-week photo log, shade from 10 a.m. to 4 p.m., and a biopsy if the same 2-3 cm patch is unchanged after 4 weeks of real sun avoidance.
Why the nose and ear margins burn first
Fur is the cat's sunscreen. The planum naseum (the bare nose leather) and the outer 5-10 mm of each pinna carry almost no coat, hold only 1-2 mm of skin over cartilage, and have little melanin in white, cream, or dilute cats. Ultraviolet B (290-320 nm) reaches the basal keratinocytes there; ultraviolet A (320-400 nm) drives the deeper dermal inflammation that thickens an ear margin. The giveaway is the border: lesions stop exactly where the coat starts, on a line you can trace with one fingertip in about 10 seconds.
Ear tips are worse than owners expect. The pinna margin is thin, poorly perfused, and angled at the sky for 8-10 hours a day. A cat that naps on a south-facing windowsill at 2 p.m. takes the same dose on the same 20 mm of tissue for years, which is why the tip of one ear is often 1-2 mm ahead of the other.
What the lesion looks like, week by week
Week 1: faint pinkness, light scaling, maybe 1 mm of crust at the nose tip. Week 2: the surface feels like sandpaper and marginal hair thins by half. Week 3: grey-white crusts 2-3 mm thick, small fissures that bead blood when the cat rubs its face on carpet, and a 3-5 mm red rim. Week 4: a raised plaque 1-3 cm across, which is when most owners finally book an appointment.
Itch ranges from none to 10 scratches an hour. Squinting, nose-rubbing, and clear discharge from 1 nostril are common. A sour smell or yellow discharge means secondary bacteria and moves the visit forward by 2 weeks.
The 4-week observation protocol
Four weeks is long enough for honest sun avoidance to change a lesion, and short enough that a tumour does not get a free season.
Photograph both sites every Sunday at the same distance (20-30 cm), in the same light, with a ruler in frame. Measure the longest axis and the height in millimetres rather than by eye. Log outdoor hours and window-sun hours separately: glass blocks UVB but passes 60-75% of UVA, so an indoor sunbather still accumulates dose. Score crust, bleeding, smell, and food intake on a 1-5 scale. At week 4, compare week 0 and week 4 images side by side, never from memory.
Read the result with 3 rules. A lesion that shrinks 50% or more is behaving like sun damage: keep the routine, re-check at 8 weeks. A lesion unchanged at 4 weeks, or larger at any point, needs a 4 mm punch biopsy plus histopathology. A lesion that ulcerates or bleeds inside 14 days skips the calendar entirely.
Four-week tracking sheet
| Week | Longest axis (mm) | Height (mm) | Outdoor hours | What changed |
|---|---|---|---|---|
| 0 | 18 | 1 | 9 | pink nose tip, fine scale |
| 1 | 19 | 1 | 7 | crust visible at 30 cm |
| 2 | 21 | 2 | 4 | hair loss on left ear margin |
| 3 | 20 | 2 | 2 | less redness, thinner crust |
| 4 | 14 | 1 | 2 | 22% smaller, no fissures |
Two columns decide the outcome: the trend in the longest axis and the outdoor hours. A lesion that grows while exposure hours fall is the pattern that earns a biopsy.
Which cats carry the most risk
Unpigmented skin is the dominant factor: white pinnae, a pink nose, or a white blaze down the face. Thin coats (Sphynx, Devon Rex, Cornish Rex) and cats clipped short for summer add risk. Age 5-12 years is the common band in first-opinion practice. Altitude above 1,500 m raises UV dose by roughly 10-12% per 1,000 m. More than 2 hours of direct sun a day between March and September is the exposure habit to change first.
Five things that mimic sunburn
Squamous cell carcinoma (SCC) starts in the same 2 sites and is the entire reason the 4-week rule exists. Eosinophilic plaque, mosquito-bite hypersensitivity, pemphigus foliaceus, and dermatophytosis are the other 4 lookalikes. Signs that point away from simple solar dermatitis: a raised rim thicker than 2 mm, an ulcer unhealed after 14 days, bleeding on light touch, or weight loss of 5% or more in 8 weeks.
Shade, timing, and sunscreen that survives grooming
Block direct sun from 10 a.m. to 4 p.m. from April through September, and treat a UV index above 3 as the trigger. Adhesive window film that cuts 99% of UV runs $30-$80 for a 2-window room and is the highest-yield purchase in this plan. Mesh run panels block only 30-50% of UV, so an outdoor enclosure needs a solid roof, not netting.
Sunscreen buys 1-2 hours at best. Use a pet-labelled, zinc-free product on the nose and ear tips 15-20 minutes before exposure and reapply every 2 hours; expect grooming to remove 70-80% of it. Skip human products with zinc oxide, PABA, or fragrance, because a cat swallows a fair share of whatever sits on its nose within 10 minutes.
The first 7 days, in order
Day 1: shoot baseline photos and open the sheet. Days 1-2: fit the window film and write shade hours on the fridge. Day 3: move outdoor time to before 10 a.m. or after 4 p.m. Day 5: check the lesion for smell and weigh the cat on a kitchen scale, since a 300 g drop on a 4 kg cat is 7.5% and worth reporting. Day 7: book the exam if there is any bleeding. A biopsy with histopathology usually lands between $250 and $600, and the /tools/cost/ page shows what else a workup adds.
Two redirects matter. A head tilt, nystagmus, or circling cat is not a skin case, and belongs with /health/vestibular-disease-cat/. A cat that stops eating for 24 hours and stops producing urine needs the checklist at /health/urinary-blockage-signs/ before anything cosmetic.
Terms mentioned in this article
Frequently asked
Can I use human sunscreen on a cat's nose?
Only if it contains no zinc oxide, no PABA, and no fragrance, and even then a pet-labelled product is safer. Cats groom their faces constantly, so a large share of any cream is swallowed within minutes. Zinc oxide is the specific hazard to avoid.
How fast should solar dermatitis improve once the cat stays out of the sun?
Redness usually drops within 10 to 14 days and crusts thin by week 3. Judge progress at 4 weeks against a baseline photo, not by feel. If the patch is the same size or larger at that point, ask for a biopsy rather than another cream.
Does a scab on the ear tip always mean cancer?
No, but the ear tip and nose are exactly where squamous cell carcinoma starts, so a scab that persists past 4 weeks of shade, bleeds on light touch, or has a raised rim thicker than 2 mm should be sampled rather than watched further.