A cat with greasy brown scale in the armpits, a musty smell and a dark waxy ear canal usually has a Malassezia overgrowth rather than a plain allergy. Cytology settles it in about 10 minutes: 3 to 5 slides from 2 or 3 sites, counted under oil at 1000x. Plan a 4-week course with a recheck at day 21 to 28, because 7 to 10 day courses relapse within 2 to 3 weeks in most cats.
What the yeast does under the scale
Malassezia pachydermatis lives in small numbers in the ear canal and skin folds of most healthy cats. It turns into a problem when surface lipids and moisture rise together: the yeast multiplies, releases lipases that split triglycerides into fatty acids, and the film smells rancid within 24 to 48 hours. The itch follows the smell. A cat that licks and scratches 20 to 40 times an hour breaks the surface barrier, which traps more moisture in the fold and feeds the next round.
The organism is oval, 3 to 8 micrometres, and buds from a broad base, so cytology reports describe peanut shapes. It does not invade living tissue. Everything visible is surface inflammation plus self-trauma, which is why topical treatment carries the case and why skipped baths fail.
Six sites, checked in order
Work down from the ear: ear canal, chin and lip folds, neck folds, armpits, groin and inner thigh, then interdigital skin and claw folds. In roughly 8 of 10 generalised cases the ear canal is involved first, and an ear that stays waxy after 2 rounds of drops is the classic missed site. Flat-faced and rex-coated cats hold more moisture in deeper folds and flare 2 to 3 times as often as cats with ordinary coat and face shape.
Map each lesion with a photo and a ruler on day 0. A 2 cm patch photographed against a scale is worth more at day 28 than any memory of how it looked.
Cytology in 10 minutes
Three sampling methods cover nearly every case: a clear tape strip pressed on dry scale, a glass slide touched to wet exudate, and a swab rolled firmly in the canal. Stain with a modified Wright or Diff-Quik type stain, examine at 1000x under oil, and count 10 consecutive fields instead of one. Most labs call more than 2 to 3 yeasts per oil-immersion field an overgrowth; the cut-off shifts with site, so ask what range the lab reports. More than 5 bacteria per field in the same view usually means a secondary infection that needs its own plan.
| Site | Method | Slides | Normal per field | Recheck day |
|---|---|---|---|---|
| Ear canal | Swab roll | 2 | 0 to 2 | 21 |
| Armpit and groin | Tape strip | 3 | 0 to 2 | 28 |
| Chin and lip fold | Impression smear | 2 | 0 to 2 | 28 |
| Interdigital skin | Tape plus scrape | 3 | 0 to 1 | 28 |
| Generalised disease | 3 sites combined | 5 | 0 to 2 | 21 |
One negative slide proves nothing. Yeast sheds in patches, so a single tap can miss a hot spot 3 cm away; sample the edge of a lesion, not the cleaned centre.
What has to be ruled out first
Three look-alikes share the same zip code, and the itch-and-sores work-up walks through them in order. Skin mites need 3 to 5 superficial scrapes, because a single scrape is negative in a large share of cases. Ringworm needs a dermatophyte culture with 10 to 14 days of incubation, or a PCR result in 1 to 3 days. Allergy needs an 8-week elimination diet or a proper dermatology work-up, and it is the reason half of recurrent cases come back 6 to 10 weeks after a good response. Diabetes and long steroid courses are the other two triggers worth a blood panel; both raise skin surface glucose and oil.
The 4-week window, week by week
Day 0 is diagnosis: cytology, scrape, weight and a lesion map. Days 1 to 14 are the heavy phase: an antifungal wash 2 to 3 times a week with 10 minutes of contact time before rinsing, plus ear cleaner and medicated drops 1 to 2 times daily for 7 to 10 days if the canal is involved. Day 14 is a photo check with the owner; if itch has not dropped by half, the plan needs a call, not more waiting.
Day 21 to 28 is the recheck: same site, same method, same magnification. Treatment continues 14 days past clinical resolution and never less than 28 days in total. Widespread or deep fold disease usually needs a 3 to 4 week oral course from the vet, with the same 28-day floor. Budget 2 to 3 cytology sets across the course at 25 to 60 dollars each, plus 1 recheck visit; the cost planner lines that up against a 4 to 8 week supply of wash.
When day 28 is still positive
Four explanations cover most failures. The diagnosis is wrong, and mites or ringworm were never excluded. The treatment was applied but not delivered: a 2-minute shampoo contact time instead of 10 is the most common version. The trigger is untreated, usually allergy or an endocrine disease. Or the yeast is unusual, in which case culture and susceptibility testing at a reference lab takes 7 to 14 days and changes the drug class.
Home changes that hold the result
Keep indoor humidity between 40 and 60 percent. Wash bedding weekly at 60 degrees Celsius or hotter, and dry it fully before the cat gets it back. Swap plastic bowls for ceramic and wash them daily, since a scratched bowl holds yeast film. Dry the folds with a soft cloth after every bath, and keep the cat at a body condition score of 5 out of 9; cats 20 percent over ideal weight have deeper folds and relapse more.
Call the vet today if
Head tilt, nystagmus or a drooping lip means the middle ear or facial nerve is involved and the case is no longer skin-deep; the pattern of those signs is covered in vestibular disease in cats. Pain on touching the ear, discharge that turns green in 48 hours, a kitten under 6 months, or spreading redness over a weekend all justify a same-day visit rather than a wait for the scheduled recheck.
Five steps that decide the outcome
- Book cytology at day 0 and insist on a written count per field, not just a positive or negative.
- Photograph every lesion with a ruler and repeat the photo at day 14 and day 28.
- Time the shampoo with a clock: 10 minutes of contact, 2 to 3 times a week.
- Recheck cytology at day 21 to 28 even if the skin already looks normal.
- Ask what the underlying trigger is; a cat that relapses twice in 12 months needs an allergy or endocrine work-up.
Frequently asked
Can I treat Malassezia with shampoo alone, without cytology?
You can start a wash, but you cannot judge it. A count per oil-immersion field at day 0 tells you whether the problem is yeast at all, and the same count at day 21 tells you whether to stop or extend. Without the second number, most owners stop at 7 to 10 days, when the skin looks better but the population is still above the 2 to 3 per field threshold.
Why four weeks instead of one?
The yeast sits in the follicle opening and in fold crevices that a bath reaches unevenly, and the barrier needs about 3 weeks to rebuild after the scratching stops. A 28-day floor with 14 days past visible resolution covers both. Stopping at day 7 leaves the surface population high enough to bloom again within 2 to 3 weeks, which is exactly the relapse pattern owners describe.
Is Malassezia contagious to other cats or to people?
It is not a reportable or classic contagious disease. The yeast is already on most cats, and what spreads is the conditions that let it grow: shared bedding that stays damp, a humid room, or an untreated allergic cat. People with a normal immune system rarely get more than an itchy patch, but a person on chemotherapy or immunosuppressants should let their doctor know a cat in the house is under treatment.