Cat Ear Dark Discharge: 3 Causes, 2 Microscope Tests

Black discharge in a cat ear comes down to 3 causes, and the texture of the debris separates them before any test runs. Dry, crumbly, coffee-ground specks mean ear mites. A greasy yellow-black film with a sour smell means Malassezia yeast. Thin dark fluid plus an ear that hurts when touched means bacteria or trauma. Two microscope checks, both done inside 10 minutes, settle which of the 3 you are treating.

The 3 causes side by side

Cause	Debris	Smell	Microscope	Clears in
Ear mites	Dry 1 to 2 mm crumbs, like coffee grounds	Faint, earthy	0.3 to 0.4 mm mites or eggs at 40x	21 to 28 days
Malassezia yeast	Greasy 2 to 3 mm wax plug, yellow-black	Sour, bready	3 or more yeasts per 400x field	14 to 21 days
Bacteria	Wet dark fluid, 1 to 2 mL	Rotting, sharp	Cocci under 1 µm or rods 2 to 3 µm at 400x	7 to 14 days
Trauma or polyp	Black-red crust, 2 to 5 mm, 1 ear only	Metallic	Red cells, no organisms, mass behind drum at 40x	2 to 5 days

Two numbers set the pace. A cat shaking its head 20 or more times an hour is bruising the flap with every shake, and a hematoma can build in 3 days. A head tilt of 15 degrees or more means the middle ear is involved, and the case stops being a cleaning problem.

Mites: dry crumbs, violent itch

Otodectes packs the vertical canal with 2 to 5 mm of coffee-ground debris. The mites are 0.3 to 0.4 mm white specks that start moving 20 to 30 seconds after the slide is warmed in your hand. The itch runs 10 to 30 scratches a minute with a hind leg, and the ear base goes scabby in 7 to 10 days.

The egg-to-adult cycle runs 21 days, so any course shorter than 3 weeks leaves the next generation alive and the discharge returns 10 to 14 days after you stop. Treat every cat in the house on the same day: mites move between cats in 2 to 4 weeks. Clean 2 to 3 times a week during treatment, not daily, because a canal kept wet is the usual setup for the yeast that follows.

Yeast: greasy film, sour smell

Malassezia turns the canal wall into a yellow-brown greasy surface you can smell from 30 cm. It rarely arrives alone: 2 weeks or more of antibiotic drops, food allergy, or atopic skin disease each push the canal toward yeast. At 400x, 3 or more yeasts per field is the usual line where treatment starts, and a sample from 2 sites beats 1 because the plug sits deeper than the film.

Expect 14 to 21 days and a recheck at day 14. If the ear is greasy again within 14 days of stopping, the trigger was never found, and the next step is an 8 to 12 week diet trial or allergy workup, not a third round of the same drops.

Bacteria and trauma: wet, painful, often one-sided

Bacterial otitis gives 1 to 2 mL of thin dark fluid and an ear tender at the base. A tilt of 15 degrees or more, nystagmus, or a drooping lip on 1 side means middle ear involvement and a ruptured drum, which also changes which drops are safe. Trauma and polyps show as a 2 to 5 mm black-red crust in 1 ear only, usually after 2 to 3 days of shaking at 20 or more an hour; that shaking raises a 5 to 15 mm hematoma in 3 days.

The 2 microscope checks

Wet mount in mineral oil. Samples from 2 sites, 1 drop of oil, 10x to scan and 40x to confirm: 0.3 to 0.4 mm mites, 0.1 to 0.2 mm eggs, dark faecal specks. Read within 10 minutes, before the oil dries.
Stained cytology. Diff-Quik or Gram, 400x, 5 fields minimum. Count yeast per field, then read bacterial shape: cocci under 1 µm in clusters against rods of 2 to 3 µm.

Hour-level triage: how fast to move

What you see	Act within	Reason
Head tilt 15 degrees or more, or nystagmus	6 hours	A tilt can fix permanently in 5 to 7 days
Ear flap swollen 5 to 15 mm and warm	24 hours	Hematoma drains well under 3 days old
Lip droop, eye that will not blink, 1 side	12 hours	Facial nerve damage, dry eye in 2 to 3 days
20 or more head shakes a minute for 2 days	24 hours	Mechanical damage to flap and canal
Black discharge both ears, no pain, eating well	48 to 72 hours	Routine cytology, 21 to 28 day course
Discharge plus 39.5 C or 24 hours off food	6 hours	Deep infection, needs systemic treatment

Home record for the day 14 recheck

Four items, written down rather than remembered. Count shakes in 60 seconds at 3 fixed times a day. Photograph each ear at the same distance and light. Score smell 0 to 3, where 0 is nothing and 3 carries 1 m. Note appetite and scratching minutes. If the score has not fallen 1 point in 14 days, the diagnosis needs revisiting, not the dose.

On the way in, do not do these

Do not push a cotton bud past the opening you can see: the horizontal canal runs 2 to 3 cm and curves, and a bud shoves debris onto the drum. No alcohol, hydrogen peroxide, or vinegar, which sting an inflamed canal and drive 20 or more shakes a minute. No human ear drops, some of which damage hearing in cats. Do not carry the cat loose in the car. If sedation is planned, follow the clinic's fasting instruction rather than guessing 4 hours or 8.

What the 2-week recheck decides

Mites gone and yeast present means the mite course worked and the yeast needs its own 14 to 21 days. Both still there at day 14 means the cleaning routine is keeping the canal wet, or the drum hides a middle ear infection. A drum that cannot be seen sends the case to CT at 1 mm slices. One clean ear and one dirty ear is the pattern that points to a polyp or tumour rather than infection.

5 steps for tonight

Photograph both ears, count shakes for 60 seconds.
Score smell 0 to 3, note whether 1 ear or both are dirty.
Wipe only the visible folds with a dry pad.
Book a visit inside 48 to 72 hours unless a red flag applies.
If mites are confirmed, treat every cat the same day.

Related: ear discharge types and what each colour means. If you suspect poisoning rather than ear disease, run the symptom past the toxic plant and food checker first.

Terms mentioned in this article

Frequently asked

How do I tell ear mites from yeast without a microscope?

Texture and smell do most of the work. Mites leave dry 1 to 2 mm crumbs that crumble between your fingers, like coffee grounds, with a faint earthy smell. Yeast leaves a greasy yellow-black film that smears, with a sour bready smell you catch from 30 cm. Itch is the second clue: mites drive 10 to 30 scratches a minute, yeast often fewer. Long-standing cases often carry both, so cytology at 400x is the only way to be sure.

Why does the ear get dirty again two weeks after treatment ends?

Two weeks is roughly when a mite egg laid on day 1 becomes an adult, so a course shorter than the 21 day cycle leaves the next generation alive and debris returns 10 to 14 days after the last dose. With yeast, a relapse inside 14 days means the trigger was never addressed, usually antibiotic drops, food allergy or atopic skin disease. Either way the answer is repeat cytology at day 14, not a longer course.

Can I clean the ear with cotton buds at home?

Only the visible folds, with a dry pad, and nothing past the opening you can see. The horizontal canal runs 2 to 3 cm and curves, so a bud pushes debris onto the drum and can bruise an inflamed wall. Never use alcohol, hydrogen peroxide or vinegar, which sting and trigger 20 or more shakes a minute. Cleaning 2 to 3 times a week during treatment is enough; daily cleaning keeps the canal wet and favours yeast.