What causes dark ear debris in cats — ear mites or an outer ear infection?

Quick answer

If the inside of your cat's ears is filled with dark, damp debris like coffee grounds, and they claw at their ears frantically or shake their head hard, ear mites (a contagious infection) are very likely. Left untreated, it can progress to an outer ear infection — get a microscopic exam at the vet immediately and start dedicated deworming treatment. In multi-cat households, isolating and treating the affected cat is essential.