Cats get ear infections when something disrupts the normal environment inside the ear canal, allowing bacteria, yeast, or parasites to multiply out of control. The most common trigger by far is ear mites, which are responsible for an estimated 85% of outer ear infections in cats. But mites are only one piece of the puzzle. Allergies, trapped moisture, growths inside the ear, and even viral infections can all set the stage.
Ear Mites: The Leading Cause
Ear mites (tiny parasites that live and feed inside the ear canal) are the single biggest reason cats develop ear infections. These mites spread through direct contact between animals, not through the environment. A kitten nursing from an infested mother, cats grooming each other, or even brief nose-to-nose contact with an infected dog can pass mites along. Indoor-outdoor cats and those in multi-pet households are at the highest risk.
Once mites colonize the ear canal, they cause intense irritation and trigger the ear to produce thick, dark, crumbly discharge that looks like coffee grounds. A large amount of black wax is a hallmark sign. The scratching and inflammation that follow create a perfect environment for secondary infections, where bacteria or yeast move in on top of the mite infestation and make things worse.
Yeast and Bacterial Overgrowth
Even without mites, the warm, moist interior of a cat’s ear canal is hospitable to microorganisms. A yeast called Malassezia is the most commonly detected infectious agent in feline ear infections, showing up in roughly 57% of cases in one large study. This yeast naturally lives on a cat’s skin in small numbers but can explode in population when the ear’s defenses are weakened.
Bacteria are the other major players. Several species are commonly involved, and they often appear alongside yeast rather than alone. When bacteria take hold, the discharge tends to shift from dark and waxy to yellowish or greenish, sometimes with a noticeable odor. Bacterial infections that reach the middle ear (the space behind the eardrum) tend to be more serious and harder to resolve than those confined to the outer canal.
How Allergies Set the Stage
Allergies are an underappreciated driver of ear infections in cats. Both environmental allergies (pollen, dust mites, mold) and food allergies cause inflammation throughout the skin, and the ear canal is lined with skin. When that lining swells, it narrows the canal, traps wax and moisture, and creates the stagnant conditions that bacteria and yeast thrive in. Pedigreed cats have a higher genetic predisposition to allergic skin disease than mixed-breed cats, which may partly explain why some breeds seem more ear-infection prone.
If your cat gets repeated ear infections that clear up with treatment but keep coming back, an underlying allergy is one of the first things to investigate. Treating only the infection without addressing the allergy is like mopping the floor while the faucet is still running.
The Shape of a Cat’s Ear Canal
A cat’s ear canal is deeper than a human’s and bends at an angle, creating a funnel shape that’s excellent at collecting sound but also traps dirt, wax, and moisture. Debris that would drain easily from a straighter canal can sit in that bend and decompose, feeding microbial growth. This anatomy is one reason ear infections in animals are far more common than in people. Cats do tend to have fewer ear problems than dogs, partly because their canals are narrower and produce less wax, but the structural risk is still there.
Polyps and Growths
Young cats, typically between eight months and a year old, can develop noncancerous growths called nasopharyngeal polyps inside the middle ear or the back of the throat. These polyps are thought to form after a respiratory virus (such as herpesvirus or calicivirus) inflames the tissue lining the middle ear. The inflamed tissue swells and keeps growing, eventually packing the middle ear space. In some cases, the polyp bursts through the eardrum into the outer ear canal.
A polyp physically blocks the ear from draining and ventilating, which virtually guarantees a secondary infection. Cats with polyps often have discharge from one ear, head tilting, or noisy breathing. The polyp itself needs to be removed for the infection to fully resolve.
Weakened Immune Systems
Cats with compromised immune systems are more vulnerable to ear infections because their bodies can’t keep normal skin flora in check. Feline immunodeficiency virus (FIV) is specifically recognized as a predisposing factor for ear disease. Cats with FIV or other chronic illnesses may develop ear infections more frequently and take longer to recover from them. If a previously healthy cat suddenly starts getting recurrent ear infections, it can sometimes be the first clue that something deeper is going on with their immune function.
Signs to Watch For
The earliest sign is usually scratching at one or both ears, sometimes accompanied by head shaking. As the infection progresses, you might notice your cat tilting their head to one side, flattening their ears, or flinching when you touch around the ear base. The ear flap may look red or swollen on the inside.
Discharge gives useful clues about what’s happening. A small amount of light brown or black wax is normal for many cats. A large amount of dark, crumbly, coffee-ground-like debris strongly suggests mites. Yellow or green discharge with a foul smell points more toward bacterial infection. Any discharge that’s new, excessive, or accompanied by behavior changes warrants a closer look.
How Ear Infections Are Diagnosed
A veterinarian will look inside the ear canal with an otoscope and almost always take a swab of the discharge for cytology, which means examining it under a microscope. This step is considered essential for every cat with ear symptoms because it reveals whether the problem is mites, yeast, bacteria, or some combination. Cytology is actually more reliable than a bacterial culture for outer ear infections, because a culture can’t distinguish between harmless resident bacteria and the ones causing the problem. If the infection has reached the middle ear, a culture is typically added to guide treatment more precisely.
Reducing the Risk
Preventing ear infections starts with addressing the most common causes. Keeping your cat on regular parasite prevention eliminates the mite risk. For cats with known allergies, managing the allergy (through diet changes for food allergies or environmental controls for atopy) reduces the chronic inflammation that invites infections back.
Routine ear cleaning is generally unnecessary for cats with healthy ears. Over-cleaning can actually strip away protective oils and irritate the canal lining, doing more harm than good. If your vet recommends cleaning, use only a product specifically designed for cats, and never insert cotton swabs into the canal. For cats that have had infections before, periodic at-home ear checks, just flipping the ear flap and looking for redness, odor, or unusual discharge, can catch problems early before they become painful.