Causes of Red Eyes in Dogs and Cats

Laurie Anne Walden, DVM

If your pet’s eye is red, have it checked by a veterinarian without delay. Eye redness is a nonspecific symptom. It’s nearly always impossible to tell whether a red eye is minor or serious without ophthalmic tests at a veterinary clinic. These are some of the conditions that cause red eyes in dogs and cats.
 
Conjunctivitis
Conjunctivitis, called pinkeye in people, is inflammation of the tissue that lines the eyelids and covers the whites of the eyes. Causes of conjunctivitis in pets include environmental irritants, allergies, skin disease, dry eye, and (especially in cats) viral or bacterial infections. Some of the conditions that cause conjunctivitis also affect the eyelids and the cornea, the clear front part of the eye.
 
Corneal Ulcer
Corneal ulcers are defects on the surface of the cornea. In dogs and cats they can be caused by things poking the eye (like turned-in eyelashes, a foreign object under the eyelid, or being swatted in the face by a cat), infections, dry eye, disorders of corneal cells, and chronic eye exposure in flat-faced animals with shallow eye sockets. Corneal ulcers and scratches are painful. Some ulcers are shallow and heal fairly quickly with treatment. Others are deep and can perforate all the way through to the interior of the eye. Diagnosis requires applying an ophthalmic stain to highlight the corneal defect.
 
Trauma
Trauma to the outer surface of the eye causes redness of the white part of the eye, similar to the redness caused by conjunctivitis. Blunt trauma to the head can cause bleeding inside the eye, which looks like dark red discoloration behind the cornea. Bleeding disorders and other conditions can also cause blood accumulation inside the eye.
 
Uveitis
Uveitis is inflammation of the interior of the eye. Uveitis doesn’t happen on its own; it’s a sign of another problem. Some of the diseases that cause uveitis affect the whole body: infections, tick-borne diseases, immune-mediated disorders, cancer, and so forth. Uveitis is also caused by eye disorders like cataracts. Uveitis is a painful condition that can lead to glaucoma. Diagnosis involves examination of the eye, measurement of eye pressure, and tests to find the underlying cause.
 
Glaucoma
Glaucoma is a disease characterized by increased pressure within the eye. The condition is inherited in some dog breeds and is also caused by uveitis, lens luxation (lens slipping out of place), and cancer of the eye. Glaucoma is painful and leads to blindness. Sudden-onset glaucoma is a medical emergency if vision is to be saved.
 
Cherry Eye
Cherry eye is the common term for prolapse of the gland of the nictitating membrane, a pink membrane (sometimes with a dark edge) at the inner corner of the eye. A tear gland within this membrane can swell and protrude past the edge. The prolapsed gland looks like a round pink or red mass at the inside corner of the eye. This is the one type of “red eye” that doesn’t require an immediate visit to the veterinarian—unless other symptoms, like squinting or eye discharge, are also present. However, a prolapsed tear gland can cause eye irritation, and tumors in this area look similar, so it should still be checked out. Prolapsed tear glands are treated surgically.
 
Photo by Céline Harrand

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