Red eyes in dogs and cats can occur for many reasons and involve multiple structures in and around the eye. Red eyes may or may not be accompanied by an eye discharge or excessive tearing (watery eyes). To better understand conditions that may cause an eye to be red, it is helpful to understand some eye anatomy. The following figures and definitions may help.

Figure 1. Side view of the eye

Anterior chamber: Area behind the cornea that is filled with a liquid called aqueous humor

Ciliary body: Structure which produces aqueous humor, the fluid in the front portion of the eye

Conjunctiva: Lining of the eyelids and the thin membrane that covers part of the sclera, which is the "white of the eye"

Cornea: Clear covering of the front of the eye

Iris: Colored portion of the eye

Lens: Structure in the eye that focuses light on the retina

Pupil: The opening in the iris that appears as a dark circle

Figure 2. Front view of the eye

Third eyelid: The eyelid that is attached to the inner corner of the eye and extends out to the side of the eye when the animal blinks; also called the "nictitating membrane"

Causes of red eyes in dogs and cats

The more common causes of red eyes in dogs and cats are shown in the Table 1 below. (This chart is not meant to be an inclusive list. The most common causes are highlighted in grey).

Table 1. Causes of Red Eyes in Dogs and Cats
Condition	Description	Symptoms	Diagnosis	Treatment
Anterior uveitis	Inflammation of the iris and/or ciliary body - many causes including trauma, bacteria, viruses and cancer	Squinting (pain); constricted pupil; iris appears swollen; cornea may be cloudy; pet avoids light; may have excessive tearing	Ophthalmic exam	Anti-inflammatory agents, medications to dilate the eye (reduces pain); specific treatments based on cause
Blepharitis	Inflammation of the eyelids due to infection, allergies, or irritants	Eyelids red and swollen	Ophthalmic exam	Specfic treatment based on cause; e.g., antibiotics, antifungals
Corneal ulcer	Breakdown of the surface of the cornea; often not visible with the naked eye	Squinting (pain); pet avoids light; may have excessive tearing	Ophthalmic exam including special exams using a fluorescent stain	Topical antibiotics and atropine; more severe ulcers may require other medications and surgery
Dry eye (kerato-conjunctivitis sicca)	Inadequate tear production	Large amounts of thick white to green discharge	Ophthalmic exam including a Schirmer Tear Test	Cyclosporine or tacrolimus and possibly artificial tears; keep eye and eyelids clean. Treatment is life long.
"Cherry eye" (prolapsed gland of the the 3rd eyelid)	The gland on the inside of the third eyelid protrudes from behind the eyelid	Red mass, resembling a small cherry appears from behind the third eyelid	Ophthalmic exam	Surgery to replace the gland
Conjunctivitis	Inflammation of the conjunctiva due to infection, allergies, or irritants	Conjunctiva very thickened and red; large amounts of discharge; normal pupil size	Ophthalmic exam including collecting cells from the conjunctiva for microscopic examination	Appropriate medications, e.g., antibiotics, depending upon cause; keep eye and eyelids clean; treat any underlying condition
Foreign body	Plant, mineral or animal material embedded in or on the surface of the eye	Object may be visible; often increased tearing	Ophthalmic exam	Remove foreign body; antibiotics and dilation of the pupil as needed
Glaucoma	Increased pressure within the eye	Eye may appear swollen; pupil is dilated and does not respond to light; pain; blood vessels in white portion of eye enlarged	Ophthalmic exam including measurement of intraocular pressure	Remove foreign body; antibiotics and dilation of the pupil as needed
Hyphema	Blood in the anterior chamber of the eye due to trauma, clotting disorders, or other eye diseases	Pool of blood in the anterior chamber; if covers part of the pupil may affect vision	Ophthalmic exam including measurement of intraocular pressure	Medications to dilate the pupil and reduce inflammation; treat underlying cause
Lens luxation	Lens of the eye becomes displaced out of its normal position; may be hereditary, congenital, due to trauma, or secondary to cataracts, uvetitis or glaucoma	Abnormal appearance to the iris; may be able to see displaced lens	Ophthalmic exam	Surgery to replace the lens
Proptosed globe	Eye is partially or completely out of the socket, usually the result of trauma	Bulging eye may be partially or totally out of the socket	Ophthalmic exam	Surgery to replace the eye; if eye severely injured, may need to be removed
Superficial keratitis	Inflammation of the cornea caused by bacteria, viruses, allergy, trauma, or constant irritation	Cornea cloudy; discharge present; squinting (pain)	Ophthalmic exam	Appropriate medications, e.g., antibiotics depending upon cause; keep eye and eyelids clean; treat any underlying condition
Tumor	Tumor behind, in, or near the eye	Signs may vary greatly depending upon the type, size and location of the tumor	Ophthalmic exam	Surgery or radiation therapy