Red eyes are a common reason for veterinary visits. Red eyes may have many causes, from mild allergic irritation to emergencies such as head trauma and glaucoma. There are far too many causes of red eyes to discuss all of them here, but I will point out a few of the more common ones.

A common cause of red eyes in both dogs and cats is conjunctivitis. Conjunctivitis describes inflammation of the conjunctiva, which is the mucous membrane that covers the inside of the eyelids and the white part of the eye. Common causes of conjunctivitis include allergies, bacterial infections and viral infections. Treatment depends on the underlying cause.

Injuries to the cornea, which is the surface of the eye, are also common. Injuries that damage the surface of the cornea often cause corneal ulcers. One of the most common tests to determine the cause of red eyes in pets is called a fluorescein stain. This test uses a special dye to determine whether a corneal ulcer is present. Corneal ulcers that affect the superficial layers of the cornea require immediate treatment in order to prevent them from extending into deeper layers. Severe ulcers that extend through the entire thickness of the cornea require emergency treatment as they may cause the eye to rupture.

A condition known as dry eye, or keratoconjunctivitis sicca, is a common condition that causes a decrease in tear production. It often results in irritation of the cornea, and is commonly recognizable by a thick, yellow/green discharge over the surface of the eye. Long-term irritation often results in decreased vision due to scarring and pigmentation on the surface of the eye. Dry eye is diagnosed by a test called a Schirmer Tear Test, which measures tear production. Treatment may involve topical medications and/or surgery. 

Glaucoma is another common, and very serious, cause of red eyes. Glaucoma is caused by increased pressure within the eye. It is a very painful condition that can quickly lead to blindness.  Symptoms may include an eye that appears cloudy, bloodshot, or that is held shut in pain.  Sometimes, the eye may be visibly increased in size, having a bulging appearance. Glaucoma causes blindness quickly, and blindness is often present in the affected eye by the time the condition is diagnosed. Glaucoma is diagnosed with an instrument that measures the pressure within the eye. If it is caught early enough, it can often be treated with topical medications and/or surgery.

The eye is a sensitive organ, and certain conditions can quickly lead to permanent loss of vision. A few minutes of a serious eye condition can mean the difference between vision and blindness, and it can be very difficult for the pet owner to differentiate between a mild and a serious condition. It is always better to be safe than sorry, especially where vision is concerned. Therefore, most cases of red eyes warrant a visit to the vet. 

Dr. Kelly Sulik owns and operates Animobile Mobile Veterinary Services in Tryon, N.C. She can be reached at animobiledvm@gmail.com.