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                                          Red eyes in dogs and cats

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). 

Dogs and cats are frequently presented to the veterinarian with the chief complaint of having a "red eye."  This "redness" typically represents inflammation of the tissues of, and around, the eye and could indicate a serious eye condition.

Some degree of redness may be normal for some breeds.  Some diseases that result in a "red eye" may require medical therapy to reduce the inflammation.  Some red eyes indication conditions where emergency surgery is necessary.

Other clinical signs that occur along with the "red eye" may include increased eye blinking, squinting, holding one or both eyes closed, rubbing of the eye, tearing or unusual discharge from the eye.  In addition, clouding of the eye, decreased vision or blindness may also occur. 
If your pet shows any of these signs, don't wait!  Have them seen by a veterinarian immediately. When it comes to diseases of the eye, time is often of the essence. A disease process, such as glaucoma, can change dramatically within the span of a few hours.

Only a complete evaluation by a veterinarian can determine how serious the problem is.  What presents as a "red eye" can be as simple as dry eye, conjunctivitis, or "cherry eye" (prolapsed gland), or as complicated as glaucoma, an eye tumor, hyphema (bleeding inside the eye), or a severe corneal ulceration.

A full history of your pet is the first thing family veterinarian will ask you to provide. When did the problem start? What is the first abnormality you noticed? Have there been any changes in lifestyle recently? Are there any incidents of trauma that you can recall? Has your pets had any health issues (eye-related or not) in the past? All these questions can help narrow down the problem. and also veterinarian will closely examine the eyes for any evidence of infection (bacterial, viral, or fungal), injury, cancer, allergy, foreign material, or lumps/bumps/ingrown lashes or eyelid deformities that might be irritating the eye, eye surface or tissues in the socket.

Meticulous examination of the eye by  veterinarian with a systematic diagnostic approach will aid in obtaining an accurate diagnosis and help determine the basis for treatment.  Your doctor may dim the room lights to perform the ocular exam to best visualize the structures of your pet's eyelids, conjunctiva, cornea, and intra ocular structures.  Advanced ophthalmic equipment may be used to examine different parts of the eye.  Additionally, examination of the retina (nerve tissue in the back of the eye) is imperative as certain diseases may only affect the tissues inside or in the back of the eye.  In addition to a complete examination, every patient should have testing performed to rule out dry eye, corneal ulcers and glaucoma.  Blood tests (complete blood count, chemistry profile, tick serology, fungal serology) and blood pressure measurements may be necessary if systemic disease is suspected.

If a diagnosis cannot be reached, a referral to a specialist may be recommended for further testing. Ophthalmologists often have more sophisticated instruments to aid in diagnoses, such as an ocular ultrasound and electroretinogram (ERG) machine to test visual capabilities.

Next, your veterinarian will perform an ophthalmological exam. All areas of the eye will be assessed, including the eyelids, conjunctiva, lens, iris, and retina. An assortment of specialized tests will check your pet’s tear production, intraocular pressure (pressure within the eye), visual capabilities, and corneal integrity. The most common eye-related complaint to the veterinarian is a “red eye”. Unfortunately, there are many reasons that an eye can appear red, some more serious than others. A full ophthalmological exam is imperative to differentiate the different causes, as well as determining the most appropriate course of treatment.


The term “conjunctivitis” refers to inflammation of the conjunctiva (the loose pink connective tissue around the eye). It is usually a mild disease with many possible causes. Symptoms are usually a red eye with some degree of discharge, ranging from clear and runny to a thick yellow/green colour. One of the most common causes of conjunctivitis is allergies due to environmental irritants such as pollen or dust. Bacterial conjunctivitis is also common, but usually secondary to other problems. Viral conjunctivitis is seen mainly in cats infected as kittens with a Herpes virus. Most cases are straight forward to treat, although chronic infections can occur depending on the nature of the cause.


Inflammation of the cornea, referred to as “keratitis”, has many causes and can present itself in many ways. The cornea is composed of four different layers sandwiched together, forming a protective barrier no more than 1mm thick. The cornea receives nourishment externally from the tears and internally from a fluid called the aqueous humor. Although the cornea is a strong structure, direct trauma can abrade the surface and allow potentially harmful organisms to carve out a crater, resulting in an ulcer. Trauma can range from a penetrating injury, such as a cat scratch, to the constant rubbing of abnormally directed eyelashes. Tears play an important role in healing the surface of the cornea. As a result, animals with low tear production or eyes that do not accommodate the proper closing of the eyelids, are at a higher risk of developing ulcers. For example, dog and cat breeds that have protruding eyes (e.g. Shih Tzu, Pug, Persian) are at greater risk of developing corneal ulcers. The inflammation caused by the ulcer is painful and can increase the size of the blood vessels around the eye, giving it a red appearance. Cases that cannot be treated with aggressive medical therapy must be referred for surgery. Left untreated, the ulcer can work its way through the entire cornea in very little time and perforate the eye.

Another disease of the cornea leading to a red eye is a condition called Keratoconjuncitivitis Sicca (KCS or “dry eye”). This is, in fact, a disease of the lacrimal gland which produces a portion of the tears. Tear film is composed of three parts: a mucous layer, a water layer, and a lipid (fat) layer. The water layer comprises 95% of the tear film, and the lacrimal gland is responsible for 70% of this. When the gland is not functioning properly, tear production falls drastically and the condition is referred to as KCS. Symptoms include: a red eye, mucoid discharge from the eye, squinting of the eye, and/or a dry look to the cornea.


The uvea is the vascular layer of the eye, with layers of blood vessels throughout. There is a barrier between the body’s circulation and inside of the eye, called the blood-aqueous border. The cells that make up the vessels in the eye are tightly bound together and do not allow any large particles into the fluid known as the aqueous humor. When the blood vessels in the uvea become inflamed, this is known as uveitis. The barriers, maintained by the blood vessels, break down and allow inflammatory chemicals and cells into the eye. Symptoms include: a red eye, squinting and severe pain, excessive tearing, and/or a protruding third eyelid. Again, there are numerous causes, including trauma (penetrating foreign body, corneal ulceration, recent surgery within the eye), pathological organisms (viruses, bacteria, parasites), autoimmune disease, cancer, and many others. A full history and careful examination may help to pinpoint the cause of uveitis. Treatment is required immediately to prevent permanent damage to the eye.


The eye can be thought of as a sink, with fluid being produced but also drained at the same rate. The pressure within the eye is maintained at a constant equilibrium, fluctuating minimally depending mostly on the animal’s blood pressure at the moment. The aqueous humor, a fluid at the front part of the eye, is produced by a structure called the ciliary body. This fluid circulates, provides nutrition to the cornea, removes waste, and then drains out of the eye at the iridocorneal angle. Two abnormalities can therefore increase the pressure within the eye: and increase in aqueous production or a decrease in outflow.

Glaucoma refers to a condition where the intraocular pressure increased above a threshold. It is more often a symptom of another underlying condition rather than a primary disease itself. Signs of glaucoma can vary, but most often the conjunctiva is red, the eye enlarged and painful, the pupil is dilated, and the cornea may have a white/blue tinge to it. It is wise to seek veterinary attention as soon as possible if glaucoma is suspected, as even a few hours can change the prognosis. Aggressive medical therapy can help stabilize the pressure within the eye for a longer period of time. Ultimately, however, the intraocular pressure in an eye with glaucoma will increase despite medical treatment.

There are many other causes of an apparently red eye not listed here, including abnormalities of the third eyelid and eyelid tumors. Any concerns should be brought to the attention of your veterinarian as quickly as possible. The eyes are a delicate organ that requires prompt attention when affected with disease. Mild conditions can often be treated easily with a full recovery. In more serious diseases, lifelong therapy may be required. Animals adapt well to reduced visual ability, but if there is a chance to save even minimal vision, it is well worth the effort.


Frequently Used Diagnostic Tests:

  • An ophthalmoscope allows thorough visualization of the tissues in the interior of the eyeball including the retina, the coating that covers the back of the eye. Other more specialized equipment such as slit lamp scopes allow for detailed evaluation of hard-to-see areas such as the internal angle of the eye, that if narrowed or blocked may lead to glaucoma, an increase in eyeball pressure.
  • Eyeball pressure can be directly measured using tonometry, a piece of equipment that is placed directly on the surface of the eye.
  • Sometimes the healthcare team will take a small amount of the discharge or swab the tissues around the eye (conjunctival swab) and examine the sample cells under the microscope (cytology), or use a swab to obtain a sample to culture the discharge for bacterial growth and antibiotic sensitivity to aid in their choice of antibiotic therapy.
  • The veterinarian may apply a dye, or flush the tear ducts to assess their function. For conditions of the cornea, topical dyes that fluoresce under ultraviolet light may be applied to check for ulcer architecture. Eye problems can have many causes, and sometimes more than one type of problem co-exists. Discharge is not specific to any one cause.

Normal or Abnormal?

Some pets are prone to chronic (ongoing) eye problems because of their breed-specific eye conformation. Short-nosed breeds such as Persians and Pugs have very prominent eyes, and they can be more prone to irritation by dust etc. because of their less protected globe position. In many of these short-nosed breeds, the tears tend to pool at the inside corner (medial canthus) of the eye, and lose moisture there. The pooling is a combination of conformation of the eye/eyelids, and a tear duct conformation that does not allow adequate tear drainage into the nose. More tears are often produced on an ongoing basis also, in order to keep the prominent eyes moist. Pooling often results in a porphyrin (pigment)-stained discharge that is sometimes pink, red, sometimes brown, or even can appear black and crusty on the fur. This is particularly noticeable in light-coloured pets, for example, white poodles.

Sometimes a discharge is related to blockage or narrowing of the tear ducts, and other times the eye problem relates to lack of adequate tear cover over the surface (cornea) of the eyes leading to dry eyes. Conditions such as pannus, a corneal pigment and blood vessel outgrowth condition, or dry eye (keratoconjunctivitis sicca) are examples of conditions that relate to exuberant inflammatory responses. A tear production test (Schirmer tear test) is typically used if dry eye is suspected to help quantify tear flow. A small paper strip is placed in the corner of the eye and the moisture wicks onto the strip to provide a measure of the number of millimetres of tears produced during timed trials.

Cataracts are opaque lenses that may develop due to genetics, old age, diabetes and many other causes. In very senior pets, they are considered a normal aging change if they develop in a pattern typifying the old age cataract.

Treatments for Eye Conditions

Because of the diverse causes of eye problems, treatment varies considerably—from surgery to topical or oral therapy. Lubricating or antibiotic eye ointment or drops will often be prescribed. Sometimes, if an allergic component is identified, a soothing steroid component may be added to the treatment regimen. Allergic eyes are an attractive place for bacteria to move in and colonize because the tissue defenses are weakened by the inflammation.

If eye preparations are prescribed, it is important to stick to the treatment schedule in order to optimize progress. This of course, can be quite a challenge in some pets that do not enjoy having things placed in their eyes! Make sure your health care team gives you administration tips to help you achieve success. Nursing care such as regular gentle cleansing of the eye area with sterile salt solution or tepid clean water to remove discharge is important to prevent dermatitis (skin inflammation and infection) in the fur around the eye and eyelids. If dermatitis around the eyes is already present, a petrolatum jelly-based ointment (with or without antibiotics) may be recommended to provide a slick barrier on the skin, so the discharge crusts do not adhere, and further irritate the area.

If needed, family veterinarian always has the option of a referral to Veterinary Ophthalmologists at MVS/OVRS/MSU.  Veterinary ophthalmologists are specially trained in various diagnostic techniques, therapies and surgical correction of all sorts of eye problems.  To learn more about eye problems in animals and veterinary ophthalmology visit American College of Veterinary Ophthalmologists.

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