Dry eye in dogs: symptoms, tests and treatment

Overview

Tears keep the cornea, the transparent front surface of the eye, moist. They remove debris and pathogens reaching the eye's surface. The tear film also helps nourish the cornea. An even, moist surface helps preserve optical clarity.

The tear film contains water, oil and mucus. In the common quantitative form of dry eye, production of the watery tear component decreases. In qualitative dry eye, the problem lies in the oily or mucous components even though water production may be normal. The tear film then fails to remain stable on the eye's surface.

The disease usually affects both eyes. One eye may develop signs earlier or more severely than the other. Most dogs benefit from treatment started before extensive destruction of tear gland tissue. However, advanced corneal changes can leave permanent visual impairment.

Watery tears are produced by both the lacrimal gland within the eye socket and the third eyelid gland. The oily component comes from the meibomian glands along the eyelid margins. It slows evaporation from the eye's surface. Mucus is produced by conjunctival goblet cells. It helps the tear film spread evenly over the surface.

Causes

The most common cause is immune-mediated tear gland inflammation: the dog's immune system damages their own tear gland tissue. Damage can affect the third eyelid gland as well as the main lacrimal gland. Destruction often progresses gradually over months or years. Medicines cannot restore gland tissue that has already been destroyed.

Other possible causes include an underactive thyroid and diabetes. Dry eye can also be associated with canine distemper virus infection. Certain medicines, such as sulfonamide antibiotics, can impair tear production. Radiotherapy around the eye is another possible underlying factor. Occasionally, a dog is born without tear gland tissue.

In neurogenic dry eye, the nerve supply to the tear gland is damaged. Inner ear disease, a tumour or an injury may be responsible. The cause of nerve damage cannot always be identified. Surgical removal of a gland also reduces the available tear-producing capacity.

Qualitative dry eye may result from chronic inflammation of the eyelid margins and meibomian glands. Long-term conjunctivitis can reduce the number of mucus-producing goblet cells. The composition of the protective surface fluid then changes even though watery tear production remains normal.

Risk factors

Dry eye can occur in any breed. It is particularly seen in small dogs and toy breeds. Susceptible breeds include American Cocker Spaniels, Lhasa Apsos, Pugs and Shih Tzus. Cavalier King Charles Spaniels and West Highland White Terriers are also susceptible. An inherited basis is suspected in several breeds.

The disease can begin at any age, but is most common in middle-aged dogs. Yorkshire Terriers are overrepresented among dogs with congenital absence of tear gland tissue. Previous removal of the third eyelid gland is important information when assessing risk.

Breeds susceptible to dry eye may also have extra eyelashes or inward-turning eyelids at the same time. These structural problems can irritate the conjunctiva and cornea in addition to the effects of drying.

Typical symptoms

Signs often develop slowly and worsen gradually. Dry eye can resemble recurring conjunctivitis. Signs visible at home include:

• Thick, sticky discharge from the eye.
• Discharge that may be grey, white, green or yellow.
• Redness over the white of the eye because of conjunctival inflammation.
• More blinking than usual or keeping the eye closed.
• A dull eye surface that has lost its shine.
• Sensitivity to light.
• Dark brown pigment appearing in the cornea with long-standing disease.

In the neurogenic form, often only one eye is dry. The nostril on the same side may also be dry. In chronic dry eye, sticky discharge is often most noticeable in the morning.

Complications and consequences

A dry surface increases friction between the cornea and eyelids. Lack of tears also weakens the eye surface's protection against bacteria. A painful corneal ulcer can develop. An infected ulcer may deepen, leading to corneal melting and perforation.

Prolonged irritation leads to corneal blood vessel growth, scarring and pigment accumulation. These changes can obscure the transparent surface and impair vision. Extensive scarring that has already developed is difficult to reverse. Untreated severe disease can lead to loss of the eye.

Tear production increases with ciclosporin treatment in about 80% of dogs whose baseline tear test is at least 2 mm per minute. With a baseline below 2 mm, the response is poorer, at about 50%. These figures describe increased tear secretion, rather than freedom from lifelong treatment. Most dogs need permanent treatment.

Diagnosis at the vet

The vet asks how long signs have been present, about previous eye operations and which medicines have been used. The eyes are examined as a whole when a new sign develops or the disease worsens. The Schirmer tear test, or STT, measures watery tear production. A test strip is placed under the eyelid for 1 minute. The result shows how far tears have wetted the strip.

A normal tear test result is about 15–25 mm per minute. A result below 15 mm may indicate dry eye. The vet interprets the reading alongside the eye signs. Testing is usually performed without topical anaesthesia so reflex tear production can also be assessed.

The cornea is stained with fluorescein when needed to look for an ulcer. In qualitative dry eye, the tear test may be normal. Tear film break-up time helps assess how well the film remains intact on the eye's surface. Examination of the eyelid margins may reveal inflammation affecting oily secretions.

The tear film break-up test measures when dark areas indicating film disruption appear on the stained surface. Normal break-up time is at least 20 seconds. A time below 5 seconds indicates an unstable tear film. Eye pressure may be measured to investigate other diseases such as glaucoma. Flushing the tear drainage passages can assess whether the outflow route is open when necessary.

Treatment

The vet selects and prescribes medicines. Treatment aims both to increase the dog's own tear production and to replace the missing tear film. Common medicines stimulating tear production include ciclosporin and tacrolimus. They also reduce inflammation of the eye's surface. Pilocarpine, which works by a different mechanism, may be used in the neurogenic form.

Lubricating drops, gels and ointments keep the eye's surface lubricated. Extra lubrication is particularly important initially, before the tear gland has responded to treatment. The treatment effect can take 2–12 weeks to begin. If the tear test has not improved within 2–3 months, the vet assesses whether to change medication.

Antibiotic drops do not replace missing tear production; they are used for any bacterial infection. Corticosteroid eye medicines are considered only if the cornea has no ulcer. An ulcer associated with dry eye may need more intensive treatment than usual. A deep ulcer may require a tissue graft supporting the eye's surface.

For severe disease responding poorly to medicines, moving a salivary duct to the eye may be considered. Saliva then moistens the eye instead of tears. However, saliva may irritate the eye's surface or the skin beneath it. Some dogs still need medication after surgery.

Thicker gels evaporate more slowly than thin artificial tear solutions. Lubricants containing preservatives may irritate a dry eye. The vet also considers tolerability when choosing a product.

After salivary duct transposition, white mineral deposits may develop on the eye's surface. The relocated duct can scar closed. In a severe, persistently painful case unresponsive to other treatment, removal of the eye may be necessary.

Early in treatment, the tear test is often rechecked about 1 month after medicines are started. If medical treatment produces no response within 3–6 months, a surgical option may be considered.

Ciclosporin inhibits the multiplication of T cells involved in tear gland inflammation. Long-term use may reduce corneal pigment and blood vessels even without increased tear production. Benefit is therefore assessed through both tear testing and examination of surface changes.

Prevention

If dry eye is suspected to be inherited, an affected dog should not be used for breeding. Treatment of cherry eye, or prolapse of the third eyelid gland, aims to preserve the gland. After complete gland removal, dry eye may develop later in about 30–40% of dogs.

Tear production must also be monitored after the gland has been replaced. Early dry eye treatment reduces the risk of permanent eye damage. Regular treatment and monitoring help keep the eyes comfortable when established disease is well controlled.

Tear production can also decrease temporarily for up to 24 hours after general anaesthesia. At the veterinary practice, the eyes are therefore protected with lubricating ointment during anaesthesia and recovery. This temporary drying is distinguished from long-term tear gland disease. Even after cherry eye repair, the later KCS risk is about 20%.

When choosing a puppy, find out about the breed's common eye diseases and any dry eye in the family.

Important for dog owners

Keep a list of all eye medicines and lubricants. Record which product is used in which eye, and ask the veterinary team to demonstrate how to apply it. Tell them immediately if giving a medicine is not possible, as this affects the practicality of treatment. Bring previous tear test results and details of medicine use to follow-up appointments.

Soften discharge with a pad moistened in warm water before gently removing it. A collar may prevent rubbing of a painful eye at the start of treatment. Also record new signs of pain and contact the vet treating your dog immediately about them.

A dog whose disease is well controlled usually has their eyes reassessed every 6–12 months. Monitoring is planned separately at the start of treatment or when signs change. At rechecks, mention if your dog avoids having drops applied or if a product seems to increase irritation.

Frequently asked questions

Why does a dry eye produce so much discharge?

Thick, mucus-like discharge can develop even when there is too little watery tear fluid. Discharge therefore does not mean the eye is adequately moist. The vet distinguishes dry eye from other causes of redness and discharge using a tear test and eye examination.

Can dry eye affect only one eye?

Yes. One-sided signs are common particularly in dry eye associated with impaired nerve supply. The nostril on the same side may also become dry. In the common bilateral disease, the eyes can also be affected to different degrees, so both should be examined.

Can treatment stop once my dog's eye looks normal?

The common form of dry eye usually needs lifelong treatment. An improved appearance may therefore show that treatment is working. Do not stop prescribed products yourself; follow the agreed monitoring plan and let the vet assess tear production at repeat measurements.

Can I use my own eye drops for my dog?

Not every human lubricating eye drop is suitable for dogs. Use only products your vet has approved specifically for your dog's eyes. In particular, tacrolimus or pimecrolimus skin preparations must not be put into the eye, even if the same active ingredient is used to treat eye disease.

Why does medication not help in the first few days?

A medicine that increases tear production can take 2–12 weeks to begin working. Lubricants protect the eye's surface during this waiting period. Increasing pain or new cloudiness still needs immediate examination and should not be dismissed as a normal part of waiting for the medicine to work.