Dry eye, or keratoconjunctivitis sicca, is a failure to produce enough tear film to keep the surface of the eye healthy. It causes thick sticky discharge, redness, squinting and a dull or hazy looking cornea. It is often treated as a recurring infection for months, and it is uncomfortable and progressive when missed.
The pattern that gives it away
The history is almost always the same. The eye gets gunky, someone cleans it, it looks better for a day, and then the discharge returns. A course of drops from a previous problem helps briefly, or does nothing. The eye is never quite right, and over months it starts to look duller and less clear than the other one.
What sounds like a series of minor infections is often one continuous problem. The discharge is not the disease. It is what the eye produces when the watery part of the tear film is missing and only the mucus part remains.
What tears actually do
Tear film is not simply water. It has three layers: an oily outer layer that slows evaporation, a watery middle layer that carries oxygen and nutrients and washes debris away, and an inner mucus layer that helps the whole film stick to the surface of the eye.
The cornea, the clear window at the front of the eye, has no blood supply of its own. It depends on the tear film for oxygen, for nutrition and for defence against bacteria. Take away the watery layer and the cornea is left exposed, undernourished and unprotected.
What follows is predictable. The eye becomes inflamed and painful. Thick ropey mucus accumulates because the mucus layer is now the dominant component. The cornea responds to chronic irritation by thickening, developing blood vessels and depositing pigment, and that pigment is what progressively clouds vision. Ulcers become more likely, and they heal less readily.
Signs to look for
- Thick, sticky, yellow-green or grey discharge, often stringy and returning within hours of cleaning
- Redness of the whites of the eye and the inner lids
- Squinting, rapid blinking, or a lid that sits half closed
- Face rubbing along furniture, or a paw going repeatedly to the eye
- A dull, lacklustre or hazy appearance to the surface, rather than a wet shine
- Brownish pigment spreading across the cornea over months
- Recurrent conjunctivitis that keeps coming back after treatment
- A dry, crusty nostril on the same side in some dogs
- Reluctance to go out in bright light, or increased sensitivity generally
The most useful clue is that dull surface. A healthy eye looks wet and reflects light cleanly. An eye with inadequate tear film looks matte, and once you have noticed the difference on one animal you will always see it.
Why it happens
The most common cause in dogs is immune-mediated: the body’s own immune system gradually damages the tear-producing glands. This tends to develop slowly and affects both eyes, though often not equally at the same time.
Other causes include certain medications, some infections, damage to the nerve supplying the gland, removal of the third eyelid gland during past cherry eye surgery, and some hormonal conditions. Breeds recognised as predisposed include Cocker Spaniels, West Highland White Terriers, Shih Tzus, Lhasa Apsos, Pugs, Bulldogs, Cavalier King Charles Spaniels and Yorkshire Terriers, though it occurs in any breed.
Cats develop it far less often, and when they do it is more commonly linked to previous or ongoing viral disease affecting the eye rather than to immune-mediated gland damage.
Diagnosis is quick and the test is simple
This is genuinely good news for owners who have been going round in circles. Tear production is measured with a small calibrated paper strip placed briefly inside the lower lid, and the result is available in about a minute. It is well tolerated and it gives a number rather than an impression.
That test is usually paired with staining the surface to check for ulceration, since ulcers change the treatment plan and are more common in these eyes. An examination of the lids, the third eyelid and the tear drainage system completes the picture, and further testing may be recommended where a specific underlying cause is suspected. All of this happens within our medical services, supported by our diagnostic services where bloodwork is indicated.
The reason this matters is that the tear test takes a minute and is not done unless someone thinks of it. Any dog with recurring eye discharge deserves to have it measured rather than treated again.
Treatment, and the part owners find hardest
For the common immune-mediated form, treatment is usually a medication that acts on the immune process affecting the tear glands, prescribed by your veterinarian, often alongside lubrication and, where there is infection or ulceration, additional treatment for those.
Two things are worth being honest about. First, it is generally lifelong. Tear production commonly falls again within weeks if treatment stops, so this is a condition you manage rather than complete a course for. Second, improvement takes time, often several weeks before tear production increases meaningfully, so the first fortnight can feel discouraging.
The practical side is a routine rather than an event. Cleaning discharge away gently with warm water and a soft cloth before applying anything, so the medication reaches the surface rather than sitting on mucus. Applying at consistent times. Rechecking tear measurements as advised, because the number guides the plan and an eye can look better while the underlying production is still inadequate.
Where a chronic eye has developed complications, or where an ulcer is not healing, additional intervention may be needed and our urgent care service is the route for anything that changes suddenly.
What not to do
Do not use leftover eye drops from a previous problem, from another pet, or from the human medicine cabinet. Some preparations that are entirely appropriate for one eye condition are harmful in another, particularly where an ulcer is present, and a steroid-containing drop used on an ulcerated cornea can cause serious damage.
Do not treat repeated discharge as a cleaning problem. Wiping it away addresses the symptom while the cornea continues to change. And do not stop treatment because the eye looks good, which is the single most common reason a well-controlled case relapses.
If your pet is squinting hard, the eye is suddenly much more painful, or the surface looks cloudy or has a visible defect, treat that as urgent rather than waiting for the next scheduled recheck.
Frequently asked questions
Why does my dog’s eye keep getting infected?
Repeated conjunctivitis in the same eye is a common presentation of dry eye rather than a series of unrelated infections. Without an adequate tear film the surface cannot clear debris or defend itself, so problems recur as soon as treatment stops. Ask for tear production to be measured; the test takes about a minute and settles the question.
How is dry eye diagnosed?
With a tear test, using a small calibrated paper strip placed briefly inside the lower lid to measure how much tear fluid is produced in a set time. It is quick and well tolerated and gives a number rather than an impression. It is normally combined with staining the surface to check for ulceration, since ulcers are more common in these eyes and change the treatment.
Is dry eye painful for my dog?
Yes. The cornea is a richly innervated surface and losing its protective film is genuinely uncomfortable, comparable to persistent grit in the eye. Dogs rarely cry out, so the discomfort shows as squinting, rubbing the face, reluctance in bright light, and a general flatness that owners often attribute to age.
Will my dog need treatment for life?
Usually yes, for the common immune-mediated form. Tear production tends to fall again within weeks of stopping, so it is managed rather than cured. That is manageable in practice, since it is generally a routine at consistent times each day, but it does mean rechecks matter and stopping because the eye looks better is the main cause of relapse.
Can I use human eye drops or leftover drops?
No. Some preparations appropriate for one eye condition are harmful in another, and a steroid-containing drop used on an eye with an ulcer can cause serious damage. Since dry eye makes ulcers more likely, this is a real risk rather than a theoretical one. Have the eye examined and use only what is prescribed for it.
Can dry eye cause blindness?
It can lead to significant vision loss if left untreated, because chronic irritation causes the cornea to scar and to deposit pigment that progressively clouds the clear window at the front of the eye. Those changes are difficult to reverse once established, which is the argument for measuring tear production early rather than treating discharge repeatedly.