Few canine eye diseases are mistaken for something harmless as often as this one. A dog wakes up every morning with thick, sticky discharge crusted in the corner of the eye. The white of the eye looks pink. The owner wipes it away, assumes it's allergies or a mild infection, and repeats the routine for months. Meanwhile the cornea — the clear window at the front of the eye — is slowly drying out, scarring, and losing transparency. The condition is keratoconjunctivitis sicca, usually shortened to KCS and known in plain English as dry eye, and it is both more common and more treatable than most owners realize.

What Dry Eye Actually Is

A dog's tear film is not just water. It has three layers: an oily outer layer from the meibomian glands that slows evaporation, a watery middle layer from the lacrimal glands that makes up the bulk of the volume, and a mucus inner layer that helps the film stick to the eye. In KCS, the watery middle layer fails. The lacrimal gland and the gland of the third eyelid stop producing enough aqueous tears, and what's left is a thick, mucus-heavy film that can't do the job.

That matters because tears aren't just for comfort. They wash away debris and bacteria, carry oxygen and nutrients to the cornea — which has no blood vessels of its own — and keep the corneal surface optically clear. Take the water away and the cornea becomes inflamed, painful, and vulnerable. The Cornell Riney Canine Health Center notes that untreated KCS can progress to corneal scarring, pigment deposition, blood vessel growth across the cornea, secondary infections, and painful ulcers — and in severe cases, permanent vision loss.

The Signs Owners Miss

The single most recognizable sign is the discharge. Because the watery component is missing, what's left is disproportionately mucus, producing the thick, ropy, yellow-green goop that owners so often read as "an infection." Other signs include:

Dry eye can affect one eye or both. It is frequently missed early precisely because the discharge responds a little to antibiotics — the drops reduce the secondary bacteria without touching the underlying tear deficiency.

Why It Happens

The overwhelming majority of cases are immune-mediated: the dog's own immune system attacks and gradually destroys the tear-producing glands. This is why so many cases are breed-linked and why treatment focuses on suppressing that immune attack rather than simply adding moisture.

Other recognized causes include drug reactions, and this is worth knowing about. Potentiated sulfonamide antibiotics — the trimethoprim-sulfa family — are well documented to cause KCS in dogs, and the damage can be permanent; smaller dogs appear to be at greater risk. A 2026 VetCompass Australia cohort study in the Journal of Veterinary Internal Medicine looked specifically at this association in dogs treated with trimethoprim sulfonamide. If your dog is on one of these drugs long-term, ask your veterinarian about monitoring tear production.

Beyond immune-mediated disease and drug reactions, KCS can follow surgical removal of the third eyelid gland (the reason modern veterinary practice repositions a "cherry eye" rather than cutting it out), canine distemper infection, damage to the nerve supply of the gland, endocrine diseases such as hypothyroidism and diabetes, chronic conjunctivitis, radiation to the head, or in rare cases a congenital absence of functional lacrimal tissue.

The Breeds at Highest Risk

Dry eye is not evenly distributed across the dog population. American Cocker Spaniels, English Bulldogs, Pugs, Lhasa Apsos, Shih Tzus, Cavalier King Charles Spaniels, West Highland White Terriers, Bloodhounds, and Pekingese all appear repeatedly in the veterinary literature as predisposed. A large UK primary-care epidemiological study published in the Journal of Small Animal Practice found dramatically elevated odds compared with crossbred dogs — on the order of fifty-fold in American Cocker Spaniels and roughly twenty- to forty-fold in English Bulldogs, Pugs, and Lhasa Apsos. Data reviewed by the American College of Veterinary Ophthalmologists put the overall prevalence across a large screened population at well under one percent, which underlines the point: this is a disease concentrated in specific breeds rather than spread thinly across all dogs.

If you own one of the high-risk breeds, especially a flat-faced one, it's reasonable to ask for a tear test at annual checkups even in the absence of symptoms.

Diagnosis Takes About a Minute

The Schirmer tear test is inexpensive, requires no sedation, and takes sixty seconds. A small calibrated paper strip is tucked inside the lower eyelid, and the vet reads how far the tears wick up the strip in one minute. Normal canine values are generally in the range of 15 to 20 mm per minute or higher. A reading below about 10 mm per minute in a dog with matching clinical signs supports a diagnosis of KCS, and results in the low single digits indicate severe disease.

Your veterinarian will usually pair this with a fluorescein stain, a dye that glows green under blue light wherever the corneal surface is damaged, to check for ulcers. Because ulceration changes the treatment plan, this step matters.

Treatment: Restart the Tears, Don't Just Add Drops

The cornerstone of modern KCS therapy is topical cyclosporine, and in refractory cases tacrolimus. Both are calcineurin inhibitors: they suppress the T-cell driven inflammation attacking the lacrimal glands, which allows surviving tissue to resume producing real tears. This is fundamentally different from artificial tears, which only substitute for what's missing.

The encouraging news is that response rates are high. Veterinary ophthalmology sources report that even dogs with Schirmer readings as low as around 2 mm still have roughly an 80 percent chance of responding to cyclosporine, though some require a higher compounded concentration than the standard 0.2 percent ointment. Dogs that fail cyclosporine may still respond to tacrolimus. Improvement is not instant — it typically takes several weeks of consistent twice-daily dosing before tear production climbs, so owners need to persist through the early weeks.

Supporting treatments usually include artificial tear gels or ointments for comfort between doses, a short course of topical antibiotics if there is secondary infection, and sometimes a mucus-thinning agent. Anti-inflammatory drops may be added, but topical steroids must never be used if there is a corneal ulcer — they can accelerate corneal breakdown. This is one reason not to reach for leftover eye medication in the cupboard. For the small subset of dogs that don't respond to medical therapy, a surgical option called parotid duct transposition reroutes a salivary duct to the eye so that saliva lubricates the cornea; it works, but brings its own complications and is reserved for genuine treatment failures.

The Part Owners Need to Hear

KCS is a management disease, not a cure-and-forget one. In the great majority of cases, treatment is lifelong. Dogs whose eyes look wonderfully normal after two months of cyclosporine will slide right back to thick discharge and an inflamed cornea within weeks if the drops stop, and each relapse risks more permanent corneal scarring. Recheck Schirmer tests — typically a few weeks after starting, then periodically — let your vet confirm the dose is doing its job.

Handled that way, the outlook is good. Most dogs diagnosed before the cornea is badly scarred become comfortable, keep their vision, and stay that way for years on a routine that costs a few seconds of your morning and evening. The dogs who lose vision to dry eye are almost always the ones whose "allergy goop" was wiped away for a year before anyone put a paper strip in the eye. If your dog's eyes look chronically gunky, dull, or red, ask for the tear test. It takes a minute.