KCS in dogs, short for keratoconjunctivitis sicca, is a chronic condition where the eyes don’t produce enough tears to stay properly lubricated. You might know it by its common name: dry eye. It’s one of the most frequent eye problems in dogs, and without treatment it can lead to painful corneal ulcers and permanent vision damage.
How Dry Eye Develops
A healthy dog’s eye is covered by a thin tear film made of three layers. The innermost layer is a mucus coating produced by cells in the conjunctiva (the pink tissue lining the eyelids). The middle layer is the watery component, produced by the lacrimal (tear) glands. The outermost layer is an oily film produced by tiny glands along the eyelid margins. Together, these layers keep the cornea moist, nourished, and protected from debris and infection.
KCS develops when one or more of these layers breaks down. The most common form involves reduced watery tear production, meaning the tear glands themselves aren’t secreting enough fluid. A second form, more typical in flat-faced breeds like Bulldogs, Pugs, and Shih Tzus, involves excessive tear evaporation. Their prominent eyes and wider eyelid openings expose more of the corneal surface, causing tears to dry out faster than they’re replaced. In either case, the result is the same: the cornea loses its protective moisture barrier and becomes inflamed.
What Causes It
The most common cause by far is immune-mediated destruction of the tear glands. The dog’s own immune system attacks the tissue that produces tears, gradually reducing output over time. Why this happens isn’t fully understood, but certain breeds are genetically predisposed, suggesting a strong hereditary component. Cocker Spaniels, West Highland White Terriers, Cavalier King Charles Spaniels, English Bulldogs, Lhasa Apsos, and Shih Tzus are among the breeds most frequently affected.
Other causes include certain medications (sulfonamide antibiotics are a well-known trigger), infections like canine distemper, hypothyroidism, surgical removal of the third eyelid gland (cherry eye repair done incorrectly), and nerve damage that disrupts signals to the tear glands. In rare cases, dogs are born with underdeveloped tear glands. Most of the time, though, a dog diagnosed with KCS has the immune-mediated form.
Signs to Watch For
The hallmark sign is a thick, yellow-green discharge that clings to the eye and surrounding fur. This isn’t a normal “eye booger.” Without adequate watery tears to flush the surface, mucus builds up and bacteria thrive, producing that sticky, often smelly discharge. Many owners initially mistake this for a recurring eye infection.
Other signs include:
- Dull, dry-looking eyes that have lost their normal shine
- Redness in the whites of the eyes and along the eyelid lining
- Squinting or holding the eye partially closed, indicating pain
- Pawing or rubbing at the face
- Dark pigment or visible blood vessels on the cornea in chronic, long-standing cases
That last sign, dark pigmentation creeping across the clear surface of the eye, is the body’s attempt to protect the chronically dry cornea. It’s a sign that the condition has been present for a while and is already causing structural changes.
How Vets Diagnose KCS
Diagnosis is straightforward. Your vet will perform a Schirmer tear test, which involves placing a small strip of absorbent paper inside the lower eyelid for one minute and measuring how far the moisture wicks up the strip. Normal dogs produce roughly 18 to 24 millimeters per minute. Values below 15 mm/min are considered diagnostic for KCS, and many affected dogs measure in the single digits. The test is quick, inexpensive, and not painful for the dog, though it can be mildly uncomfortable.
Your vet will also examine the cornea with a special dye (fluorescein) to check for ulcers, which are open sores on the eye’s surface that can develop when dryness leaves the cornea vulnerable to scratches and infection.
Treatment and What to Expect
The primary treatment is a topical medication that suppresses the immune attack on the tear glands, allowing them to recover and resume tear production. Cyclosporine is the most widely used option, typically applied as an ointment or oil-based drop twice daily. It works by calming the immune response in the tear glands and also stimulates mucus production, which helps stabilize the overall tear film.
About 75% of dogs respond well to cyclosporine, showing a meaningful increase in tear production within the first month. For the roughly 25% that don’t respond, tacrolimus is an alternative that works through a similar mechanism but can sometimes succeed where cyclosporine didn’t. Dogs already on cyclosporine without adequate improvement have shown significant gains after switching.
Your vet will typically recheck the Schirmer tear test about one month after starting treatment to see how the tear glands are responding. Once your dog is stable, expect follow-up visits every 6 to 12 months to monitor progress and catch any changes early. Most dogs need these medications for life, since stopping treatment usually allows the immune attack to resume and tear production to drop again.
Artificial tear supplements are often used alongside the immune-modulating drops, especially early on when the glands haven’t yet recovered. These provide temporary moisture but don’t address the underlying problem on their own.
What Happens Without Treatment
Untreated KCS is not just uncomfortable. It’s a progressive condition. The chronically dry cornea becomes increasingly inflamed, and the body responds by growing blood vessels and depositing dark pigment across the eye’s surface. Over time, this pigmentation can cover enough of the cornea to significantly impair vision.
More urgently, a dry cornea is fragile. Without the protective tear film, even minor irritants or a paw rubbed across the face can cause a corneal ulcer. Shallow ulcers are painful but treatable. Deep ulcers can perforate the eye entirely, which is an emergency that may require surgical repair or, in the worst cases, removal of the eye. Early and consistent treatment prevents this cascade.
When Surgery Is Considered
For dogs with severe KCS that doesn’t respond to any topical medication, a surgical option called parotid duct transposition exists. This procedure reroutes a salivary duct from the mouth to the eye, so saliva provides moisture in place of tears. It sounds unusual, but it can be effective for dogs who have run out of other options.
The surgery does come with potential complications. The duct can scar and narrow over time, reducing flow. Some dogs produce excessive saliva to the eye, particularly around mealtimes, requiring the owner to wipe the overflow. Mineral deposits from saliva can also form on the eye’s surface. Because of these tradeoffs, parotid duct transposition is reserved for cases where medications have clearly failed, not used as a first-line approach.
Living With a Dog With KCS
For most dogs, KCS is very manageable once treatment starts. Applying eye drops or ointment twice a day becomes part of the routine, similar to any other daily medication. Many dogs show noticeable improvement within weeks: less discharge, brighter-looking eyes, and less squinting or face rubbing.
Keeping the eyes clean between treatments helps. Gently wiping away discharge with a warm, damp cloth prevents buildup that can harbor bacteria. Staying consistent with the medication schedule matters more than almost anything else. Dogs that receive steady treatment typically maintain comfortable, functional eyes for years. Those whose owners stop treatment during a “good phase” often relapse, sometimes with complications that are harder to reverse the second time around.