Why Does My Dog Have Dry Eye? (KCS Causes Explained)

Your dog has dry eye because the glands that make the watery part of the tear film have stopped producing enough of it. In most dogs the underlying reason is immune-mediated: the dog's own immune system slowly damages the lacrimal gland and the gland behind the third eyelid, and tear output falls over months rather than overnight. Other established causes include certain medications (sulfa-containing antibiotics are the classic example), damage to the nerve supply that tells the gland to produce tears, surgical removal of the third eyelid gland years earlier, glands that never developed properly in some small breeds, and injury from infections such as canine distemper.

The clinical name is keratoconjunctivitis sicca — KCS — and it is one of the most common ocular surface diseases veterinarians see in dogs. If your dog has thick, sticky discharge that keeps coming back, book a veterinary exam rather than waiting it out. Dry eye is diagnosed with a one-minute test, and the cornea does not improve on its own while you wonder.

The tear film your dog is missing

Yes — dogs absolutely get dry eyes, and the mechanism is the same one that makes human dry eye miserable, just more visible because dogs cannot tell you their eyes sting.

A healthy tear film has three layers working together. The outer oily layer comes from the meibomian glands along the eyelid margins and stops tears evaporating. The thick middle watery layer comes from the lacrimal gland above the eye and the gland of the third eyelid, and it carries oxygen, antibodies and nutrients across the cornea. The inner mucin layer, produced by goblet cells in the conjunctiva, grips the eye's surface so the whole film spreads evenly with each blink.

Dog dry eye disease is a failure of that middle watery layer. When aqueous production drops, the oil and mucus keep arriving but have nothing to float on, so the eye produces a ropey yellow-green goo instead of a clean film. That is why owners often describe the problem as too much discharge, not too little moisture — the discharge is the dry eye.

Underneath the mess, the cornea is the part that suffers. Corneal tissue has no blood vessels of its own; it is fed and defended by tears. Strip that away and the surface dries, inflames, and tries to protect itself the only ways it can — by growing blood vessels inwards, laying down brown pigment, and thickening. Dry eye is progressive: the longer the cornea goes without a proper tear film, the more scarring and pigment it accumulates, and that pigment does not clear on its own — which is why a sticky-eyed dog needs a tear test rather than another week of wiping.

Why tear production drops: the common causes

Most dogs arrive at dry eye by one of a handful of routes.

Immune-mediated gland damage. By far the most common cause. Immune cells infiltrate the tear-producing glands and replace functional tissue with scar tissue. It is usually gradual, usually affects both eyes (though rarely at the same rate), and is more common in middle-aged and older dogs. Some dogs with this form also have other immune-mediated conditions, which is one reason your vet may suggest broader bloodwork.

Breed predisposition. Certain breeds are over-represented, including Cocker Spaniels, English Bulldogs, West Highland White Terriers, Shih Tzus, Lhasa Apsos, Pugs, Pekingese, Cavalier King Charles Spaniels, Boston Terriers and Yorkshire Terriers. Flat-faced breeds carry a double burden: prominent eyes with wide lid openings mean more tear film evaporating from more exposed cornea.

Drug-induced. Sulfonamide antibiotics are the best-documented offenders, and some dogs lose tear production while taking them. Certain other drugs have been associated with reduced tear output too. This form is sometimes reversible once the drug is stopped — but that decision belongs to the veterinarian who prescribed it. Never discontinue a prescribed medication on your own.

Neurogenic dry eye. The lacrimal gland is switched on by a parasympathetic nerve supply travelling with the facial nerve. Ear disease, head trauma, tumours or surgery can interrupt it. The giveaway is a dry, crusty nostril on the same side as the affected eye, because the same nerve branch supplies the nasal glands.

Removal of the third eyelid gland. That gland contributes a meaningful share of the watery tears. Dogs whose cherry eye was managed decades ago by excising the gland — rather than repositioning it, which is the standard approach now — are at higher risk later in life.

Congenital alacrima. Some dogs, particularly small and toy breeds, are born with underdeveloped tear glands. These cases show up young, often in one eye, and tend to be severe.

Infection and systemic disease. Canine distemper virus can damage the glands directly. Endocrine conditions such as hypothyroidism and diabetes mellitus have been associated with reduced tear production, as has chronic inflammation of the conjunctiva. Anaesthesia and some sedatives temporarily suppress tear production, which is why eyes are lubricated during surgery.

What dry eye looks like day to day

Owners usually notice the discharge first. Dog dry eye symptoms tend to appear in this order:

• Thick, sticky mucus — yellow, grey or green, gathering in the inner corner and along the lid margins, often returning within hours of being wiped away
• Redness of the conjunctiva, the pink tissue lining the lids and the whites
• Squinting, excessive blinking or holding the eye half-closed, especially in bright light or wind
• Rubbing and pawing at the face, or dragging the eye along furniture and carpet
• A dull, lacklustre cornea that has lost its wet, glassy shine — the surface looks matte or hazy rather than polished
• Recurrent conjunctivitis that clears while drops are being used and returns as soon as they stop
• Brown or black pigment creeping in from the edges of the cornea in longer-standing cases, sometimes with fine blood vessels growing across the surface
• Corneal ulcers, which can appear suddenly and are painful and sight-threatening
• A dry, crusted nostril on the same side, pointing towards the neurogenic form

To know whether your dog has dry eye, look for the combination: discharge that reappears quickly, a surface that has lost its shine, and an eye that looks sore rather than merely weepy. A single goopy morning is not dry eye. Daily goop for weeks, in a predisposed breed, in a middle-aged dog, very often is. Talk to your veterinarian with those observations in hand — a photo taken before you wipe the eye is genuinely useful.

Dry eye, conjunctivitis or allergies?

This is the differential that catches most owners out, because dry eye looks exactly like conjunctivitis — and in fact causes it. The distinguishing features matter.

Feature	Dry eye (KCS)	Infectious or allergic conjunctivitis
Discharge	Thick, ropey, mucoid to mucopurulent; rebuilds fast after cleaning	Watery, clear or lightly mucoid; sometimes purulent with bacterial infection
Corneal surface	Dull, matte, often pigmented or vascularised over time	Usually clear and glossy
Which eyes	Commonly both, often asymmetric	One or both, depending on cause
Course	Chronic and progressive without management	Often resolves or flares seasonally
Response to treatment	Returns promptly when prescribed therapy stops	Often resolves and stays resolved
Contagious	No	Sometimes — viral and bacterial causes can spread

Is dog dry eye contagious? No. KCS itself cannot pass between dogs or from dog to human, because it is a failure of the dog's own glands. The important caveat is that some of the causes and complications are transmissible — canine distemper is contagious, and secondary bacterial infection on a dry ocular surface can be. If you have a multi-dog household and several animals have sore eyes at once, that points away from dry eye and towards something infectious, which is a same-week veterinary visit.

How a veterinarian confirms it

The diagnosis hinges on the Schirmer tear test. A calibrated paper strip is tucked into the lower conjunctival sac of each eye and left for exactly one minute while tears wick along it. The distance the tears travel, measured in millimetres, is read against established reference ranges — lower readings indicate more severe aqueous deficiency, and your vet interprets the number alongside the clinical picture. The test is quick, needs no sedation and is uncomfortable for a few seconds at most.

It rarely stands alone. Expect a fluorescein stain to check for corneal ulceration, which changes the management plan immediately and rules certain medications out. Your vet may also measure intraocular pressure, examine the lids and third eyelid for anatomical problems, and check the nostril on the affected side. Where the history suggests it, bloodwork for thyroid function or other systemic disease may follow, and a drug review will look at anything your dog has been prescribed recently.

Treatment sits entirely with your veterinarian. Prescription tear-stimulating medications such as topical ciclosporin and tacrolimus are the mainstay for immune-mediated cases, often lifelong, supported by lubricants and, where needed, antibiotics for secondary infection. Follow-up tear tests track whether production is responding. Surgical options exist for dogs that do not respond. None of this is guesswork territory — it is a prescription pathway, and the eye is unforgiving of delay.

Key Takeaways

• Dry eye means the glands producing the watery layer of the tear film have failed; the clinical name is keratoconjunctivitis sicca (KCS).
• Immune-mediated gland damage is the most common cause, followed by drug reactions, nerve injury, past third eyelid gland removal, congenital gland underdevelopment and infection.
• The hallmark signs are thick recurring mucus, redness, squinting and a cornea that has lost its glassy shine.
• Several breeds — Cocker Spaniels, Bulldogs, Westies, Shih Tzus, Pugs and others — carry a higher risk, as do middle-aged and senior dogs.
• KCS is not contagious, but some of its causes and complications are.
• Diagnosis is a one-minute Schirmer tear test plus a corneal stain; management is prescription-based and usually lifelong.

Where supplements fit, and where they don't

It is worth being direct: dry eye is an ocular surface disease managed with prescription therapy, and nothing on a supplement shelf replaces a tear-stimulating medication your veterinarian prescribes. Be wary of products marketed for "eye health" that lean on vague wording — if a label will not tell you what is in it and how much, it is telling you something.