Red Eyes and Eye Discharge in Dogs

Why the eye is not an organ you watch and wait on

With diarrhoea or a cough it is often reasonable to observe for a day or two before deciding. The eye is different. The cornea is a layer less than a millimetre thick, and damage underneath it is frequently irreversible. Ophthalmology references warn consistently that a sudden rise in intraocular pressure — acute glaucoma — can compress the optic nerve and lead to blindness within hours to days. Apply the timeline you would use for a digestive upset and the sight you could have saved may already be gone by the time you arrive.

What makes it harder is how well dogs hide eye pain. A person says so immediately; a dog blinks a little more often, avoids bright rooms, or rubs a paw across its face. What looks to an owner like "a bit more discharge than usual" can be a painful scratch on the cornea. For that reason eye problems are safer judged by which signs appear together than by how bad the eye looks.

Any one of the following means the same day, regardless of colour or quantity of discharge.

By contrast, slightly increased clear tearing with the eye opening normally, no redness, no rubbing and no worsening over a day sits closer to the watch-and-see side. That judgement is provisional, though; if it lasts more than two days, look for the cause.

Reading discharge by colour and texture

Discharge is not a diagnosis. It is the result of the eye responding to something. Colour and texture help narrow the field but do not settle it on their own. Treat the table below as a starting point.

The two most often confused are yellow discharge and grey mucus. Both get lumped together as "lots of gunk", but they call for different responses. Thick yellow discharge points towards infection; grey mucus that stretches like a thread suggests the watery part of the tear film is missing, leaving mucus behind in higher proportion. The second will not resolve on antibiotics alone — tear production itself has to be raised — so cycling through drops without testing simply wastes time.

Reddish-brown staining is a different matter again. It is less discharge than a record of tears that have spilled onto the fur, and the approach depends on whether the problem is too many tears or blocked drainage. That subject has its own article: see dog tear stains.

A red eye, split five ways

"His eye is red" is the phrase owners use most, and it is exactly where the paths diverge. Here are the five conditions most often in play. Bear in mind that two of them can be present at once.

The most practical column is the pupil. Uveitis is inflammation inside the eye and constricts the pupil; acute glaucoma leaves it dilated and slow to respond to light. Simply dimming and brightening the room to see whether both pupils move at the same size and the same speed gives you one more piece of information to hand the vet. It is a clue, not a diagnosis.

Sources describe intraocular pressure slightly differently. Cornell University College of Veterinary Medicine puts normal canine pressures at a maximum of roughly 20 to 28 mmHg and says readings above 40 to 50 mmHg require emergency treatment, while the MSD Veterinary Manual describes acute glaucoma as pressures well above 40 to 60 mmHg. The wording differs; the point does not. Pressure cannot be judged at home — it has to be measured with a tonometer. Primary glaucoma is well recognised in the American Cocker Spaniel, Basset Hound, Chow Chow, Shar-Pei, Samoyed and Siberian Husky, and when one eye is affected the second eye is often involved eventually, which is why preventive treatment of the other eye gets discussed.

Corneal ulcers — one dog in a hundred, eleven times the risk if flat-faced

Ulceration of the cornea is far more common than most owners expect. In 2017 the VetCompass team at the Royal Veterinary College analysed the clinical records of more than 100,000 dogs in England and found that almost one dog in every hundred is affected by a corneal ulcer each year. What matters more than the frequency is the outcome. In the same study around 60% of affected dogs needed pain relief, about one in five needed surgery, and the condition contributed to death in over 10%. This is not a scratch to shrug off.

Risk depends heavily on face shape. The same work found that brachycephalic dogs carried more than eleven times the risk of non-brachycephalic breeds, and pugs more than nineteen times the risk of crossbred dogs. Purebreds were over twice as likely as crossbreds, and spaniel breeds over three times as likely as non-spaniels. The reason is structural: an eye that sits forward in a shallow socket exposes more cornea, the lids may not close over it completely, and hair from a nose fold can touch the surface directly. Add an uneven tear film and small irritations turn into wounds.

What you notice at home is typically a dog that suddenly holds one eye shut, tears more, avoids light and paws at its face. The difficulty is that a shallow scratch and a deep one look much the same. In the clinic a drop of fluorescein stain settles into the damaged area and glows green; before that test, the two are hard to tell apart. Melting ulcers, where enzymes or bacteria break the cornea down quickly, can deepen within a day — which is why a flat-faced dog holding an eye shut is best treated as an emergency.

Stopping the rubbing alone reduces the damage. Put a cone on before you leave for the clinic. Reaching for human eye drops in the meantime is the opposite of helpful, for reasons set out in the last section.

When there are not enough tears — dry eye and cherry eye

Keratoconjunctivitis sicca (KCS, dry eye) is a shortage of the watery component of the tear film. Cornell University College of Veterinary Medicine describes the most common cause as immune-mediated inflammation of the tear-producing glands, with chronic allergies, endocrine disease, congenital factors, removal of the third eyelid gland, infections, drug toxicity and radiation treatment also implicated. Breeds noted as more commonly affected include the American Cocker Spaniel, English Bulldog, Lhasa Apso, West Highland White Terrier, Cavalier King Charles Spaniel, Shih Tzu and Pug.

Diagnosis is by Schirmer tear test, a standardised paper strip tucked inside the lower lid for one minute; clinical references put normal canine production at more than 15 mm per minute. Treatment does not stop at wetting the eye with artificial tears — cyclosporine or tacrolimus drops are used to raise tear production itself. The response figures are specific: one clinical review reports improvement in 81.8% of dogs on cyclosporine, with roughly a 50% response rate when the starting Schirmer value is below 2 mm/min against about 80% when it is 2 mm/min or above. In other words, the more gland function is left, the better it works — so finding it early is the prognosis.

Two things commonly catch owners out. First, judging the effect takes a long time. References advise a recheck at one month and hold that treatment failure can only be declared after twelve weeks of consistent application. Stopping after a fortnight because "it isn't working" leaves nothing to judge by. Second, this is usually lifelong management; stopping once the eye looks better often brings it straight back.

Related to tears is the prolapsed third eyelid gland, known as cherry eye. The red lump appearing at the inner corner is alarming to look at, but what matters is what that gland does. VCA notes that it produces up to 50% of the watery portion of the tear film. That is why the old approach of cutting it out is no longer recommended and the gland is instead repositioned and tacked back in place — removal substantially raises the risk of dry eye later. Even repositioning carries a reported re-prolapse rate of about 5 to 20%, and dogs affected on one side often go on to prolapse the other. Cocker Spaniels, bulldogs, Boston Terriers, Beagles, Lhasa Apsos, Shih Tzus and Pugs are among the breeds most often seen.

Systemic infections such as canine distemper are also listed among the causes of conjunctivitis and reduced tear production, so keeping vaccinations on schedule is not unrelated to eye health. If you are unsure when the next one is due, check with the dog vaccination schedule calculator.

Cloudy eyes, and what never to do at home

When an older dog's eyes start to look cloudy, most owners think cataract. In fact much of what is seen in senior dogs is nuclear sclerosis, a hardening of the lens nucleus. The American College of Veterinary Ophthalmologists notes that it is usually seen after eight to ten years of age and is not thought to cause significant visual impairment. Lens fibres compress inward over a lifetime, the centre becomes dense, and it takes on a bluish cast. No treatment is required.

A cataract is another matter. The lens itself turns opaque, vision goes as it progresses, and leaking lens protein can cause uveitis or secondary glaucoma. So nuclear sclerosis can be watched, while a cataract needs following. The problem is that they cannot be reliably distinguished from the outside; ACVO advises an eye exam with the pupil dilated. Shining a torch at home will not settle it. If you are not sure whether your dog has crossed into the senior range, run the numbers through the dog age calculator — past seven or eight, it is worth adding an eye check to the annual visit.

Finally, what to do and what never to do before you get to the clinic.

• Reasonable — wiping discharge from around the eye outward with sterile saline on clean gauze; a cone to stop the rubbing; flushing with lukewarm water or saline immediately after something has gone in; writing down when the signs started and how they have changed.
• Never ① — steroid eye drops. With a wound on the cornea, a steroid impairs healing and encourages infection, and the ulcer can deepen. The catch is that the wound is invisible before a fluorescein stain.
• Never ② — leftover drops from last time. There is no guarantee the old diagnosis matches today's cause, and an opened bottle kept for months may be contaminated.
• Never ③ — touching the eye itself. Do not try to push back protruding tissue or pull out an embedded foreign body at home. Damage to the globe does not come back.
• Never ④ — human painkillers. However uncomfortable the dog looks, human anti-inflammatories cause poisoning.

Knowing the order of tests helps. It usually runs Schirmer tear test → fluorescein stain → pressure measurement, and the sequence has a reason: any drop instilled into the eye changes the tear reading, so the Schirmer goes first. Slit-lamp examination, fundus examination, culture or PCR follow as needed. Which is another reason putting drops in on the way to the clinic muddies the results.

Everything here is general information to help you orient yourself; the same sign means different things depending on age, breed and underlying disease. An accurate diagnosis has to come from a veterinarian. That is especially true of eyes. When you are unsure, phoning the clinic beats reading further — and with eyes, those few hours change the outcome.

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