The Dog Owner's Guide to Canine Conjunctivitis

At face value conjunctivitis may appear a simple condition: After all, what’s so complicated about an eye infection?

But to assume conjunctivitis is just a matter of infection is to do this important problem a disservice. Indeed, there are many reasons for a dog to have a sore, red eye, and it’s important to understand that left untreated the consequences may be dire for the dog’s eyesight.

A Real Life Sticky Case

My favorite example of not taking things at face value was a dog brought to see me with a closed eye. The owner had reached a diagnosis of “conjunctivitis” because the dog’s eye had a runny discharge, but just 30 seconds into the physical examination and I discovered all was not what it seemed. In fact, the dog had a one-inch long twig trapped in the deep pocket between the lower eyelid and his cornea!

Ok, this isn’t entirely fair, but you get the point. Just because an eye is sore, doesn’t mean it’s infected.

What is Conjunctivitis?

Indulge me for a moment, because I love the etymology of words: I think it gives us a deeper understanding about what a word means and “conjunctivitis” is no exception. The term derives from “conjunctiva”- meaning the membranes covering the external eye, and “-itis” – meaning inflammation.

In other words conjunctivitis means a sore, red eye.

To understand conjunctivitis fully means to ask why the eye is inflamed. And this is where the misunderstanding creeps in, because infection is just one reason, and trauma, allergy, disease, and dry eye also need consideration.

On the Look Out for Conjunctivitis

Eyes are precious, and in a worst case scenario if something goes wrong, a pet could lose their eyesight.

Take the example of the dog with a twig in his eye. If the owner had neglected to bring him in, the stick would have rubbed at the surface and caused an ulcer. If that ulcer perforated, the dog would have lost his eye. As it was, it took seconds to remove the stick without any ill effects. But had the owner not thought to bring his dog in…well who knows what would have happened.

Long story short, if your dog has a sore eye get him checked by a veterinary professional. In fact, if you see any of the following, get him seen.

  • Discomfort: This might be rubbing the eye with a paw, or scooting his face along the ground. Blinking repeatedly, and/or holding the eyes closed are sure signs of discomfort.
  • Redness: The white of the eye should be white: From pink through to brick red, this is not normal and should be checked.
  • Swelling: Does the area around the eye or the eyelids look puffy or swollen?
  • Discharge: Is the eye watering more than the other? Whilst a yellow-green discharge indicates infection, other abnormal discharges need checking on – such as a gluey or tacky discharge.

Causes of Conjunctivitis

So if there’s more to conjunctivitis than infection, what are we looking at?

1)    Irritants: My personal bugbear is seeing a dog with his head out of the window in moving vehicle. High speed grit or dust easily damages the surface of the eye, and that’s without the trauma of air moving at 60 mph hitting the cornea. If you don’t want to risk your dog’s eyes, then keep his head inside the car at all times.

2)    Trauma:  This doesn’t have to be dramatic, and includes a branch hitting the dog in the face or a cat scratch to the eye.

3)    Anatomy: Some breeds are born with extra folds of skin (think Bassett and Bloodhound) which allows hairy skin to roll forward and down into the eye. Others havein-turned eyelids (a condition called entropian) which mean the eyelashes scratch the eye’s surface each time he blinks.

4)    Dry Eye:  Tears are vital to the health of the eye. A lack of tear production means a dry, itchy, and poorly moisturized eye

5)    Allergy:   Just as we can get streaming eyes with hay fever, some dogs also have allergies to air borne agents, which results in reddened eyes.

6)    Infection: OK, fair enough, infection does have a part to play, bacterial and viral infections being the major players.

Reaching a Diagnosis

The veterinarian has a number of tools at their disposal, not least of which is looking carefully observation. The first step is to look at the dog from a distance, and check if the dog’s eyes are the same size and shape, and if there are anything unusual of the eyelids.

The vet then completes a physical examination of the eye and eyelids, using an ophthalmoscope. This instrument has a light source, lens, and magnification so at to see all parts of the eye in great detail, including the leading edge of the eyelids.

If necessary, the vet measures tear production. This is done using a special piece of blotting paper to soak up the tears, called a Schirmer tear test.

Another basic test is to put drops of a dye called fluorescein into the eye.Fluorescein stains damage on the surface of the eye bright green, and helps to show the size and depth of any ulcers.

Veterinary ophthalmologists have ever more sophisticated equipment and techniques, and can assess the quality (not just the quantity) of the tear fluid.

Are all Dogs Equal?

When it comes to conjunctivitis, not all dogs are created equal.

I’ve already mentioned breeds with droopy faces such as Bloodhounds. Indeed, other breeds have poor eyelid conformation of which the St Bernard is notorious for having “diamond eye”, where the eyelids are too generously sized and sag as a result. This exposes the delicate lining of the eyelid and makes it more prone to irritation, trauma, or infection. [$]

Breeds such as the Shar Pei and the Cocker spaniel often have in-turned eyelids, which rub on the surface of the eye and cause distress. Imagine having grit in your eye 24/7 for the whole of your life and you begin to appreciate the problem. [$]

Other risk factor include have long hair, so the Tibetan terrier with a shaggy mop runs a risk of hair in the eye. Also active dogs that regularly run through undergrowth or in woods are at greater risk of trauma related conjunctivitis.

And then there are the breeds how are prone to allergies, such as West Highland white terriers, Labradors, and spaniels, who not only have itchy skin but sore eyes.

Individual Conditions in More Depth

Some causes of conjunctivitis are relatively self-explanatory, such as trauma or grit in the eye. But let’s take a more detailed look at certain medical conditions.

Dry Eye

If you’ve been on a long haul flight, you probably experienced hot, itchy eyes. This is because the air conditioning dries out the surface of the eye, robbing it of the soothing lubrication of tear fluid.

Some dogs don’t produce enough tear fluid, which leaves their eyes permanently sore. This deficiency is because the body’s immune system attacks the cells which produce tears. Genetics also play a part with dry eye being common in breeds like the West Highland white terrier and the German shepherd. Ultraviolet can aggravate the condition – an argument for doggy sunglasses? And generally dry eye is a condition of “control rather than cure”.

Left untreated, over time the dry eye protects itself by laying down brown pigment on the surface of the cornea which eventually impairs the dog’s vision. (A bit like wearing spectacles with the lenses painted brown) [&]


If you own a dog with allergy issues you’ll be all too familiar with the symptoms of scratching and excessive licking of the feet, legs, and tummy. However, allergies also show up as red, itchy, and watery eyes, which the dog scoots along the ground or rubs with a paw.

In the long term these dogs need their allergy issues addressing, but in the short term anti-inflammatory drops can settle the irritation.


In the same way skin becomes infected, so can the eye. The bugs that cause the problems are often those that are natural inhabitants of the canine body, including Staphs, Streps, and Proteus. Sometimes the bugs can be a bit more unusual and difficult to get rid of such a Pseudomonas, Mycoplasma, or E.coli.

If the infection isn’t responding to a regular antibiotic ointment then the vet will swab the eye to collect a sample of bacteria to send away for culture. This helps the clinician to choose an antibiotic effective against that particular bug.

Treating Conjunctivitis

Treatment is most commonly with topical eye ointments, which deliver medication right where it’s needed. For treatment to be successful it is important to know if this is a simple infection or other factors are involved, and then address the complete picture.

There are many different types of eye ointment or gel available, which are designed to treat different conditions. They commonly contain ingredients such as:

  • Antibiotics: To treat bacterial infections
  • Anti-inflammatories: If the eye is inflamed (because of trauma or allergy) these drugs reduce the soreness and the urge to rub. These anti-inflammatories are either from the corticosteroid family, or the NSAID (non-steroidal anti-inflammatory) group
  • Artificial Tears:  A simple solution for dry eye is to mimic natural tears by regularly dropping in “false” tears. The drawback is these need putting in frequently, although newer formulations are coming through which require once or twice daily application.
  • Immune Modulators: The gold standard treatment for dry eye is a drug developed on the back of organ transplant medicine, cyclosporine. Cyclosporine prevents organ rejection, and in this case switches off the body’s immune attack on the tear-producing glands. Cyclosporine is applied twice a day initially, and the results can be life-changing for the dog – however, it comes at a price because the medication is expensive.

Complications of Conjunctivitis

One of the most worrying complications of conjunctivitis is the development of a corneal ulcer. The cornea (the front of the eye) is clear so that light can pass through, but this means a compromise in terms of toughness. The way the cornea is “built” leaves it vulnerable to damage and ulcer formation.

It’s useful to think of an ulcer in a similar way to a blister on your heel. Your new shoe rubs your heal and a blister forms. You persist in wearing the shoe and the blister pops, leaving an erosion or “hole” in your skin. This damage to the protective skin of your heel is similar to an eye ulcer.

Corneal ulcers cause two potential problems. The first is that if they heal poorly they can leave scar tissue which then impairs vision (equivalent to a pair of badly scratched spectacles). The second is that if the ulcer is deep, there is a risk of the eye rupturing – with unthinkable consequences.

This is one of the reasons it’s so important for your dog to see a veterinarian if he has a sore eye: Not only are you saving him discomfort but you are protecting his eyesight. The likelihood is that if the problem is treated early, ulcer formation can be prevented and you can save yourself much heartache and preserve your pet’s vision.


  • Veterinary Ophthalmology. Gelatt. Publisher: Williams & Wilkins
  • Canine entropian. Miller & Albert. Comp Cont Ed Pract Vet 10, 431-438
  • Topical administration of cyclosporine for treatment of keratoconjunctivitis sicca in dog. Morgan & Abrams. JAVMA 199
  • Ophthalmology for the veterinary practioner. Stades. Publisher: Schleutersche