Eye infections in dogs are usually caused by bacteria, viruses, or fungi that take hold after something else has already compromised the eye’s natural defenses. A scratch from a thorn, an eyelid that rolls inward, or a condition that reduces tear production can all create the opening an infection needs. Understanding the root cause matters because treating the infection alone, without addressing what triggered it, often means it comes back.
Bacteria, Viruses, and Fungi
One of the most important things to know about bacterial eye infections in dogs is that bacteria almost never cause the initial problem. Unlike in humans or cats, bacterial infection is not a recognized cause of primary conjunctivitis in dogs. Instead, bacteria like Staphylococcus and Streptococcus species move in after something else has already damaged or irritated the eye. A corneal scratch, chronic dryness, or an allergic reaction breaks down the eye’s protective barriers, and bacteria colonize the vulnerable tissue. This is why your vet will look for an underlying cause rather than simply prescribing antibiotics.
Viruses, on the other hand, can directly infect the eye. Canine herpesvirus-1 and canine adenovirus 2 can both infect the tissue lining the eye and cause primary conjunctivitis. Dogs with systemic viral illnesses like canine distemper or canine influenza may also develop eye inflammation, though these dogs are visibly sick in other ways as well, with respiratory symptoms, fever, or lethargy.
Fungal eye infections are less common but do occur, particularly in regions where certain soil fungi are prevalent. These infections tend to be more serious and harder to treat than bacterial ones.
Injuries and Foreign Objects
Physical trauma is the single most common trigger for corneal ulcers in dogs. Scratches from branches or thorns during walks, rough play with other animals, and accidental pokes are all frequent culprits. When the outermost layer of the cornea is damaged, bacteria that would normally sit harmlessly on the eye’s surface can contact the unprotected tissue beneath. From there, infection can deepen the ulcer, and in severe cases, bacteria can “melt” through the cornea, risking rupture of the eye itself.
Foreign objects work the same way. A grain of sand, a grass seed, or a bit of plant material trapped under the eyelid creates constant irritation. The longer it stays, the more damage it does, and the greater the chance of secondary infection. If your dog is suddenly pawing at one eye, holding it shut, or tearing excessively, a trapped foreign body is one of the first things to suspect.
Eyelid and Eyelash Abnormalities
Some dogs are born with structural problems in their eyelids that create lifelong irritation and repeated infections. Entropion, where part or all of the eyelid rolls inward, is one of the most common. The inverted lid pushes the eyelashes directly against the surface of the eye. Over time, this constant rubbing causes irritation, corneal scarring, pigmentation changes, and ulcers that are prone to infection.
Eyelash problems are a separate but related issue. Distichiasis is a condition where extra eyelashes grow from the oil gland openings along the eyelid margin. Many dogs with distichiasis have no symptoms at all, but when the extra lashes are stiff or numerous, they can scratch the cornea with every blink. A more painful variant, called ectopic cilia, involves lashes that grow through the inner surface of the eyelid and poke directly into the cornea. These can cause significant pain and rapid ulceration.
All of these structural problems share a common thread: they create ongoing mechanical damage to the eye’s surface, giving bacteria a consistent entry point. Surgical correction is often the only way to break the cycle.
Dry Eye and Reduced Tear Production
Tears do more than keep the eye moist. They contain antibodies and enzymes that fight off bacteria, flush debris away, and deliver oxygen and nutrients to the cornea. When tear production drops, a condition called keratoconjunctivitis sicca (commonly known as dry eye), the eye loses all of those protective functions at once.
Dry eye in dogs is typically caused by inflammation that damages the tear glands, reducing the watery component of the tear film. Without adequate tears, the cornea dries out, becomes irritated, and develops a thick, sticky discharge. Left untreated, dry eye leads to scarring, abnormal blood vessel growth across the cornea, and recurring secondary infections that can eventually cause permanent vision loss. Certain breeds, including Cocker Spaniels, Bulldogs, and West Highland White Terriers, are especially prone to this condition.
Why Flat-Faced Breeds Are at Higher Risk
Brachycephalic breeds like Pugs, French Bulldogs, Shih Tzus, and Boston Terriers face a perfect storm of eye vulnerabilities that veterinary ophthalmologists group under the term brachycephalic ocular syndrome. The core issue is anatomy: their eye sockets are shallow, which pushes the eyeballs forward and makes them far more exposed than in longer-muzzled breeds.
This creates several overlapping problems. The eyelid openings are often too large to cover the eye fully, a condition called lagophthalmos. Many of these dogs cannot completely close their eyes even when they blink or sleep. The exposed cornea dries out in the areas the lids can’t reach, creating a vulnerable zone for injury and infection. On top of that, many flat-faced dogs have reduced corneal sensitivity, meaning they blink less often and may not react to pain even when a serious injury is present.
The nasal skin folds common in these breeds can also push hair directly against the cornea, adding constant friction. The combined result is a high rate of corneal ulcers that can progress rapidly. In brachycephalic dogs, ulcers are more likely to become infected, deepen quickly, or melt through the cornea compared to other breeds. Chronic irritation also leads to pigmentary keratitis, a dark film that grows across the cornea and can gradually block vision.
Environmental Irritants and Allergens
Allergies don’t directly cause infections, but they create conditions that make infections more likely. When a dog’s immune system reacts to pollen, dust, mold, or certain foods, the eyes become inflamed, itchy, and watery. A dog that rubs its face on carpet or paws at its eyes to relieve the itch can easily scratch the cornea or introduce bacteria from its paws.
Chemical irritants work differently. Cigarette smoke, air fresheners, perfumes, and household cleaning sprays can all irritate the delicate tissue of the eye directly, causing redness and excess tearing. Prolonged or repeated exposure weakens the eye’s surface defenses. Keeping dogs away from freshly sprayed cleaning products and well-ventilated during any exposure to smoke helps reduce this risk.
How to Tell an Infection Is Developing
A small amount of clear, watery discharge from a dog’s eyes is normal. The shift to watch for is a change in color or consistency. Clear, runny discharge that turns yellowish-green and thick, with a mucus-like texture, is a strong signal that bacteria have moved in. Redness in the white of the eye, swelling of the tissue around the eye, squinting, and increased pawing at the face are all common signs.
Some infections develop gradually from chronic conditions like dry eye or entropion, while others appear suddenly after an injury. Rapid onset, especially if your dog is holding the eye completely shut, suggests a corneal ulcer or foreign body that needs prompt attention.
How Vets Identify the Cause
Because so many different problems can lead to the same symptoms, vets use a series of quick, painless tests to narrow down the root cause. A Schirmer tear test measures tear production by placing a small paper strip inside the lower eyelid for about a minute. Low readings point toward dry eye as the underlying issue.
Fluorescein staining involves placing a drop of orange dye on the eye’s surface. The dye highlights any damage to the cornea, making ulcers and scratches visible under blue light that would otherwise be invisible to the naked eye. If the dye appears in the dog’s nostril afterward, it confirms the tear drainage system is working properly.
These baseline tests, along with a pressure reading to check for glaucoma, give the vet a clear picture of what’s happening beneath the surface discharge. The goal is always to identify and treat the underlying cause, not just the infection sitting on top of it. An infection that keeps coming back after treatment is almost always a sign that the trigger, whether structural, environmental, or immune-related, hasn’t been addressed.