What Is Conjunctivitis in Dogs?
Conjunctivitis is the inflammation of the conjunctiva — the thin, transparent membrane that lines the inner surface of the eyelids and covers the white part of the eye. When this tissue becomes irritated, infected, or inflamed, it causes redness, discharge, and discomfort that can range from mild to severe. Conjunctivitis is one of the most frequently diagnosed eye conditions in dogs, but it is important to understand that it is a symptom rather than a diagnosis in itself. Identifying the underlying cause is essential for choosing the correct treatment.
Types of Conjunctivitis in Dogs
Infectious Conjunctivitis
Bacterial conjunctivitis is caused by organisms such as Staphylococcus, Streptococcus, and Pseudomonas. It can occur as a primary infection or secondary to another eye problem. Viral conjunctivitis may accompany systemic illnesses such as canine distemper or canine herpesvirus infection in neonatal puppies. Infectious conjunctivitis is more likely to spread between dogs kept in close quarters, such as in kennels or multi-dog households.
Allergic Conjunctivitis
Allergies are a very common cause of conjunctivitis in dogs. Environmental allergens such as pollen, dust mites, and mould spores trigger an immune response in sensitive individuals. Allergic conjunctivitis often accompanies other signs of atopic dermatitis, such as itchy skin, paw licking, and ear inflammation. Both eyes are usually affected simultaneously, and the condition tends to be seasonal or recurring.
Follicular Conjunctivitis
This form involves the development of small lymphoid follicles on the inner surface of the third eyelid. It is thought to be a response to chronic antigenic stimulation — essentially an ongoing immune reaction to persistent irritants or allergens. Young dogs are most commonly affected. The follicles have a rough, cobblestone appearance and can cause irritation and mucoid discharge.
Common Causes of Conjunctivitis
Beyond the categories above, a range of factors can trigger or contribute to conjunctivitis in dogs. These include foreign bodies such as grass seeds or grit lodged under the eyelid, eyelid abnormalities like entropion (inward rolling of the eyelid), blocked tear ducts, exposure to smoke or chemical irritants, and secondary effects of systemic disease. In some breeds, conformational features — such as prominent eyes in Pugs and French Bulldogs, or excessive facial skin folds — predispose dogs to chronic eye irritation.
Signs and Symptoms to Watch For
The most obvious sign of conjunctivitis is redness of the conjunctival tissue, which may give the eye a bloodshot appearance. You may also notice:
- Discharge from the eye, which can be watery, mucoid, or thick and purulent depending on the cause
- Squinting or partially closed eyes, indicating discomfort or photophobia
- Swelling of the eyelids or the tissue around the eye
- Pawing at the eye or rubbing the face along furniture or the ground
- Crusting around the eye margins, especially after sleep
Discharge character can offer clues: watery discharge is more typical of allergic or early viral conjunctivitis, while yellow-green, thick discharge suggests bacterial involvement.
Why Differential Diagnosis Matters
This is one of the most important points any dog owner should understand. Several serious eye conditions can mimic the signs of simple conjunctivitis, and using the wrong treatment — or delaying appropriate care — can result in permanent vision damage.
Glaucoma
Glaucoma involves increased pressure within the eye and is a veterinary emergency. Early signs can resemble conjunctivitis — redness, apparent discomfort, and a slightly cloudy appearance. However, glaucoma causes irreversible damage to the optic nerve if not treated urgently. A vet can measure intraocular pressure using a tonometer to rule this out.
Keratoconjunctivitis Sicca (Dry Eye)
Keratoconjunctivitis sicca, or KCS, occurs when the tear glands fail to produce enough tears. This leads to chronic eye irritation, thick mucoid or mucopurulent discharge, and eventual corneal damage. KCS is frequently mistaken for conjunctivitis or recurring eye infections. Diagnosis requires a Schirmer Tear Test, a simple in-clinic procedure. Treatment differs significantly from conjunctivitis — it involves lifelong tear stimulant medication such as ciclosporin eye drops rather than antibiotics.
Entropion
Entropion is the inward rolling of one or both eyelids, causing eyelashes and fur to rub against the cornea. It causes redness, tearing, squinting, and secondary infection — all signs that look like conjunctivitis. Without corrective surgery, the corneal irritation can progress to ulceration and scarring.
Treatment Options
Treatment depends entirely on the underlying cause, which is why veterinary assessment is non-negotiable before starting any medication.
- Bacterial conjunctivitis is typically treated with topical antibiotic eye drops or ointment, such as chloramphenicol or fusidic acid, prescribed by a vet following examination
- Allergic conjunctivitis may be managed with topical anti-inflammatory drops, oral antihistamines, or addressing the underlying atopic disease with longer-term allergy management
- Follicular conjunctivitis may require mechanical debridement of the follicles under sedation or topical steroid treatment
- Foreign bodies are removed under local anaesthetic, and any secondary infection treated accordingly
- Dry eye requires lifelong tear stimulation therapy and artificial tear supplementation
- Entropion is corrected surgically once the dog is of appropriate age and size
Never Use Leftover Human Eye Drops
It can be tempting to reach for human eye drops — particularly antibiotic or antihistamine preparations — when your dog's eyes look sore. This is strongly inadvisable without specific veterinary guidance. Some human formulations contain preservatives or active ingredients that are toxic to dogs. Steroid-containing eye drops, which are common in human medicine, can catastrophically worsen corneal ulcers if used inappropriately. Using the wrong product can cause permanent vision loss. Always consult your vet before applying anything to your dog's eyes.
When to Seek Urgent Veterinary Attention
Some eye signs require same-day or emergency veterinary care. Contact your vet immediately if your dog has a suddenly painful or closed eye, a visible cloudiness or colour change in the eye, a visible foreign body, trauma to the eye, or signs of severe pain such as crying or extreme restlessness. These signs may indicate glaucoma, corneal ulceration, or penetrating injury — all of which are emergencies that cannot wait.
Summary
Conjunctivitis in dogs is common and treatable, but it should never be assumed to be a minor issue without proper diagnosis. The wide range of underlying causes — from simple allergies to serious structural problems or systemic disease — means a thorough veterinary examination is always the right first step. With the correct diagnosis and treatment, most dogs recover fully and quickly.