Conjunctivitis in Dogs and Cats

Laurie Anne Walden, DVM

Photo by Antonio Lapa on Unsplash

The conjunctiva is a thin membrane that covers the insides of the eyelids and the white part of the eye. Inflammation of the conjunctiva, or conjunctivitis, makes the eye look red. Conjunctivitis is sometimes called pinkeye. This name is a bit misleading, though, because lots of other eye disorders—some very serious—also cause redness of the eye.
 
The conjunctiva is a mucous membrane, like the membranes lining the nose and mouth. It protects the eye, helps lubricate the surface of the eye with tears, aids in eye movement, and is part of the system that heals damage to the cornea.
 
Causes
Conjunctivitis has many possible causes in dogs and cats:

• Viral infections: very common in cats
• Bacterial infections: common in cats, usually secondary to another cause in dogs
• Other infections (parasites, fungi, etc)
• Allergies: very common in dogs, less common in cats
• Tear film deficiency (dry eye)
• Blocked or inflamed tear duct
• Immune-mediated eye disorders
• Environmental irritants
• Foreign object in the eye
• Tumor in or near the eye

​Signs
Signs include eye redness, squinting, blinking more than usual, and eye discharge. The discharge can be watery and clear or it can be cloudy, yellow, or greenish. The tissues around the white of the eye and the insides of the lids might look puffy. The animal might rub or paw at the eye because of itching or discomfort.
 
Diagnosis
Conjunctivitis can’t be diagnosed just by the appearance of the eye. Redness and discharge are part of the eye’s response to any abnormality, so other problems need to be ruled out before conjunctivitis can be diagnosed. Diagnosis includes a general physical examination, an eye examination with an ophthalmoscope, and a corneal stain to identify ulcers or other damage to the cornea. Other tests include measuring tear production, measuring eye pressure (to rule out glaucoma), and sometimes swabbing or scraping the conjunctiva to obtain samples for laboratory analysis.
 
Treatment
Conjunctivitis is treated with topical medication (eye drops or eye ointment); oral medication is sometimes also used. The type of medication depends on the cause of the conjunctivitis. Antibiotics are usually part of the treatment, either to eliminate a primary bacterial infection or to prevent a secondary bacterial infection. Cats with known or suspected herpesvirus conjunctivitis sometimes need antiviral medication.
 
Eye medications used to treat conjunctivitis often contain a steroid (such as dexamethasone or hydrocortisone) to reduce inflammation. However, steroids interfere with corneal healing, so before a steroid-containing medication is used, the cornea must be stained and examined with an ophthalmoscope to be sure there is no corneal ulcer or scratch.
 
If your pet has eye medication from an earlier episode of conjunctivitis, don’t use it to treat a new episode of eye redness unless your veterinarian has instructed you to. And never use any over-the-counter (nonprescription) eye product in an animal without first consulting your veterinarian.
 
Photo by Antonio Lapa on Unsplash

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