Canine conjunctivitis is a common eye ailment. It may occur alone or secondary to another eye disease. Finding and treating the underlying eye problem can prevent or diminish future episodes of conjunctivitis. In some instances no cause is ever defined for the conjunctivitis, but there are a variety of diseases that can produce conjunctivitis. Keratoconjunctivitis sicca (KCS or dry eye). With KCS there is inadequate production of the watery component of the tears. As a result, the surface of the eye becomes dry, irritated, inflamed, and infected. Signs of dry eye include a thick, ropey mucus-type discharge, corneal scarring, and sometimes squinting. The conjunctiva is usually red and inflamed.
Upper respiratory diseases, as with kennel cough. These infections involve both bacteria and viruses. Signs of conjunctivitis are often present in both eyes, and other signs such as coughing, sneezing, nasal discharge, lethargy, fever and decreased appetite may be noted.
Mechanical irritation. Usually this chronic irritation is due to problems in the development of eyelid and eye lashes. Eyelids may be rolled inward, which causes the eyelashes to rub continuously against the cornea. Loose and drooping eyelids may not be able to close completely and may lead to dry eye. Some dogs may have eyelashes that grow in the wrong direction and rub against the cornea. Some dogs may even have eyelashes that grow from areas other than the eyelids, which may be directed at the cornea and cause constant irritation.
Foreign matter. Pieces of sand, plastic, metal, or grass can lodge under the eyelids and create a profound irritation of the eye.
Environmental irritants. Cigarette smoke, dust, exhaust fumes, household chemicals, lawn and garden sprays, pollen and other plant material, may cause conjunctivitis. Such irritants may cause a certain type of conjunctivitis, called follicular conjunctivitis, particularly in young, growing dogs.
Infection and inflammation of the eyelids and cornea. Because the conjunctiva is physically adjacent to both the eyelids and the cornea, any infection or inflammation of these tissues may result in conjunctivitis. Examples include corneal ulcers, certain forms of keratitis, blepharitis (inflammation of the eyelids), and skin diseases that affect the eyelids.
Allergies. Allergy-related conjunctivitis is common in the dog and is most often associated with atopy (inhalant allergies). With allergic conjunctivitis, the conjunctiva becomes red, the eyes are often itchy, and watery discharge may be seen.
Parasites. Parasites on the surface of the eye are rare in North American, but occasionally the Cuterebra fly larva may grow near the eye, or small Thelazia worms may occur on the surface of the eye.
Primary bacterial infections. Without associated eye disease, these infections are a rare cause of conjunctivitis. It is much more common for bacteria to take advantage of inflamed conjunctiva, and then to invade this inflamed tissue to create a secondary infection.
Trauma to the conjunctiva, eyelids, cornea or eye itself.
Inflammation from within the eye. Occasionally outward extension of inflammation can reach the conjunctiva, resulting in conjunctivitis. In these instances inflammation within the eye is the primary concern.
Any illness. Conjunctivitis may also develop anytime a dog is ill and not feeling well. Discharge from the eyes is common in diseases causing lethargy and fever.