In a recent post, we delved into the most common reasons for cats to have conjunctivitis, or a red, irritated lining of the eyeball and eyelids. It’s a common saying in veterinary medicine that cats are not small dogs, and this holds true for ocular disease s. While contagious diseases are the most common cause of feline conjunctivitis, mechanical irritation or primary ophthalmic disease are more likely to cause conjunctivitis in dogs. Once conjunctivitis occurs in dogs, it can lead to a secondary bacterial infection. This means that topical antibiotics may be needed, but that the underlying cause of the conjunctivitis must also be addressed.
Mechanical irritation includes some of the disease processes discussed in the first eye post, including entropion and ectropion (eyelids which are folded too far in or too far out, respectively). Other anatomic abnormalities include trichiasis, in which eyelashes or hair grow towards the eye, and ectopic cilia, in which an eyelash grows on the inside of the eyelid instead of the outside. Breeds with prominent facial folds can be more prone to trichiasis, since hair from their faces can rub on their eyes. For most pet owners, knowing the specific terminology associated with eyelash abnormalities is not important. What is relevant is the importance of a careful professional assessment of the eye if conjunctivitis is present. These abnormalities may require good lighting, magnification, and experience to identify and correct. Until the offending tissue is removed or corrected, the conjunctivitis will not improve. Sometimes, the mechanical irritation could be foreign material in the eye, and once it is manually removed, the condition resolves.
Some dogs also suffer from primary allergic conjunctivitis. For humans, itchy, runny eyes are one of the most common manifestations of seasonal allergies. Allergic conjunctivitis is less common in dogs, but can occur. When it does, it usually happens in conjunction with other signs of allergies, especially irritated or infected skin. Some dogs will respond to symptomatic therapy with eyewashes or topical steroids. Others may require systemic treatment for their allergies, usually including oral medications or injections.
Another common cause of conjunctivitis in dogs is keratoconjunctivitis sicca (KCS), more commonly known as dry eye. This condition is caused by insufficient tear production, so the eye is not sufficiently lubricated and protected by a coating of tears. It can occur in cats, but is less common. Affected animals usually have red, irritated eyes, with thick yellow or greenish discharge. The chronic exposure of the eye can lead to corneal ulceration (scratches to the clear front part of the eye), which then results in pain and squinting. Most animals with chronic dry eye have visible changes to the eye, like pigmentation of the cornea or blood vessels growing over the front of the eye. These can interfere with vision, and may eventually cause blindness. The cornea also contains many nerves, and untreated dry eye is undoubtedly painful for the pet. If your veterinarian is concerned that your pet may have dry eye, he or she will likely perform a Schirmer tear test. This involves inserting a small strip of wicking paper into the inside of the eyelid. The test measures tear production, and a low value is consistent with dry eye.
Why do certain animals become affected by dry eye? The most common cause of KCS is immune-mediated. The body misidentifies cells in the tear gland as foreign, and begins to attack them. When enough of the tear gland is damaged, tear production decreases. KCS can also be a side effect of certain drugs, damage to the nerves supplying the tear glands, or certain infectious or endocrine diseases.
Although dry eye can be painful and even debilitating if not addressed, it often responds well to treatment. Medications have two goals — lubricate and protect the eye, as well as increase tear production. Usually, artificial tears are applied to the eyes four times a day. There are two other common medications used to treat dry eye: tacrolimus and cyclosporine. Both are available as eye drops, and have immune modulating effects which encourage greater tear production. While most dogs respond well to treatment, it’s important to recognize that dry eye cannot usually be cured, and that medication will need to be given lifelong. Pets with KCS will require occasional recheck appointments to monitor their tear production.
In cases that do not respond to medical management, surgical treatment may be considered. There is a surgery called a parotid duct transposition, in which the exit of the salivary duct is moved to empty onto the surface of the eye. Essentially, the eye can then be lubricated by saliva instead of tears. This procedure is typically performed by a specialist, and is reserved for severe cases of KCS.
Conjunctivitis can also be secondary to diseases inside the eye, like glaucoma (increased pressure inside the globe) or uveitis (inflammation of the inside of the eye). In these cases, the irritation of the outside of the eye is just the visible sign of a larger problem. Further testing, like checking the pressure of the eye and examining the retina, will be performed during the eye exam to rule out these types of issues.
Regardless of the type of conjunctivitis, dogs with irritated eyes often have an increase in ocular discharge. Goopy eyes don’t just look unappealing, they can cause irritation of the skin under the eyes. To avoid facial dermatitis, the area can be gently cleaned with a warm, moist washcloth. Depending on the length of the fur, clipping the hair directly under the eyes may be helpful as well.
Whatever the cause of your pet’s runny eyes, checking in with your veterinarian is an important step! With appropriate treatment, he or she can soon be bright-eyed and bushy tailed once again.
Lewin, A., Bentley, E., Miller, P., & Herring, I. (2016, September 2). Keratoconjunctivitis Sicca. Retrieved February 4, 2019, from https://www.vin.com/Members/Associate/Associate.plx?DiseaseId=506.