Unlike humans, when dogs and cats have a "red eye", they do not have a bacterial conjunctivitis or "pink eye". They are not susceptible to the bacteria which causes pink eye in children. When a veterinary patient is presented with what is described as conjunctivitis, several conditions must be considered.
Conjunctivitis simply means that the conjunctiva, the moveable white tissue seen when we look at each other¹s eyes, is inflamed. It does not describe the cause. Conjunctivitis is described in people as an itchiness and is described as not painful.
In the dog, causes of a red eye include keratoconjunctivitis sicca, glaucoma, uveitis, episcleritis, foreign bodies, eyelash disease and allergy. In the cat, causes of a red eye include keratoconjunctivitis sicca, uveitis, herpesvirus infection , eosinophilic conjunctivitis, chlamydia infection, mycoplasma infection, foreign bodies and allergy.
Episcleritis is an inflammation of the outermost layer of the external wall of the eye (sclera). Episcleritis is divided into three classifications depending on the clinical appearance: erosive, proliferative or diffuse. Erosive episcleritis is the most severe in that the wall of the eye is "eaten" by inflammation and the wall of the eye can rupture. This type of epislceritis requires surgery to strengthen the wall of the eye. Proliferative episcleritis appears as a thickening of the outer wall of the eye or a thickening of the inner eyelid.
The arrow indicates a proliferative lesion of episcleritis.
These lesions can lead to corneal ulcers adjacent to the lesions due to an area of drying occurring when the tear film cannot spread evenly over the eye. Alternatively, the lesion may itself spread into the cornea resulting in episclerokeratitis.
In this dog, the pink, fleshy lesion has spread into the cornea resulting in episclerokeratitis.
Diffuse epislceritis appears as an area of redness which may worsen in degree of redness and enlargement of area. Episcleritis is most commonly seen in the Collie, Rottweiler and American Cocker Spaniel although any breed of dog can develop the condition. Ofttimes, an area of cholesterol may be present in the outermost layers of the cornea following the inflammation of episcleritis. Diagnosis of epislceritis is by biopsy (if proliferative) or cytologic examination. Treatment of epislceritis is either steroids, cyclosporine or a combination of both. Additionally, oral antibiotics and vitamin B6 may be given. If these do not work, then an oral immunesuppressive medication (Imuran®) is used.
Allergic conjunctivitis may be due to pollen, house dust mites, molds, mildew and food allergy as well as odors from new carpets and recently finished floors. There is no way to diagnose an allergy directly. For this reason, allergic conjunctivitis is a diagnosis of exclusion, that is we exclude all other causes and what is left is allergy. The treatment for allergic conjunctivitis is either steroids or nonsteroid antiinflammatory drugs (NSAIDs). If a cause can be determined, the elimination of the cause will often result in the resolution of the allergy.
Cats can develop a specific type of conjunctivitis known as eosinophilic conjunctivitis. The diagnosis is made by performing a conjunctival scraping and examining the cells with a microscope. The predominant type of cells is a white blood cell known as the eosinophil. It is called this because of the way it retains stains. Although the cause for eosinophilic conjunctivitis is unknown, several causes of eosinophilic conjunctivitis have been proposed. These include herpesvirus, allergy and other virus infections. Herpesvirus must be ruled-out as a cause because treatment for eosinophilic conjunctivitis is topically applied steroids or oral steroids or megesterol acetate.
Cats may develop a respiratory condition (called URI) from herpes, mycoplasma or chlamydia. The most common cause of conjunctivitis in the cat is Herpesvirus. This virus is a common cause of upper respiratory disease in cats. It is passed in droplets when the cat sneezes or coughs. Other causes of respiratory disease and conjunctivitis are Chlamydia and Mycoplasma. Like viruses, they live within the cells of the respiratory tract and the conjunctiva. These conditions are most commonly found in the kitten, rather than older cats but can be found in any cat of any age. Whilst these pathogens are not extremely common, they do cause a lot of problems, especially in catteries. These organisms are smaller than bacteria and larger than viruses. Herpes is discussed in another handout. Chlamydia is an infection with a rickettsial agent. Rickettsia agents are not really bacteria, although they can be killed with specific antibiotics and not really viruses although they act like viruses because they need to live inside cells. Chlamydia is in the same family as the agents which cause Rocky Mountain Spotted Fever. Mycoplasma is similar in that it lives on the outside of cells and needs cellular material to live and can be killed by specific antibiotics. Both of these organisms cause URI and conjunctivitis, which can be quite dramatic. Neither of these can cause corneal ulcers. Both can be a simultaneous infection with herpes. We can detect chlamydia and mycoplasma by scraping the white of the eye and examining the stained slide using a microscope and looking for characteristic findings (inclusions). The inclusions are not commonly found in scrapings from patients with a long-standing or chronic condition. Finding the inclusions is usually diagnostic although other laboratory tests can confirm the findings, especially in chronic cases. Treatment is with either antibiotics applied to the eyes or oral Zithromax®. If medication is given and the patient doesn¹t respond as expected, tests for herpes may be warranted.
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Dennis Hacker via E-mail:
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