It is important to differentiate whether the enlargement of the blood vessels that cause an eye to appear red represent superficial or internal inflammation. Superficial inflammation often arises with surface irritation or infection. Internal inflammation is inherently more serious in that it involves deeper structures within the eye. Deep inflammation is more often associated with diseases that threaten vision.Causes Conjunctivitis is one of the more common causes of red eye in dogs. It may occur when the eye becomes exposed to environmental irritants (dust, smoke, cleaning agents, and chemicals), when the eye is dry, or when the lids do not protect the eye properly.
Conjunctivitis can also occur in association with allergies, skin diseases and other ocular diseases, such as eyelid deformities and diseases of the third eyelid.
Blepharitis is inflammation of the eyelids. Blepharitis can be part of a more widespread skin inflammation or may only involve the eyelids. Causes are numerous and include allergies, parasitic, fungal or bacterial infections, and certain immune diseases. Blepharitis usually also causes inflammation of the adjacent conjunctiva.
Inflammatory diseases of the third eyelid may also cause the eye to be red. See the related article on Protrusion of the Third Eyelid in the dog.
Keratitis is inflammation of the cornea. With keratitis, the blood vessels of the conjunctiva and sclera may be enlarged, and blood vessels may migrate from these tissues into the cornea. In general, keratitis is a more serious finding than conjunctivitis. Keratitis may be accompanied by ulceration or opacification of the cornea.
Anterior uveitis refers to inflammation of the inner vascular layer of the eye, including the colored iris and related structures. This is a serious and potentially vision-threatening disease. Uveitis may indicate that your dog has a systemic disorder that involves other organs within the body.
Glaucoma is an increase in intraocular pressure or pressure within the eye. Glaucoma is diagnosed by measurement of intraocular pressure with a special instrument called a tonometer. Glaucoma is sometimes a painful disease and may also be associated with anterior uveitis.
Infections, inflammation and tumors behind or around the eye, in the nose or face may also cause an eye to become red.