Red Eyes in Dogs and Cats
Veterinary & Aquatic Services Department, Drs. Foster & Smith

Red eyes in dogs and cats can occur for many reasons and involve multiple structures in and around the eye. Red eyes may or may not be accompanied by an eye discharge or excessive tearing (watery eyes). To better understand conditions that may cause an eye to be red, it is helpful to understand some eye anatomy. The following figures and definitions may help. 

​Anterior chamber: Area behind the cornea that is filled with a liquid called aqueous humor
Ciliary body: Structure which produces aqueous humor, the fluid in the front portion of the eye
Conjunctiva: Lining of the eyelids and the thin membrane that covers part of the sclera, which is the "white of the eye"
Cornea: Clear covering of the front of the eye
Iris: Colored portion of the eye
Lens: Structure in the eye that focuses light on the retina

Pupil: The opening in the iris that appears as a dark circle

Third eyelid: The eyelid that is attached to the inner corner of the eye and extends out to the side of the eye when the animal blinks; also called the "nictitating membrane"

Causes of red eyes in dogs and cats
The more common causes of red eyes in dogs and cats are shown in the Table 1 below. (This chart is not meant to be an inclusive list. The most common causes are highlighted in grey).

Table 1. Causes of Red Eyes in Dogs and Cats