The eye is one of the most injury-prone and fragile structures on the body, no matter the species, and understanding what some of the more common ailments – discharge, squinting, swelling, and redness – indicate can help pet and livestock owners act appropriately when an ocular issue arises.
The Eye’s Structure
Understanding the basic structure of the eye can lead to a better understanding of eye ailments. The clear outer layer of the eye is called the cornea and it can protect more vital parts of the eye from injury, but this means it’s prone to injuries such as scratches, known as corneal abrasions. Tears emanate from glands in the eye corners and they serve to keep the eye moist, while also flushing away debris and irritants. The eyelids help to distribute the tears and along with the eyelashes, help to protect the eye.
In some animals, like the dog, a third eyelid is present and in these cases, a condition called “cherry eye" can arise. This occurs when there's a prolapse of the tear gland associated with the dog's third eyelid. Infection is often associated with cherry eye, as discomfort from the inflammation triggers pawing and rubbing of the eye, and this in turn, introduces bacteria which leads to infection.
The next layer below the surface is called the anterior chamber that is filled with a fluid called the aqueous humor. At the rear of the anterior chamber is the colored portion of the eye – the iris – which surrounds the pupil, the black center of the eye that acts as a lens, contracting and dilating to control the amount of light that enters the eye.
Past the anterior chamber is the vitreous humor, a thick liquid through which light passes after it enters via the pupil. The light and image that the eye views lands on the rear wall of the eye, and this is where the retina is located. The retina is covered with rods and cones, which process light and color. The image that lands on the retina is then sent to the brain via the optical nerve, which runs from the rear of the eyeball. Nerves also run directly from one eye to another, so if only one eye detects light, the other will also dilate or contract accordingly.
The Eye’s Appearance
An animal’s eyes should be clear and free of discharge, although it’s not uncommon to see some tearing on occasion when the eye is exposed to dust, wind or other minor irritants.
The eyelashes should project outwards and they should not come in contact with the eye when blinking occurs. The cornea should appear clear and free of abnormalities, which can take the form of small white marks, often similar in appearance to a how a small hair may appear if it becomes caught in the eye. The white of the eye may have a slightly tan appearance, but it should not appear bloodshot.
Blinking should occur simultaneously in both eyes. Squinting or excess blinking in one or both eyes is abnormal.
Signs of a Problem
An animal’s eyes are prone to injury through normal, everyday activities, and even a seemingly minor injury can result in blindness, so understanding how to detect a problem is key.
“Pet owners need to be familiar with how their pet normally looks and acts, whether it’s the eyes or another body part – if you know what ‘normal’ is, it will be easy to detect an abnormality. I recommend examining your pet once a week, and after activities like walks, car rides, hikes and play,” explained Dr. Michael Levine, DVM.
Any sign of an abnormality is cause for concern and a vet visit is recommended, within 12 hours if possible. Signs of a problem include: discharge (often green or yellow in appearance), squinting, tearing, swelling or redness of the eyelids or of the actual eyeball, a bloodshot appearance, protrusion of one or both eyes, and the appearance of debris or any other abnormalities on the cornea.
An immediate vet visit is recommended in the event that irritation occurs after a chemical or other substance, like soap, comes in contact with the eye.
