Why do cats have a third eyelid and what does it mean if it’s showing?

Many pet parents don’t know their cats have a third eyelid. That’s because when it’s visible, it’s often a sign something’s wrong.


Why do cats have a third eyelid and what does it mean if it’s showing?

If you’re like most cat parents, you’ve seen your pet wake from a deep slumber and noticed a translucent layer over his eye. Though this milky membrane disappears after a few quick blinks, you are not imagining it: Cats, and many other mammals, have a third eyelid.

Officially called a nictitating membrane, our pets’ third eyelids serve as an extra layer of protection when they run through tall grass or during a fight with another animal. This membrane also helps moisten to eye and swipe away particles such as pollen that can cause irritation. Many other mammals, including dogs, have this third eyelid, and scientists theorize that humans once had it, too. You may be wondering why you don’t see it more often, but this is a good thing: Other than brief moments after sleep, spotting Kitty’s extra eyelid can be a sign something is wrong. The membrane may become visible due to an infection, such as pink eye, or trauma to the eye, such as a scratch on the surface of the cornea. Dental disease or abscesses may affect the nerve connected to your cat’s eye and can also cause a visible third eyelid, too. Oral infections can be in the back of your pet’s mouth, where they are not visible, so it is important to seek a veterinarian’s help if you’re noticing your pet’s third eyelid. Other causes behind a visible third eyelid include Haw’s Syndrome, a protrusion of both eyelids without an identifiable cause, and called Horner’s Syndrome, caused when the nerve connected to the eye is impacted. If you can see part of your pet’s third eyelid, bring him to the veterinarian to diagnose the problem. If only one membrane is visible, the problem is likely related to that eye, ruling out some illnesses and systemic issues. If both are showing, however, it could be a sign your cat is sick. Do not delay visiting the vet any longer than needed, as cat’s with a visible third eyelid are often in discomfort or pain. Your veterinarian can check for the above ailments through a comprehensive exam to diagnose Kitty’s problem and prescribe a cure.

Cats’ third eyelids are a functional part of their anatomy that help clean and protect their eyes. If you notice just a small sliver of this membrane on close examination, do not fret. However, if your cat’s third eyelid suddenly becomes visible, it is important to take Kitty to the vet to diagnose the problem and treat the source of his pain.