The Third Eyelid

Third Eyelids can be visible with stress, infection, or foreign material. A good exam is important & essential.



I must say that I am very worried about my Maine coon kitten. This morning, when we woke up, we found that one of her eyes had an off-white coating on most of the eye. It doesn’t seem to bother him (he’s not scratching at it or grooming it or anything else) but it really worries me. It looks like the stuff that is usually in the inner corner of his eye. My daughter has said that when other cats she has had developed this condition they died. Can you tell me what causes this condition and what can or should be done about it?


Dr. Nichol:

I’m glad you and your daughter have taken this cat’s symptoms seriously. Here is the long and short of this situation: What you describe is called a protracted third eyelid. The third eyelid is actually a normal structure that all cats and dogs have. It also goes by the name nictitans or nictitating membrane. What’s important about it is its protective function. On the back side of the third eyelid is a patch of lymph tissue like a mini lymph node; thus it helps clean up infections and debris. In addition the third eyelid can easily slide up over the entire cornea (the clear front part of the eye). Most people never notice that their pets even own such equipment because the third eyelid normally sits tucked neatly in the inside corner of the eye. In this position it can move up to shield the eye from injury or help heal it if it gets injured. Pretty nifty. Too bad we don’t have them. If you look closely at the inside corner of your eyes you will see a small pink tissue that is an evolutionary vestige of a third eyelid. It’s all we have left of ours. Maybe we humans have just gotten too good for such things. Oh how I yearn for simpler times…


Enough nostalgia; back to your question. What does it mean when you actually notice that your cat has a third eyelid? If it’s visible on only one side it suggests an infection or injury to that eye only. On the other hand, if both third eyelids are protracted we need to be concerned about internal disease. We call it a sign of systemic illness because a disease in some other part of the body is responsible for the symptom. Diseases that can cause protracted third eyelids in cats include any physical or emotional stress as well as severe problems like feline leukemia and F.I.P. infections. The entire list of possibilities would be quite long. Suffice it to say that any stress can do it.


So here is my advice: Have your daughter take this kitty to a veterinarian with lots of experience with cats. A thorough physical exam alone could answer the question. If not screening lab work like a blood count, chemistry panel, and urinalysis are in order to look for indicators of internal disease. In addition testing for leukemia virus is essential. The last big concern, F.I.P. virus, can be investigated if physical signs and initial lab work suggest it.


So how sick is this kitty? The truth is that without a good exam and lab work I can only guess. Her problem may be as simple as a minor stress or as severe as a terminal illness. But like any possibly serious disease early diagnosis and treatment is your best hope. Good luck.