Canines have Third Eyelids?

Do all dogs have one? 

Yes, all dogs have them, most other animals as well, except humans and pigs, who do not.  The other name for the third eyelid is the nictitating membrane.

Are they different for different breeds?

– in general they are the same in different breeds, sizes and pigmentation may vary from breed to breed

Why is the third eyelid important? What's its purpose?

  • functions as a “windshield-washer” for the cornea (the clear window in front of the eye), underneath the conjunctival covering of the third eyelid, there is a very thin T-shaped piece of cartilage, which acts like a device to wipe water off of windows, and it clears debris and mucus off of the cornea
  • the gland of the third eyelid contributes a significant portion of the tear film that bathes the eye, in addition to that which produced by the lacrimal gland *
  • the third eyelid also contains tissue which secretes immunologic agents to help fight infection *
  • functions to protect the cornea from injury

Are dogs born with one or do they develop over time?

– yes, they are born with them

Have dogs always had one?

– yes

When should a dog owner be concerned about the eyelid (when it protrudes, infected etc)?

A dog owner should be concerned if the gland suddenly “pops” forward, this condition is called “cherry eye” because it looks as if there is a small red cherry on the inside corner of the eye.  The correct term is “prolapsed gland of the third eyelid”. When this happens, the supporting cartilage of the membrane folds over and the third eyelid loses its windshield wiper function.
The gland can become very inflamed in this exposed state. If left untreated, it can sometimes lose some of its tear function, resulting in keratoconjunctivitis sicca, or “dry-eye”, a chronic, painful and debilitating condition.  In other cases, the gland retains its ability to make tears but it just looks red and unsightly.
Replacement, rather than excision, of a prolapsed nictitans gland is currently the preferred treatment of choice. Removal of the gland or, in some cases, the entire third eyelid is sometimes indicated for cancer or other specific conditions.

Other problems of the third eyelid include allergic conjunctivitis, autoimmune diseases, scrolling of the edge of the third eyelid.
Sometimes, when a dog is not feeling well, is dehydrated or otherwise ill, both third eyelids might be elevated.  The third eyelid can be elevated any time the eye is painful, from a corneal ulcer or glaucoma, or dry eye.  Often people see this and think that the eye is “rolling back into the head”. The eye is probably not rolling back, but the third eyelid is elevated and covering the eye.

If so, when should he call a vet?

When an owner notes any sudden change in the appearance of an eye they should call their veterinarian immediately: redness, cloudiness, ocular discharge, squinting, rubbing the eye, change in vision, protruded third eyelid. Any of these conditions should mean an immediate call and rapid medical attention.

Does it get worse as the dog gets older?

Depends on the condition. While an exposed third eyelid may stay fairly quiet over time, it also may become more inflamed and be a source of irritation and discomfort. Tumors may develop on the third eyelid, especially as dogs get older,  that can be either benign or malignant. Development of tumors, however, can happen whether or not the third eyelid gland is exposed.