By Jennifer Copley (Last Updated 6 December 2011)
Cats and dogs have a white third eyelid (also called the nictitating membrane or haw) that is capable of moving over the eye like a windshield wiper, protecting the cornea. It’s usually only visible when a pet is ill or has a painful eye. Some healthy cats do keep their third eyelids visible when resting, but retract them quickly when something alerts or startles them.
Reasons why the third eyelid may show include the following:
- Cherry eye (a condition caused by prolapse of the nictitating membrane, where the membrane appears as a red mass in the corner of the eye closest to the nose)
- Keratoconjunctivitis sicca (dry eye syndrome)
- Neurologic or systemic diseases, including infections affecting the whole body
- Relaxation of the muscles surrounding the eyeball due to tranquillizers or illness
- Something irritating the eye (such as dirt, grass seed, etc.)
- Tumour, inflammatory disease, infection, injury, or cyst affecting the eye
- Weight loss
If the third eyelid is showing and the eye is bulging, there may be an eye abscess, tumour, or hematoma (bleeding behind the eye). If the eye is sunken, there could be many different causes, ranging from tetanus to dehydration.
If the nictitating membrane is only showing in one eye, the problem is probably localized to that eye. When both eyes are affected, it’s more likely to be something affecting the entire body.
Treatment depends on the underlying problem. If you believe that your pet may be ill, consult a veterinarian, particularly if there are any additional symptoms.
Dr. Noelle McNabb (2011) states that you should not use human eye medications such as Visine® on cats because they are unlikely to help, and they may even interfere with the veterinarian’s ability to make an accurate diagnosis.
This article is provided for informational purposes only and is not intended as a substitute for veterinary consultation and care.