Why Do Dogs Have a Third Eyelid? Essential Eye Care Explained

Unlike humans, who have just upper and lower eyelids, dogs and many other animals possess a third eyelid known as the nictitating membrane. This delicate, translucent fold of tissue is located at the inner corner of their eyes, beneath the standard eyelids, and plays an essential role in protecting and maintaining eye health.

What Is the Third Eyelid?

The third eyelid, or nictitating membrane, is unlike the upper and lower eyelids as it is made of a thin, moist membrane rather than skin covered with fur. It varies in appearance, often appearing greyish or almost clear, depending on your dog’s coat, and is usually hidden unless your dog is sleepy or unwell. It also harbours a gland critical for tear production.

Key Functions of the Third Eyelid in Dogs

1. Protection: It shields the eye from scratches, dust, and debris, particularly when dogs run through undergrowth or rough terrain. This helps prevent injury to the delicate eyeball.
2. Lubrication: The tear gland within the third eyelid produces approximately 30% to 60% of a dog's tears, helping to keep their eyes moist and comfortable.
3. Cleaning: It sweeps across the eye surface, clearing away dirt and contaminants to ensure the eye remains clear and healthy.
4. Immune Defence: The glands produce antibodies that help fight infections, giving an added layer of defence against microbes.

Is the Third Eyelid Visible in All Dogs?

In healthy dogs, the third eyelid is rarely noticeable as it sits tucked away beneath the main eyelids. However, you might see a small, pale membrane at the inner corners of your dog’s eyes when they are very relaxed, just waking, or going to sleep. Notably, certain breeds with prominent eyes, such as the pug, may show it more frequently.

Common Third Eyelid Problems and What They Mean

If you frequently see your dog’s third eyelid or it appears swollen, red or inflamed, it could indicate an underlying health issue. Some common problems include:

• Cherry Eye: This occurs when the gland within the third eyelid prolapses, causing a red, raised mass at the inner corner of the eye, common in breeds like pugs and pug.
• Ectropion: An eyelid disorder that may expose the third eyelid abnormally.
• Infections or Allergies: Both can inflame the third eyelid, making it appear red and sore.
• Neurological or Systemic Illnesses: Conditions such as Horner’s syndrome or dehydration can cause the third eyelid to become visible, often signalling the dog feels unwell.

If you notice persistent third eyelid visibility, redness, or discomfort in your dog's eyes, it is important to consult your vet promptly for accurate diagnosis and treatment.

How To Care For Your Dog’s Eyes and Third Eyelid

1. Observe regularly: Keep an eye on your dog’s eyes for any changes in colour, swelling, or unusual appearance of the third eyelid.
2. Keep the eyes clean: Gently wipe away any discharge with a damp cloth, but avoid poking or applying pressure.
3. Protect from hazards: Prevent your dog from playing in areas with sharp plants or debris that might injure their eyes.
4. Schedule veterinary check-ups: Regular visits and prompt attention to eye issues help maintain eye health.

By understanding your dog’s third eyelid and monitoring its health, you contribute significantly to their wellbeing and comfort.

When to See a Vet

Always seek veterinary advice if you notice unusual or persistent third eyelid protrusion, changes in eye colour (especially redness), discharge, or signs of eye discomfort such as squinting or pawing at the eye. Early intervention can prevent serious eye complications.