TL;DR: A visible third eyelid in cats usually indicates irritation or illness and requires a veterinary exam within 24–48 hours, though immediate care is needed if accompanied by bleeding or lethargy.
What is a cat’s third eyelid and why is it visible?
Cats and kittens possess a unique anatomical feature called the nictitating membrane, commonly known as the third eyelid. This membrane is located in the inner corner of the eye and serves to protect the eyeball and distribute tear film. Under normal circumstances, you should barely see it. When it becomes visible, it is often a sign that the cat's eyeball has retracted slightly into the socket or that the membrane is responding to irritation or systemic illness.
Is my cat showing their third eyelid a veterinary emergency?
- The urgency level for a visible third eyelid is typically Medium.
- While it is rarely a life-threatening emergency on its own, it is a clear indicator that something is wrong.
- You should schedule a veterinary appointment within 24 to 48 hours.
- Seek immediate urgent care if the protrusion is accompanied by intense squinting, bleeding, a change in pupil size, or extreme lethargy.
Why should I take a photo of my cat's eyes for a triage consultation?
- Capturing a clear, well-lit photo of your cat's eyes is one of the most helpful steps you can take for a remote triage.
- A photo allows a professional to see if the issue is unilateral (one eye) or bilateral (both eyes).
- This helps differentiate between a localized injury and a systemic issue like Haw's syndrome or dehydration.
- It helps us assess the color and thickness of the membrane to determine the level of inflammation.
Clinical Context (Merck Veterinary Manual)
In cats, the presence of a prominent third eyelid can be associated with various systemic diseases. Common pathogens like feline herpesvirus-1, Chlamydia, and Mycoplasma frequently manifest as acute or recurrent conjunctivitis. Other systemic conditions such as feline infectious peritonitis, toxoplasmosis, feline immunodeficiency virus, and feline leukemia virus can present with ocular manifestations, including anterior and posterior uveitis, retinal detachment, and secondary glaucoma. Eyelid inflammation may also be linked to systemic Demodex cati, D. gatoi, Notoedres cati (scabies), ringworm, and immune-mediated skin diseases.
Chapter: Ophthalmology
Source: The Merck Veterinary Manual, 11th Edition (Page 490)