Why Is My Cat's Third Eyelid Showing? Haw Syndrome, Horner's, Parasites and What to Do

A visible third eyelid (the "haw") can mean anything from normal sleep to serious illness. Learn medical and behavioral causes, red flags, and when to see your vet.

When to See a Vet

If your cat's third eyelid (the nictitating membrane, often called the "haw") is visible for more than a few hours, comes on suddenly, affects one eye only, or is accompanied by any of the following, seek veterinary care promptly:

• Eye pain: squinting, pawing at the eye, excessive blinking, or blepharospasm
• Ocular discharge (especially green/yellow) or blood
• Sudden vision changes, bumping into objects, dilated or very small pupil
• Signs of systemic illness: lethargy, decreased appetite, vomiting, diarrhea, fever
• Neurologic signs: head tilt, circling, weakness, collapse

What is the "third eyelid" (the haw)?

Cats, like many mammals, have a nictitating membrane — the third eyelid — located in the inner corner of each eye. It helps protect the cornea and distributes tears. Normally you should not see much of it when a cat is awake and alert. When it becomes visibly protruded across or partially across the eye, people describe the eye as "showing the haw." The reason for that protrusion can be medical, or much less commonly, related to behavior or environment.

Medical causes

A number of medical problems can make the third eyelid more visible. Key conditions include:

• Conjunctivitis and ocular infections: Viruses (feline herpesvirus/FHV-1), bacteria, chlamydophila and mycoplasma can inflame the conjunctiva and eyelids and cause the third eyelid to protrude. Infected eyes often have redness, discharge and squinting. (Merck Veterinary Manual)

• Corneal ulcers and eye trauma: Painful corneal injuries cause the eye to close and the third eyelid often rises to protect the surface.

• Horner's syndrome: Loss of sympathetic nerve input to the eye produces a classic set of signs — droopy upper eyelid (ptosis), a sunken eye (enophthalmos), a constricted pupil (miosis) and a protruding third eyelid. Horner's can be idiopathic, or secondary to ear disease, cervical or chest trauma, middle ear infections, or masses along the sympathetic pathway. (Merck Veterinary Manual)

• Ophthalmic mass or foreign body: Tumors of the orbit, eyelids or bulbar conjunctiva can mechanically push the third eyelid forward.

• Systemic illness and general malaise: Cats that are dehydrated, weak, febrile or otherwise systemically ill (including severe parasitism in kittens) may hold the third eyelid partially forward. This is a nonspecific sign of illness in many species.

• Neurologic disease: Lesions affecting cranial nerves or central pathways can produce third eyelid protrusion.

• Horner-like syndromes due to thoracic disease or trauma: The sympathetic pathway runs from the brainstem down the neck and into the chest; disease along that path can cause ocular signs.

• Prolapse of the nictitans gland ("cherry eye"): A separate problem where the gland of the third eyelid becomes exposed and swollen; this appears as a pink mass in the inner corner. More common in dogs but can occur in cats.

• Eye surgery or anesthesia: Post-operative third eyelid elevation can occur transiently after anesthesia or ocular surgery.

Behavioral and non-medical causes

Although medical causes are most common and should be ruled out first, a few non-disease reasons may explain a temporarily visible third eyelid:

• Sleep or deep rest: When a cat is very sleepy or deep-sleeping, the third eyelid occasionally becomes slightly visible.
• Mild irritation or environmental debris: A scratchy breeze, dust or an eyelash may cause transient protrusion while the eye blinks and clears.
• Breed or individual anatomy: Some cats have naturally more visible third eyelids at rest; this is uncommon.
• Temporary stress or fatigue: A stressed or exhausted cat may appear "tucked" in the face and show more of the third eyelid.

How to tell medical vs behavioral causes

Look for these patterns to decide whether the problem is likely medical (requires immediate vet assessment) or possibly benign:

• Onset: sudden onset, especially with pain or discharge, is usually medical. A slow or intermittent appearance with normal behavior may be less urgent but still warrants evaluation.
• Unilateral vs bilateral: many ocular problems (corneal ulcer, conjunctivitis, trauma, foreign body) begin in one eye. Horner's is typically unilateral. Bilateral Haw elevation without other signs might be more consistent with sleepiness or systemic illness.
• Accompanying signs: medical causes often come with redness, discharge, squinting, changes in pupil size, neurological signs, or systemic signs (fever, vomiting). Behavioral causes usually lack these.
• Pupil size and eyelid position: if the affected side has a constricted pupil, ptosis (drooping eyelid), and a sunken look, Horner's is likely. If the pupil is dilated and the eye is blind, neurologic causes or severe ocular disease are possible.
• Response to handling: a painful eye will cause your cat to resist examination, protect the eye, or hide; a comfortable cat is more likely to allow inspection.

Why might parasites cause the haw to show?

Heavy gastrointestinal parasitism (especially in young kittens) can cause weakness, anemia, dehydration and general malaise. A sick kitten may partially close its eyes and let the third eyelid protract. In addition, systemic infections (including some parasites) can predispose cats to secondary ocular infections. Treating the underlying parasitism and supporting hydration often improves general signs — but the eye itself must still be examined for concurrent ocular disease. (Merck Veterinary Manual: gastrointestinal parasites overview)

What to observe and record for your vet

Bring the following information or note it before your appointment:

• Exact timeline: when you first saw the third eyelid, whether it came on suddenly or slowly
• Which eye(s) is affected: left, right, or both
• Any discharge: color (clear, white, yellow, green), amount, and whether it crusts overnight
• Pupil sizes and response to light (if you can safely check) — is one pupil smaller?
• Any signs of pain: pawing at the eye, squinting, reluctance to be touched around the head
• Recent trauma, fights, or exposure to other sick cats
• Vaccination status, indoor/outdoor status and any recent deworming or known parasite exposure
• Appetite, activity level, vomiting, diarrhea, coughing, sneezing, ear symptoms
• Any medications or home treatments you've tried (including topical agents)

Next steps — practical action plan

When might it resolve on its own?

• Transient causes such as sleepiness, mild irritation or a brief environmental exposure can resolve within hours.
• Some cases of idiopathic Horner's syndrome in cats may improve spontaneously over weeks to months, but the underlying cause should be investigated because some triggers (ear infections, neck trauma, thoracic disease) need treatment.
• Conjunctivitis or infectious causes usually require veterinary treatment; they rarely resolve quickly without therapy and may worsen.

Red Flags — Seek emergency care now

Seek immediate emergency veterinary care if your cat has any of the following along with third eyelid protrusion:

• Severe eye pain (constant squinting, vocalization, pawing at the eye)
• Sudden loss of vision or rapid decline in vision
• Bleeding from or around the eye, or a clearly bulging globe
• Neurologic collapse, seizures, severe weakness or breathing difficulty
• High fever, collapse, or signs of shock

Sources and further reading

• Merck Veterinary Manual — Horner's syndrome; Conjunctivitis in cats; Gastrointestinal parasites overview: https://www.merckvetmanual.com
• Gelatt K. Veterinary Ophthalmology (classic reference textbook).
• American Veterinary Society of Animal Behavior (AVSAB) — resources on animal behavior and the importance of medical evaluation before behavioral attribution: https://avsab.org/resources/

Key takeaways

• A visible third eyelid (the "haw") is a clinical sign, not a diagnosis — it may indicate ocular pain, neurologic disease (for example Horner's), infection, trauma, or systemic illness.
• Always seek veterinary evaluation if the haw is persistent, sudden, unilateral, or accompanied by discharge, pain, vision changes or systemic signs.
• Some benign causes (sleepiness, mild irritation) resolve quickly, but it is not safe to assume a medical cause is not present without an exam.
• Do not use over-the-counter human eye medications; photograph the eye and record a timeline to help your veterinarian.
• Red flags such as severe pain, bleeding, sudden blindness or neurologic signs require emergency care.

Frequently Asked Questions