Red, swollen eyes with discharge or squinting usually point to conjunctivitis, which is inflammation of the conjunctiva, the thin tissue that lines the eyelids and covers the front of the eye. The term describes the inflammation, not its cause. Feline herpesvirus is a common trigger, but bacteria, irritants, foreign material, corneal injury, and other eye diseases can produce the same signs. That is why a veterinary eye examination comes before any treatment, and why the right treatment depends on what the exam finds.
What conjunctivitis looks like in a cat
The most common signs are:
- Redness or swelling of the conjunctiva, the pink tissue inside the eyelids
- Watering, or discharge that ranges from clear and watery to thick and yellow or green
- Frequent blinking or squinting
- Sensitivity to light
- A swollen third eyelid, which is the pale membrane in the inner corner of the eye. When it swells, it can partly cover the eye.
Sneezing or nasal discharge can appear alongside eye signs when an upper respiratory infection is involved. Because these signs overlap across many conditions, they cannot tell you the cause on their own.
Causes
Infectious causes
Cornell University’s College of Veterinary Medicine lists feline herpesvirus type 1 (FHV-1), feline calicivirus, Chlamydophila felis (older references use Chlamydia felis), and Mycoplasma among frequent infectious causes. Viral infections can also be complicated by secondary bacterial infection, according to VCA Animal Hospitals, so a cat may have more than one problem at once.
Noninfectious and secondary causes
- Dust and chemical irritants
- Allergens
- Foreign material under an eyelid
- Entropion, where the eyelid rolls inward so the hair rubs the eye
- Corneal injury or corneal ulcer
- Dry eye
- Glaucoma and uveitis, which are serious inflammatory or pressure conditions inside or around the eye
- Eyelid or ocular tumors
Treat eye redness in one eye with the same seriousness as redness in both. The MSD Veterinary Manual notes that infectious disease, foreign material, and other local problems can all begin in just one eye, so a one-eye presentation does not identify the cause.
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Age, household setting, and latent infection
Cornell describes feline conjunctivitis as especially common in young animals and in multi-cat environments, where contagious infections spread more easily. Thomas Kern, DVM, associate professor of ophthalmology at Cornell’s College of Veterinary Medicine, puts it this way: “Although conjunctivitis can occur in older cats, this is an infection that occurs primarily in young animals.”
Herpesvirus does not always clear after the first infection. It can stay dormant in the body and reactivate, often in connection with stress or illness. Dr. Kern’s description of the pattern is that most affected cats eventually develop immunity and stop having episodes, while cats that carry the virus may have recurrences periodically, “just as cold sores appear from time to time in humans who carry it.”
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When to see a veterinarian
Cornell advises veterinary care whenever a cat shows apparent eye discomfort together with discharge. Book an appointment promptly. Seek same-day care if the eye is held closed, the cat appears to be in marked pain, the swelling is severe, or an injury such as a scratch is possible. A corneal ulcer can threaten the eye, and it needs treatment that matches its depth and severity.
When you call or visit, tell the clinic how long the signs have lasted, whether one or both eyes are affected, whether the cat has sneezing or nasal discharge, whether there are other cats in the home, and whether the cat has recently been ill or under unusual stress. These details help the veterinarian narrow the list of causes.
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How the veterinarian diagnoses the cause
VCA’s diagnosis summary states that “diagnosis is based primarily on medical history and physical examination.” Depending on what the exam shows, a veterinarian may then use some of the following:
- Fluorescein corneal stain to detect ulcers or surface injury to the cornea.
- Tear production measurement to check for dry eye.
- Intraocular pressure measurement to screen for glaucoma.
- Conjunctival sampling or biopsy when an infection or growth needs to be examined directly.
- Tear-duct flushing when drainage problems are suspected.
- Blood testing when a systemic cause is possible.
- PCR testing for a suspected infectious agent. A herpesvirus PCR can help identify active infection, but results can be unhelpful when the virus is latent rather than actively shedding.
Treatment depends on the cause
Bacterial infection, suspected herpesvirus, corneal ulceration, foreign material, eyelid abnormalities, and other eye disease are not interchangeable situations, and they do not share one standard treatment. The table below shows how the cause shapes the approach.
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| Suspected cause | What the veterinarian checks | Treatment direction |
|---|---|---|
| Nonspecific or bacterial infection | Eye exam, stain, and sampling if indicated | VCA describes broad-spectrum topical medication for some nonspecific infectious presentations |
| Feline herpesvirus | History, exam, and PCR when useful | Specific antiviral or other treatment chosen for the diagnosis. There is no cure; care aims to reduce how severe and how often episodes occur, and to address eye damage or secondary disease. |
| Corneal ulcer or injury | Fluorescein stain | Treated as urgent, with ulcer-specific care chosen by the veterinarian |
| Foreign material or entropion | Eyelid and eye exam | The physical cause is addressed; the specific procedure depends on the finding |
| Dry eye | Tear production measurement | Treated as a tear-film problem, not as simple conjunctivitis |
| Glaucoma or uveitis | Intraocular pressure and wider eye exam | Managed as serious eye disease in its own right |
Using medication safely
- Follow the veterinarian’s instructions for the prescribed product and the full course.
- Do not restart eye medication left over from a previous episode without advice. It may be the wrong product for the current cause.
- Avoid generic eye products that may mask symptoms and delay the examination the cat needs.
L-lysine is not an established treatment
L-lysine is often suggested for feline herpesvirus. VCA states that evidence for reducing herpes-related conjunctivitis is limited, and that the stress of administration can worsen signs. MSD says results vary and that lysine is not first-line treatment. It should not replace veterinary care.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why herpesvirus conjunctivitis tends to return
Cornell reports that up to 97% of cats are exposed to feline herpesvirus during their lives, and that the virus causes lifelong infection in up to 80% of exposed cats. These figures describe exposure and infection; they do not measure how many cats develop conjunctivitis.
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VCA’s feline herpesvirus conjunctivitis overview, a page published around 2022, gives a narrower estimate for recurrence: fewer than 45% of adult cats with latent herpesvirus infection develop recurrent ocular disease, and about 80% of infected cats may become permanent carriers. Treat these as estimates from a single source rather than predictions for an individual cat. Many uncomplicated cases do improve, and the course depends on the diagnosis made at the exam.
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