What exactly is eosinophilic keratitis in cats?

Eosinophilic keratitis is an inflammatory condition in which a specific type of white blood cell called an eosinophil, along with mast cells and lymphocytes, infiltrates the cornea and sometimes the adjacent conjunctiva. The result is a raised, pink to white, often vascularized plaque or thickened patch on the surface of the eye that can resemble a growth even though it is inflammatory, not cancerous, tissue (Merck Veterinary Manual, 2023). The exact trigger for this immune response is not fully understood, but many veterinary ophthalmologists consider it part of a broader group of feline eosinophilic disorders that also includes eosinophilic granuloma complex on the skin and mouth.

The disease usually affects one eye, though the Cornell Feline Health Center notes that bilateral involvement occurs in a minority of cases and that recurrence in the same eye or spread to the second eye is possible over a cat's lifetime (Cornell Feline Health Center, n.d.). It is classified separately from simple conjunctivitis because the inflammation specifically targets the cornea itself, which carries a higher risk of scarring and vision changes if the disease is allowed to progress unchecked for weeks or months.

What are the first signs an owner usually notices?

Most owners first notice squinting (blepharospasm), a pinkish or grayish film creeping in from the edge of the cornea, or new blood vessels visibly branching across what should be a clear window of the eye. Discharge is common and is often described as thick, mucoid, or curd-like rather than the thin watery discharge typical of a simple upper respiratory infection (Merck Veterinary Manual, 2023). Some cats paw at the affected eye or rub it against furniture, which can worsen irritation and increase the risk of a secondary corneal ulcer.

Because the raised, irregular plaque can look pale pink to white and slightly bumpy, it is sometimes mistaken by owners for a foreign object stuck to the eye or even a tumor. The affected eye may also appear smaller or partially closed compared to the normal eye due to discomfort. Signs can wax and wane over days to weeks, and a cat with a history of herpesvirus flare-ups or chronic sneezing bouts is not unusual to also develop this corneal condition during or after a respiratory episode.

What causes it, and is feline herpesvirus really involved?

The precise cause remains unclear, but feline herpesvirus type 1 (FHV-1) is strongly suspected as a trigger or contributing factor in many cases, since a large proportion of adult cats carry latent herpesvirus and stress or immune changes can reactivate it (Cornell Feline Health Center, n.d.). Rather than being a direct infection at the plaque site, the current thinking is that herpesvirus antigens or chronic low-grade irritation provoke an abnormal, exaggerated eosinophilic immune response in genetically or immunologically predisposed cats.

Any age or breed can be affected, but veterinary reports commonly describe onset around 2 to 5 years of age, with no strong sex predisposition documented in the literature (Merck Veterinary Manual, 2023). Environmental stressors, multi-cat households, and previous upper respiratory disease are frequently cited as risk factors that increase the likelihood of a flare, similar to the pattern seen with herpetic conjunctivitis and feline herpetic dermatitis.

How do veterinarians diagnose eosinophilic keratitis?

Diagnosis starts with a thorough ophthalmic exam, often using a slit lamp or magnification to characterize the raised plaque, its vascularization, and its location on the cornea versus conjunctiva. Veterinarians typically perform fluorescein staining first to rule out or identify a concurrent corneal ulcer, since eosinophilic keratitis can occur alongside ulcerative disease and the treatment approach differs if an ulcer is present (AVMA, n.d.).

A definitive diagnosis usually requires a corneal cytology sample, collected by gently scraping the surface of the plaque with a blunt instrument or cytobrush under topical anesthetic, then examining the sample under a microscope for eosinophils, mast cells, and lymphocytes (Cornell Feline Health Center, n.d.). Some cases are also tested for feline herpesvirus DNA via PCR, though a positive result does not confirm herpesvirus as the direct cause since latent virus is common in the general cat population.

What treatments actually work for this condition?

Topical corticosteroids, such as ophthalmic prednisolone acetate or dexamethasone drops, are the mainstay of treatment and are usually applied multiple times daily during the active phase, then tapered gradually over weeks once the plaque resolves (Merck Veterinary Manual, 2023). Steroids are only used once an active corneal ulcer has been ruled out, because steroids can worsen ulcerative disease and delay healing if used incorrectly.

For cats that cannot tolerate steroids, respond poorly, or have suspected herpesvirus involvement, topical cyclosporine or tacrolimus ointment is a common alternative or add-on therapy, and some veterinarians combine this with an oral antiviral such as famciclovir when herpesvirus reactivation seems likely (Cornell Feline Health Center, n.d.). Treatment courses often run four to eight weeks, and lifelong intermittent flare-ups mean many cats need repeat courses of medication rather than a single cure.

What does day-to-day home care and monitoring look like?

Owners are typically responsible for administering eye drops or ointment on a strict schedule, which can mean three to four applications a day during the initial weeks, so setting phone reminders or building a fixed routine around meals helps with compliance. An e-collar (cone) is often recommended temporarily if the cat paws at the eye, since self-trauma can convert simple keratitis into an ulcerated, more painful lesion requiring more intensive treatment (AVMA, n.d.).

Regular rechecks, usually every two to four weeks during active treatment, let the veterinarian confirm the plaque is shrinking and adjust the steroid taper rather than stopping medication too abruptly, which is a common cause of rebound flares. Keeping a simple photo log of the eye on a phone can help both owner and veterinarian track subtle changes in size, color, and vascularization between visits.

What's the long-term outlook, and does it come back?

Most cats respond well to appropriate treatment, with visible improvement in the plaque within one to two weeks of starting therapy and full or near-full resolution over four to eight weeks (Merck Veterinary Manual, 2023). However, eosinophilic keratitis has a recognized tendency to recur, sometimes months to years later, in the same eye or occasionally the opposite one, particularly in cats with a history of herpesvirus-associated respiratory disease.

Long-term management for cats with frequent recurrences may include maintenance use of low-dose topical cyclosporine, environmental stress reduction, and prompt treatment of any new upper respiratory flare, since early intervention at the first sign of squinting tends to shorten and lessen the severity of each episode (Cornell Feline Health Center, n.d.). Vision loss is uncommon when the disease is caught and treated early, but chronic untreated cases can progress to corneal scarring that permanently affects clarity of vision in the eye.