Uveitis in cats is most often caused by an underlying infection, with Toxoplasma gondii, feline immunodeficiency virus (FIV), feline leukemia virus (FeLV), and feline infectious peritonitis (FIP) topping the list. In a retrospective study of 96 cats with uveitis, infectious diseases accounted for about 32% of cases, tumors for 23%, and roughly 30% were classified as idiopathic, meaning no cause could be identified despite testing. The remaining cases were split between direct eye trauma and corneal infections that spread inward.
What Uveitis Actually Is
The uvea is the middle layer of the eye, made up of three structures: the iris (the colored part), the ciliary body (which produces the fluid inside the eye), and the choroid (a blood-vessel-rich layer lining the back of the eye). When any of these structures become inflamed, that’s uveitis. Anterior uveitis affects the iris and ciliary body at the front of the eye. Posterior uveitis involves the choroid in the back, and because the choroid sits right against the retina, retinal damage almost always accompanies it. When the entire uvea is inflamed, it’s called panuveitis.
What makes feline uveitis tricky is that the eye inflammation is rarely the primary problem. It’s usually a signal that something else is going on in the body, whether that’s a systemic infection, a tumor, or an immune system gone haywire. Identifying the underlying cause is the whole challenge.
Toxoplasma: The Most Common Infectious Cause
Toxoplasma gondii, the single-celled parasite best known for its link to undercooked meat and cat litter, is the infectious agent most frequently tied to feline uveitis. In one study of 93 cats with uveitis that had no obvious external cause, 78.5% had blood evidence of Toxoplasma exposure, a rate significantly higher than in healthy cats from the same area. Among cats with internally-driven uveitis, about 61% had Toxoplasma-specific antibodies in their blood, and the vast majority of those also had the same antibodies inside the fluid of their eyes, strongly suggesting the parasite was directly responsible for the inflammation.
Many cats carry Toxoplasma without ever showing symptoms. The parasite becomes a problem when the immune system can’t keep it in check, allowing it to reactivate and trigger inflammation in the eye and other organs.
Viral Infections: FIV, FeLV, and FIP
Three major feline viruses are well-established causes of uveitis. FIV was found in nearly 23% of cats with uveitis in the study mentioned above, while FeLV appeared in about 6%. Feline coronavirus antibodies at high levels (suggesting FIP) were present in roughly 4% of cases. Each virus causes eye problems through a different mechanism.
FIV weakens the immune system over time, making the eyes more vulnerable to opportunistic infections and immune-mediated inflammation. FeLV can cause uveitis directly or promote the development of lymphoma inside the eye. FIP tends to produce the most dramatic eye disease: severe, widespread inflammation affecting the entire eye, often with a hazy cornea, heavy debris floating in the eye fluid, retinal detachment, and a distinctive pattern of large, greasy-looking clumps of inflammatory cells stuck to the inner surface of the cornea. This pattern reflects the intense, granulomatous nature of FIP inflammation.
Bacterial and Fungal Causes
Bartonella henselae, the bacterium behind cat scratch disease in humans, has been increasingly recognized as a cause of uveitis in cats themselves. Feline herpesvirus 1 may also trigger intraocular inflammation in some cats, though the evidence for this is still developing.
Fungal infections are another important category, especially in certain parts of the United States. Cryptococcosis, histoplasmosis, blastomycosis, and coccidioidomycosis can all spread to the eye through the bloodstream. Which fungi pose a risk depends heavily on geography. Histoplasmosis is concentrated in the Ohio and Mississippi River valleys, blastomycosis in the Great Lakes region, and coccidioidomycosis in the desert Southwest. If your cat lives in or has traveled through one of these areas, fungal infection moves higher on the list of suspects.
Tumors Inside the Eye
Cancer accounted for nearly 23% of uveitis cases in the retrospective study, making it the second most common identified cause after infection. The most frequent tumor type is iris melanoma, which often starts as a flat, freckle-like dark spot on the iris. Some of these spots never change, but others gradually become raised, multiply, and thicken the iris. As the tumor grows, it causes chronic uveitis, a misshapen pupil, and sometimes secondary glaucoma. Iris melanoma in cats is aggressive: the estimated rate of spread to other organs is 63%.
The second most common eye tumor in cats is trauma-associated sarcoma. This highly malignant cancer develops in eyes that were injured years earlier, with an average gap of about five years between the original injury and the cancer diagnosis. Signs include ongoing uveitis, bleeding inside the eye, rising eye pressure, and visible white or pink masses. These tumors can invade along the optic nerve and spread to nearby lymph nodes. Other possible tumors include growths arising from the ciliary body and lymphoma linked to FeLV that settles in the eye.
Trauma and Lens-Related Inflammation
Direct injury to the eye from fights, falls, or foreign objects can trigger uveitis, though trauma is actually one of the less common causes in cats compared to infections and cancer. It accounted for about 6% of cases in the retrospective study. Corneal infections that penetrate deeper into the eye were responsible for another 8%.
Lens-induced uveitis is a separate category that occurs when proteins from inside the lens leak out into the surrounding eye fluid. This can happen after a cataract forms or if the lens capsule is damaged by trauma. The immune system treats these lens proteins as foreign, launching an inflammatory response inside the eye.
Idiopathic Uveitis: When No Cause Is Found
About 30% of cats with uveitis receive an idiopathic or immune-mediated diagnosis after testing fails to pinpoint a specific cause. These cats tend to be younger, with an average age around four years. In many of these cases, the immune system appears to be attacking the eye’s own tissues without an identifiable trigger. It’s possible that some of these cats have infections that current tests simply can’t detect, or that an earlier infection resolved but left behind an ongoing immune reaction.
How Uveitis Is Recognized
The signs of uveitis vary depending on which part of the eye is affected. Anterior uveitis, the more common form, typically causes a red, painful eye with a noticeably small pupil. The fluid inside the front of the eye becomes cloudy as proteins and inflammatory cells leak in. Sometimes pus or blood pools visibly at the bottom of the eye’s front chamber. Clumps of inflammatory cells may stick to the inside of the cornea, appearing as small dots.
Eye pressure drops during uveitis because the inflamed ciliary body produces less fluid. In a healthy cat’s eye, pressure typically runs between 15 and 25 mmHg. In an actively inflamed eye, it often falls below 10 to 15 mmHg. Paradoxically, if the inflammation goes on long enough, scar tissue can block the eye’s drainage system and cause pressure to rise, leading to secondary glaucoma.
Posterior uveitis is harder to spot without specialized equipment. A veterinarian examining the back of the eye may see grayish discoloration where inflammation has caused swelling or cellular buildup in the retina. In more severe cases, portions of the retina can detach.
Why Identifying the Cause Matters
Treating uveitis without knowing its cause means only addressing the inflammation while potentially missing a life-threatening disease. A cat with FIP-related uveitis needs antiviral treatment. A cat with Toxoplasma needs antiparasitic medication. A cat with iris melanoma may need the eye removed before the cancer spreads. And a cat whose uveitis is truly immune-mediated needs long-term anti-inflammatory management to prevent vision loss.
The standard workup for a cat with uveitis typically includes blood testing for FIV, FeLV, Toxoplasma antibodies, and feline coronavirus antibodies. Depending on the cat’s history and location, fungal testing and Bartonella screening may be added. A thorough eye exam, and sometimes ultrasound of the eye, helps evaluate for tumors or retinal detachment. Even with a full panel of tests, roughly a third of cases remain unexplained, which is why repeated monitoring matters. Some causes, particularly slow-growing tumors, only become apparent over time.