Uveitis with or without chorioretinitis is one of the most frequent and significant ophthalmic diseases in cats. In recent reports, between 38% and 70% of the cats with uveitis have concurrent systemic disease. The most important causal agents are corona virus (feline infectious peritonitis), retrovirus (feline leukemia virus, feline immunodeficiency virus), Toxoplasma gondii, and various mycoses. Feline herpes virus is probably also a cause of uveitis. Prompt diagnosis of underlying viral infection in cats with ophthalmic disease is paramount for accurate diagnosis and prognosis and is required for appropriate therapeutic decision-making.
The intraocular structures are normally separated from the choroidal and retinal blood vessels by two barriers; the blood-aqueous barrier and the blood-retinal barrier. In inflammation or infection, both barriers may collapse and permit the passage of larger molecules, inflammatory cells and blood cells into the eye.
The eye does not contain lymphoid tissue, but lymphocytes are normally present in the uvea. Thus, the uvea may act as a regional lymph node, which can be stimulated by antigen. After an episode of inflammation, immunologic competent cells are retained as "memory lymphocytes", which can be reactivated.
Acute anterior uveitis (iritis) is a painful condition causing photophobia and blepharospasm. Episcleral hyperemia is less pronounced in the cat than in the dog. Expression of pain is not always obvious in cats, but the affected cat may be less active than normal, and the owner may report some squinting. The degree of corneal edema may vary, and miosis may be less obvious in cats than in dogs with acute uveitis. Especially in cats, neovascularisation on the iris surface rapidly develops, with visible blood vessels ("rubeosis iridis") and change in the iris color. Intraocular pressure is reduced. The composition of the exudate determines the appearance of the aqueous in the anterior chamber. Chronic inflammations are characterised by mononuclear inflammatory cells. Immune complexes may sediment in the anterior chamber and attach to the ventral part of the inside of the cornea. These are termed keratic precipitates. As a complication, fibrinous exudate may cause the iris to adhere to the lens (posterior synechia). Both this and inflammatory cells and fibrin blocking the iridocorneal angle can cause secondary glaucoma.
Inflammation of the choroid causes less dramatic clinical signs, and may easily be overlooked, but there will be some episcleral hyperemia present. Inflammatory swelling of the choroid may cause fluid to accumulate behind the retina, resulting in partial or total retinal detachment. The retina may also be affected by concurrent inflammation, chorioretinitis, which is a common entity in many cases of posterior uveitis.
Uveitis can be associated with many systemic diseases, and may in some cases be the first clinical sign observed. Thus, a thorough work-up is therefore recommended in all cats presented with uveitis.
Feline infectious peritonitis (FIP)
FIP may present in a granulomatous (dry) and a productive (wet) form. The granulomatous form of FIP, which is caused by a partial cell-mediated immune response, is most often associated with ocular signs, and uveitis may even be present without concurrent signs of systemic disease. Keratic precipitates due to immune complex formation are a characteristic finding in FIP-related uveitis. Granulomatous processes are frequently present in the uvea as well as in other tissues. Diagnosis may be difficult, as presence of elevated coronavirus titer may also imply infection with the related coronavirus enteritis. Elevated plasma globulins are a frequent finding in the non-effusive form of FIP. Vaccination against FIP has not been very successful until now. The following illustration shows the outcomes of a feline coronavirus infection, dependent on the host cell mediated immune response.
Natural FIP infection
Feline Leukemia virus (FeLV)
Lymphoma is defined as a lymphoid malignancy that originates from solid organs. About 70% of cats with lymphoma are FeLV-positive. Prevalence of FeLV in outdoor cats is reported between 3.5 and 5.8%. Ocular changes linked to FeLV are related to the ability of the virus to induce immunosuppression, hematologic changes and tumor formation, and include nodular uveal tumors, often with secondary uveitis, retinal dysplasia, retinal hemorrhages and pupillary changes. Abnormalities in pupil shape include spastic pupil, D-shaped pupil and reversed D shaped pupil. These changes probably represent FeLV infection of the autonomic ganglia for the parasympathic portion of the third cranial nerve, which delivers one branch to each of the two pupillary constriction muscles of the iris. Diagnosis may be made through commercially available diagnostic test kits. Affected cats may be treated with multiple-agent chemotherapy. Studies have shown that vaccination is effective in protecting a significant proportion of cats from FeLV challenge.
Feline immunodeficiency virus
Feline immunodeficiency virus (FIV) infection in cats causes disease virtually indistinguishable from that caused by HIV-1 in humans. The anatomical distribution of FIV includes mucosal interfaces, lymphoid organs, sites of haematopoiesis, circulating mononuclear cells, and the central nervous system. FIV infection can occur in all ocular tissues and may be manifestations of direct viral effects or occur secondary to viral-related malignant transformation. FIV may cause a mild uveitis that can be aggravated by co-infection with Toxoplasma gondii. A positive FIV status has also shown to be associated with lymphoma and it is possible that this infection may predispose to the development of lymphoid neoplasia. FIV is most often transferred through bite wounds; thus, intact outdoor male cats are over represented. In-clinic diagnostic kits for serology are widely used for establishing a diagnosis, but may sometimes give false negative results.
Feline herpes virus
Feline herpes virus (FHV-1) is a very common pathogen; studies in the USA have shown that about 75% of the adult cat population is seropositive. Latency is established in the trigeminal ganglia, and a chronic carrier state develops with intermittent virus shedding. Recurrence is common, especially with stress or other systemic disease. The most common ocular change is keratitis, but recently it hat been shown that FHV-1 may also be a cause of uveitis in the cat.
The prevalence of toxoplasmosis in the cat population has been difficult to determine. General infection in cats is usually subclinic, but the disease has been well documented in both kittens and adults. The demonstration that IgA, IgG and IgM titres to the organism can be found in many uveitis-affected cats has led to heightened interest in toxoplasmosis as a cause of feline uveitis. Still detection of serum antibodies cannot be used alone to document ocular toxoplasmosis. T.gondii DNA has been detected by PCR in aqueous humor of naturally exposed and experimentally infected cats. Uveitis presented as chorioretinitis is the most frequent ocular sign, but anterior uveitis does occur. It is, however, unclear why some cats exposed to infection develop ocular disease and some do not. There may be differences in ocular tropism of T.gondii strains, and it has also been hypothesized if ocular toxoplasmosis in cats may be related to infection in utero or in the neonatal period. Toxoplasmosis uveitis is treated both with anti-inflammatory agents and appropriate antibiotics.
The most common cause of fungal infection in the cat is Cryptoccus neoformans. Other fungi include Histoplasma capsulatum, Blastomyces dermatitidis and Coccidioides immitis. The most common ocular manifestation of these diseases is posterior uveitis (choroiditis); however, anterior uveitis is sometimes present and is usually secondary to the inflammation in the posterior segment. The prognosis for cats with systemic fungal diseases is better after the advances in antifungal therapy. The prognosis for return of vision for eyes affected with systemic fungal disease is still guarded.
Other agents reported to as possible cause of uveitis in cats include Bartonella spp, Mycobacterium tuberculosis and the parasites Toxocara cati, Dirofilaria immitis and fly larvae.
1. Barnett KC, Crispin SM,(1998) Feline Ophthalmology, Saunders, London.
2. Chavkin MJ, Lappin M, Powell CC, Roberts SM, (1993), Seroepidemiologic and clinical observations of 93 cases of uveitis in cats, Prog Vet Comp Ophthalmol, 2: 29-36.
3. Davidson MG, Nasisse MP, English RV et al.(1991) Feline anterior uveitis: a study of 53 cases, J Am Anim Hosp Assoc, 27: 77-83.
4. Formston C. (1994), Retinal detachment and bovine tuberculosis in cats, J Small Anim Pract, 35: 5-8.
5. Gerds-Grogan S, Dayrell-Hart B, (1997) Feline cryptococcosis: A retrospective evaluation, J Am Anim Hosp Assoc, 33: 118-122.
6. Guillermo Couto C. What is new on feline lymphoma? J Feline Med Surg 2001;3:171-176.
7. Harbour DA, et al. (2002) Protection against oronasal challenge with virulent feline leukaemia virus lasts for at least 12 months following a primary course of immunisation with Leukocell 2 vaccine. Vaccine 26; 2866-2872.
8. Maggs DJ, Lappin MR, Nasisse MP.(1999) Detection of feline herpesvirus-specific antibodies and DNA in aqueous humor from cats with or without uveitis. Am J Vet Res 60:932-936.
9. Powell CC, Lappin MR. (2001) Clinical ocular toxoplasmosis in neonatal kittens. Vet Ophth 4: 8792.