Feline uveitis: A review of its causes, diagnosis, and treatment - Veterinary Medicine
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Feline uveitis: A review of its causes, diagnosis, and treatment
Many disorders—both ocular and systemic—can result in inflammation of the uveal tract. By quickly narrowing the causes and initiating appropriate treatment, you might be able to save an affected cat's sight.


VETERINARY MEDICINE


Mycotic diseases

Cryptococcosis, blastomycosis, histoplasmosis, and coccidioidomycosis have all been associated with feline uveitis.

Cryptococcosis. Cryptococcosis is one of the most common mycotic diseases in cats. Basidiospores of Cryptococcus neoformans cause the disease; the mode of transmission is thought to be inhalation.37 Pigeon droppings serve as the principal reservoir for the yeast.37 Affected cats typically develop respiratory and cutaneous manifestations, but they may also exhibit neurologic signs secondary to direct extension of the organism through the cribriform plate.37 Ocular signs unrelated to neurologic disease are thought to occur secondary to hematogenous dissemination of the organism and include both anterior uveitis and chorioretinitis.37,38 Chorioretinal lesions vary in appearance from single to multifocal and pinpoint opacities to large circular lesions.37

Blastomycosis. Blastomycosis is caused by Blastomyces dermatitidis, a dimorphic fungus often isolated from bat and pigeon feces.37,39,40 Infection occurs primarily by inhaling aerosols containing infective spores.37 Although the disease appears to be most prevalent in areas around water, such as the Mississippi, Missouri, and Ohio river valleys,37 it has been observed in solely indoor cats in dry geographic regions.40 Most affected cats present with systemic signs, including anorexia, lethargy, weight loss, and respiratory impairment.37,39 Central nervous system signs and cutaneous lesions may also be seen in a few cases.37 Ocular lesions may include severe aqueous flare, posterior synechiae, keratic precipitates, rubeosis iridis, severe retinal detachments with subretinal pyogranulomas, and intraretinal pyogranulomas.37 Lesions described as pyogranulomatous chorioretinitis have also been reported in the posterior segment of the eye.39

Histoplasmosis. Histoplasmosis is caused by Histoplasma capsulatum, a dimorphic fungus most commonly found in bat and bird feces.37,41 Infection occurs primarily by inhaling aerosols containing infective spores.37,41 Most affected cats present with vague systemic signs, including anorexia, lethargy, weight loss, fever, and anemia.37 Infrequent findings include pulmonary involvement, skeletal infection, and cutaneous lesions.37 Ocular involvement is thought to be more common with disseminated histoplasmosis than with other feline systemic mycoses.37 Ocular lesions may include mucoid ocular discharge, blepharospasm, conjunctivitis, granulomatous blepharitis, endophthalmitis, chemosis, anterior uveitis, chorioretinitis, retinal detachment, and secondary glaucoma.37,41,42

Coccidioidomycosis. Coccidioidomycosis is caused by Coccidioides immitis, a dimorphic fungus found in soil in the southwestern United States.37,43,44 Transmission occurs primarily by inhaling aerosols containing infective spores but can also occur after direct inoculation of the organism into the skin.37,43 Clinical signs typically include draining skin lesions, abscesses, subcutaneous granulomatous masses, and regional lymphadenopathy.37,43 Ocular lesions may include fibrinopurulent exudates in the anterior, posterior, and vitreous chambers; pyogranulomatous endophthalmitis; diffuse granulomatous chorioretinitis44 ; and retinal detachment.37

Diagnosis and treatment. The aforementioned mycotic diseases can be diagnosed based on clinical findings, the results of serologic testing, and demonstration of an organism by cytology or histology.37 In cats with ocular lesions, aqueous aspirates are rarely rewarding, but histologic examination of an enucleated eye37,39 or vitreous or subretinal aspirates37 is likely to demonstrate organisms.

In addition to nonspecific topical therapy for uveitis (see sidebar titled "Nonspecific therapy for uveitis"), azole antifungal therapy with or without adjunctive amphotericin B therapy has been effective.37,43,45 Fluconazole is the azole of choice in cats since it is associated with the fewest side effects7 and has good penetration into the eye. It is recommended in cases of cryptococcosis and coccidioidomycosis at doses of 25 to 50 mg/cat orally every 12 hours or 5 to 15 mg/kg orally every 12 to 24 hours.2,37 Histoplasmosis and blastomycosis have been effectively treated with itraconazole administered orally at 5 mg/kg every 12 hours, but reversible hepatotoxicosis can occur.4,37

Continue antifungal therapies with azoles for one month after clinical signs resolve.37 Therapy is typically long-term and may last six months or longer.37 Cats with severe disease should be adjunctively treated with parenteral amphotericin B.37 This medication is typically reserved for severely affected patients since it can be nephrotoxic. As such, blood urea nitrogen and serum creatinine concentration monitoring is recommended.37

Protozoal diseases

Toxoplasma gondii is an obligate intracellular coccidian parasite.46 Cats, the definitive host, acquire toxoplasmosis by ingesting T. gondii cysts in prey animals.46 Systemic signs of infection include vague clinical signs, such as lethargy, anorexia, weight loss, and weakness; short-lived signs, such as a self-limiting, small-bowel diarrhea; and more severe signs, such as ataxia, seizures, icterus, abdominal effusion, and cardiac arrhythmias.46 The seroprevalence of T. gondii infection in cats with uveitis has been reported as high as 80.2%.47 Both the organism's DNA and antibodies to the organism have been detected in aqueous humor, confirming that T. gondii can directly infect the eye.47,48 Intraocular inflammation is thought to occur secondary to organism replication or intraocular hypersensitivity induced by exposure of antigen-specific intraocular lymphocytes to circulating T. gondii antigens.3 In addition to anterior uveitis, T. gondii can cause chorioretinitis and retinal vasculitis.

Several diagnostic methods are available, including serology, fecal examination, aqueous humor PCR testing,48 and aqueous humor antibody detection.47 But the only means to definitively diagnose the disease is to demonstrate the organism on ocular histologic examination. The latter may prove difficult as the organism has rarely been identified.3 In addition to nonspecific therapy for uveitis (see sidebar titled "Nonspecific therapy for uveitis"), cats infected with T. gondii should be given clindamycin hydrochloride46,47 at a dose of 12.5 mg/kg orally twice a day for two to three weeks.47 Clindamycin slows the replication rate of the organism but is unlikely to clear it from the body.47

COMPLICATIONS


Figure 9. A complete cataract secondary to chronic anterior uveitis. Numerous pigmented foci are present on the anterior lens capsule surface as a result of previous anterior synechić. Rubeosis iridis is also evident.
Uveitis can lead to secondary glaucoma because aqueous humor flow through the pupil or out of the iridocorneal angle becomes impaired. Secondary glaucoma has been reported to occur in up to 50% of cats with uveitis secondary to systemic disease.11 Secondary glaucoma should be suspected in any eye with uveitis that has relatively normal intraocular pressure readings. Treat secondary glaucoma with carbonic anhydrase inhibitors and beta-blockers to decrease aqueous humor production.


Figure 10. Posterior lens luxation secondary to chronic anterior uveitis. Arrowheads denote the superior aspect of the luxated lens.
Cataracts may develop after inflammatory mediators diffuse across the lens capsule (Figure 9).2 Chronic uveitis is the leading cause of feline cataracts and is often associated with posterior synechiae, rubeosis iridis, and peri-iridal inflammatory membranes.11 Additionally, inflammatory products within the aqueous humor can result in the breakdown of lens zonular fibers and, consequently, lens luxation (Figure 10).2

CONCLUSION

Feline uveitis can occur secondary to ocular disorders or can be a manifestation of a systemic disease. The disease processes that can lead to uveitis, although discussed individually in this article, can occur concurrently in a patient. In many cases, ocular lesions are the first and only clinical signs of systemic disease. A thorough history and physical and ophthalmic examinations are necessary to obtain a diagnosis. Early diagnosis and treatment can help preserve a cat's vision.

Jennifer Chang, DVM
Center for Veterinary Health Sciences
Oklahoma State University
Stillwater, OK 74078

Renee Carter, DVM, DACVO
Department of Veterinary Clinical Sciences
School of Veterinary Medicine
Louisiana State University
Baton Rouge, LA 70803


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