The etiology of eosinophilic keratitis is uncertain. It has been proposed that the condition is related to the Eosinophilic Granuloma complex of cats, by reason of the similar histopathologic findings and response to treatment. It is generally accepted that cats produce an exaggerated response to certain chronic allergic stimuli evidenced by an abundant infiltration of inflammatory cells including plasma cells, lymphocytes, mast cells, eosinophils and histiocytes into the area of stimuli.
Ketring has noted cats with typical eosinophilic keratitis in one eye and typical herpetic keratitis in the other eye. He has also noted cats treated successfully for herpetic keratitis subsequently developing eosinophilic keratitis, and he questions that the chronic history of corneal irritation and ulceration presented in cases of eosinophilic keratitis may be suggestive of previous herpetic keratitis.
A recently published study demonstrated that 33% of cats with eosinophilic keratitis were positive for feline herpesvirus when corneal and conjunctival scrapings were tested using a viral immunoflurescence test. Another study reports that 76% of samples from cats with eosinophilic keratitis were positive using a Polymerase Chain Reaction Test for feline herpesvirus.
The typical clinical presentation of cats with eosinophilic keratitis is a proliferative white to pink plaque affecting a variable portion of the cornea, usually beginning at the temporal limbus but the nasal limbus may also be affected. There is a thick cottage cheese like deposit which is lightly adherent to the corneal surface, and is readily scraped off. There is associated corneal edema, superficial vascularization, conjunctivitis, mucoid discharge, and some blepharospasm and prolapse of the nictitans. Some cases have an attendant reduction in Schirmer Tear Test values.
Cytological examination of the cheeselike corneal deposits reveals a predominance of eosinophils, a few neutrophils, non cornified epithelium and lymphocytes. Deeper corneal scrapings and histologic sections often reveal predominantly plasma cells and lymphocytes, with mast cells, eosinophils and histiocytes present in fewer numbers. In some cases the scrapings are devoid of eosinophils. Some authors have proposed that such cases are have a different pathogenesis and have used the term Proliferative Keratitis. Others suggest that eosinophils may be transient and therefore this is a different stage of the same disease process.
The initial reported case of eosinophilic keratitis was tentatively diagnosed as corneal neoplasia, and the mass was removed by keratectomy. Once the surgical wound had reepithelialized, the eye was treated with subconjunctival and topical steroids. Once the medication was discontinued, the condition recurred. Recognizing similarities between the histologic findings of this case of eosinophilic keratitis and the eosinophilic granuloma complex, Brightman et. al. utilized megestrol acetate (Ovaban, Schering) for the treatment of the recurrence of the condition in this cat with excellent results. The dosage used was 5 mg daily for 30 days, at which time the dosage was reduced to 5 mg every other day. Other authors reported varying dosage regimes of megestrol acetate which would provide clinical remission.
In recent years, however, the adverse side effects of megestrol acetate therapy have received attention in the veterinary literature. These include increased appetite and weight gain, behaviour change, mammary hyperplasia, adrenocortical suppression and diabetes mellitus. As a result, some authors advocate the use of megestrol acetate only in cases refractory to topical anti inflammatory therapy, while others suggest using megestrol acetate at lower dosages and in combination with topical steroid. Most cases of eosinophilic keratitis will respond to topical dexamethasone alcohol or prednisolone acetate, when applied twice daily, and reduced to first once daily and then every other day over a two to three month period. It is my practice to use megestrol acetate only in cases refractory to treatment with topical corticosteroids. The dosage I use is 0.5 mg/kg daily for seven days, then reduced to every other day for 3 doses, then once weekly for maintenance. I concurrently use a topical steroid such as Dexamethasone Alcohol (Maxidex, Alcon) two times daily. In most cases, if the response is satisfactory, I will attempt to discontinue the megestrol acetate after two to three months, and use topicals for maintenance. Should the condition recur, I will then use megestrol acetate for long term maintenance at 2.5 mg weekly.
It is recommended when collecting a corneal scraping for cytology, that a sample also be collected for a herpesvirus polymerase chain reaction (PC R) test. This test is a very sensitive indicator of the presence of feline herpesvirus. If the test is positive, concurrent use of antiviral medications is advisable. Follow this link for more information on Feline Herpesvirus Keratitis
This page prepared using: