An eosinophil is a type of white blood cell. It is seen in all mammalian blood. It is often increased in numbers in patients with allergies and parasitic infections. Cats have several types of dermatologic (skin) eosinophilic diseases: Eosinophilic plaque, eosinophilic or "rodent" ulcer and linear granuloma. The eosinophilic ulcer occurs on the upper lip with the eosinophilic plaque and linear granulomas occurring on the limbs and abdomen. These dermatologic conditions are called the "Eosinophilic Granuloma Complex" or EGC. Over the years, a condition has been recognized which is also unique to cats: Eosinophilic keratitis.
In the eye of a patient with eosinophilic keratitis, the conjunctiva (white part of the eye) will initially be reddened initially. Next, a few white "cottage cheese" deposits will develop on the cornea (clear part of the eye) and/or on the conjunctiva immediately adjacent to the cornea. Finally, the cornea will become pink to red and a white plaque will develop on the cornea. If left untreated, the cat can become blind in the eye!
An area of eosinophilic keratitis on the dorsal aspect of the cornea of a cat (arrow).
The cause of eosinophilic keratitis is not known for certain. The cause of EGC is likewise unknown. Eosinophilic keratitis and EGC may be similar diseases due to similar pathologic (biopsy) findings and response to treatment. Pathology results reveal an infiltration of inflammatory cells including lymphocytes, mast cells, plasma cells, eosinophils and histiocytes into the area of biopsy. Over the years, we have noticed several cats with simultaneous infections with herpesvirus and eosinophilic keratitis. In recently published papers, one third of cats with eosinophilic keratitis were positive for feline herpesvirus when corneal and conjunctival scrapings were tested using a viral immunofluorescence test (IFA) and 76% of cats with eosinophilic keratitis were positive using a Polymerase Chain Reaction (PCR) Test for feline herpesvirus.
Diagnosis of eosinophilic keratitis is by cytological examination of a scraping of the cornea and conjunctiva. Examination reveals eosinophils, neutrophils, lymphocytes and corneal epithelium. Deeper corneal scrapings or pathology sections, obtained by performing surgery, often reveal plasma cells and lymphocytes, with mast cells, eosinophils and histiocytes. In some patients, the scrapings are devoid of eosinophils. In these patients, a different cause is suspected and the term Proliferative Keratitis (PK) has been used. As was stated above, some patients with eosinophilic keratitis also have herpesvirus. It is highly recommended that when collecting a corneal scraping for cytology, that a sample also be collected for a herpesvirus PCR and IFA test. When performed concurrently, these tests are very sensitive indicators of the presence of feline herpesvirus. If these tests are positive, concurrent use of antiviral medications is necessary.
The first reported case of eosinophilic keratitis was thought to be a corneal neoplasm (cancer). The mass was removed by a superficial lamellar keratectomy. Once the surgical wound had healed, the eye was treated with subconjunctival and topical steroids. When the medication was stopped, the condition returned. Due to similarities between the pathology findings of this case of eosinophilic keratitis and the eosinophilic granuloma complex, veterinary ophthalmologists used a birth control medication, 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. The 5 mg daily or every other day is an extremely high dosage and can lead the cat to develop diabetes mellitus or other side effects such as increased appetite and weight gain, behavior change, mammary gland enlargement and adrenal gland suppression. These side effects are similar to those caused by steroids in cats. Because of these side effects, Dr. Hacker recommends that megestrol acetate be used at much lower doses when appropriate. Alternatively, topical antiinflammatory therapy may be used. Most patients with eosinophilic keratitis respond when topical dexamethasone or prednisolone acetate is applied 2-to-3 times daily. Treatment frequency is reduced as the patient responds to the medication.
When accurately diagnosed and treated, eosinophilic keratitis is a readily controlled disease. It may require lifelong treatment for the kitty. Each case is different.
Have any questions on this subject? Contact
Dennis Hacker via E-mail:
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