Treatments Advance For Corneal Disease, Injuries

The equine amnion has been proven to help heal corneal injuries and diseases.

Retinal degeneration (inactive chorioretinitis).

Photos courtesy of Dr. Noelle McNabb

Cutting-edge treatments and new methods to treat equine corneal diseases and injuries, equine recurrent uveitis and fungal keratitis are on the horizon. Some are even available now or will be available in the future for general practitioner use.

New Ways to Heal

Corneal disease and injury are common in horses, and injuries can be complicated and difficult to heal. Researchers are working on utilizing different biomaterials to help the eye heal itself.

The equine amnion, which has been used successfully in other parts of the body, has been proven to help heal corneal injuries and diseases. The amnion is obtained during Cesarean sections and normal deliveries, then cleaned and processed with antibiotics. Ophthalmologists cover the cornea with the amnion, which acts as a self-sacrificing dressing. 

“The amnion has different antibiotic and anti-scarring compounds—although research is ongoing to discover what those are in horses,” says Dennis E. Brooks, DVM, PhD, Dipl. ACVO, professor of ophthalmology at University of Florida College of Veterinary Medicine.

“It’s adjunctive therapy; you still need the eye drops,” Brooks continues. “But the amnion reduces enzyme activity and scarring, which is the major problem with horses. We’ve applied amnion after corneal ulcers, corneal transplants and corneal injuries. We’ve sutured it and glued it on sick horse corneas. We’ve also experimented with freeze-drying it and crushing into a solution to use as eye drops. We’ve also attached horse stem cells to the amnion, which is making a difference in speeding healing in ulcerated horse eyes.”

A commercially available product called ProKera (a sutureless cryopreserved human amniotic membrane preparation, which resembles a contact lens) is already used in human eyes for curbing inflammation and promoting corneal surface healing. In human eyes, the membrane stays in place until the eye is healed. But in horses, Brooks says, the amnion should remain for only a week to 10 days and then fall off.

“This is because the bacteria in the tear film attack the amnion so the horse has time to heal itself,” Brooks explains. “In other words, we’re misdirecting the bacteria and fungi to the amnion so Mother Nature can heal the horse’s eye. The horse eye knows how to heal. In fact, some therapy in the past may have interfered in the healing process. Severe eye problems with no therapy whatsoever have managed to heal themselves in a few horse eyes.”

Right now the amnion is available only through a few veterinarians in Canada and the United States. And because many practitioners aren’t comfortable with suturing the eye, Brooks’ goal is to simplify the procedure. He and his colleagues are working on creating a glue for practitioners to use to secure the amnion onto a standing horse.

Corneal Transplants

Advances in Equine Eye Protection 
Fly masks have been invaluable for minimizing the eye’s exposure to direct sunlight and keeping pests away. But now there’s a mask called the Eyesaver, designed for horses with active eye diseases (http://www.jorvet.com/).
 
“Each mask includes a hard eye cup (either right or left side) that is protective of fragile eyes,” says Noelle McNabb, DVM, Dipl. ACVO, consulting ophthalmologist at Peterson & Smith Equine Hospital in Ocala, Fla.

The cups have holes for airflow, but they also have three sets of replaceable faux sheepskin rings on the inside that snap in to soak up the moisture.

“Horses can get scalding wounds on the side of their faces from excessive tearing and installation of medications, and the rings and terry cloth inserts soak up that moisture,” McNabb says. "These masks help tremendously when you’re treating an eye injury or infection, trying to minimize self-trauma and rubbing of the SPL and after ocular surgery.”

Corneal transplants have become successful in horses during the past decade, but ophthalmologists have refined the technique.

“Initially we replaced the full thickness of the cornea,” Brooks says. “Now we can replace the part of the cornea that is diseased, restoring and saving the horse’s vision. So if the middle of the cornea is diseased, we only replace the middle. The horse will have a small scar but the rest of the cornea will be fine. Ten years ago, we just removed many of these eyes. Now we don’t have to do that.”

Equine Recurrent Uveitis

Equine recurrent uveitis (ERU), or “moon blindness,” is the leading cause of blindness in horses. It is an immune-mediated inflammation of one or both eyes that is relapsing and remitting in nature. Noelle McNabb, DVM, Dipl. ACVO, consulting ophthalmologist at Peterson & Smith Equine Hospital in Ocala, Fla., explains that usually this occurs after an initial ocular infection or injury. ERU can develop at any age but often appears between 4 and 8.

“Subsequent flare-ups of uveitis follow an unpredictable course, developing weeks to months after quiescence from the initial attack,” Dr. McNabb says. “Clinical signs may vary depending on the location of the uveitis. Inflammation of the posterior uvea may be less clinically evident than anterior uveitis. Horses with ERU commonly present with corneal cloudiness, increased tearing, blepharospasm, miosis and some degree of eyelid swelling. As the episodes become more severe and frequent, the eyes can be blinded from secondary synechiae, cataract, retinal degeneration and/or detachment and glaucoma.”

Conventional treatment of ERU includes the use of topical and systemic anti-inflammatory drugs to control active inflammation. However, until recently, there has been no means of preventing recurrent uveitic attacks typical of ERU. 

Change is on the horizon for horses with this disease. The immunosuppressant drug cyclosporine has been identified as an ideal drug to prevent the recurrence of immune-mediated ocular inflammation.

“Cyclosporine suppresses the activity of T-lymphocytes, which is the most common infiltrating cell in the uvea of horses with ERU,” says McNabb. “However, cyclosporine applied topically as a drop or ointment does not penetrate the cornea and conjunctiva, and therefore cannot get inside the eye to treat uveitis.”

To facilitate intraocular drug delivery, researchers headed by Brian Gilger, DVM, MS, Dipl. AVCO, professor of ophthalmology at North Carolina State University, developed a suprachoroidal ocular delivery device that allows slow release of cyclosporine for three to five years. Experimental studies show that the implants are safe and effective in decreasing uveitis. The device is experimental, and is not yet FDA approved for use in horses.

“The clinical study reports and ongoing patient evaluations have shown excellent control of ocular inflammation,” says McNabb. “It’s been very gratifying to see the implants reduce the inflammatory attacks and help to preserve vision in horses with ERU that until recently, we had very limited ability to control.”

Medication for Fungal Keratitis

Many veterinary compounding pharmacies now make topical anti-fungal medications for equine fungal keratitis. Previously, practitioners often had to make do with over-the-counter vaginal yeast creams, silver sulfadizine or dilute betadine solution, among others.

“These preparations often result in significant ocular irritation and discomfort as they not designed for ophthalmic use.” says McNabb. “Some effective anti-fungal medications are available commercially. But they can be cost prohibitive due to volume of medication needed to treat a horse for several weeks and/or may not be conducive to delivery to the eye using a subpalpebral lavage system.”

Now practitioners can call a compounding pharmacy experienced in ophthalmic compounding and order anti-fungal medications based upon fungal culture results, formulated as either a solution or ointment and that are physiological for the eye.

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