Emerging Equine Eye Therapies
Potentially one of the most delicate aspects of the horse’s anatomy is the eye. Offset on the equine skull, it has very little protection from trauma or the elements. And, at some point in their lives, horses will likely develop eye issues. That could be a corneal ulcer that clears up after a few weeks of four daily treatments or a more serious condition requiring enucleation, or removal, of the eye to alleviate pain. Equine ophthalmologists have made many advancements in recent years, producing new treatment methods for a number of eye abnormalities. I sat down with several veterinarians who live and breathe equine eyes to share exciting up-and-coming treatments.
Low-Dose Gentamicin Intravitreal Injection
Not uncommon to horses, uveitis is inflammation of the uvea, or the inside of the eye. Causes are many, including bacterial (e.g., leptospirosis), viral, and parasitic infections, trauma, and immune-mediated diseases. Appaloosas are predisposed to the condition. When clinical signs linger and flares become frequent, the condition can evolve into equine recurrent uveitis (ERU).
Equine recurrent uveitis is difficult to treat, to say the least. Each horse can respond differently to standard topical and oral treatments, and if an underlying infection exists, diagnosis and treatment are vital to the horse’s well-being. While veterinarians find success using topical steroid and anti-inflammatory medications, daily to twice-daily treatment can be prohibitive to some horses or owners.
That’s where low-dose gentamicin injections come into play. “This is an injection of an antibiotic into the back (vitreal) chamber of the eye,” says Megan Cullen, DVM, of the Kansas State University College of Veterinary Medicine’s ophthalmology department, in Manhattan. “A very small dose of gentamicin has been shown to reduce recurrence of uveitis in horses.”
The procedure is delicate, requiring careful hygiene, heavy sedation, regional anesthesia, and familiarity with intraocular anatomy. It carries some risk of complications, such as cataracts or detached retina, and the injection is best done by a trained veterinary ophthalmologist using a precise dose of preservative-free antibiotic. Veterinarians have found great success with low-dose gentamicin intravitreal injection in equine cases, though its exact method of action remains unclear.
“It is speculated that gentamicin has an immunomodulatory effect on the eye,” says Cullen. “Given that horses respond to this injection even without evidence of infection with Leptospira and that recurrent bouts of inflammation in horses with ERU are thought to be an autoimmune response against ocular structures, it is unlikely that the bactericidal effect of gentamicin is the reason for improvement.”
Indeed, the procedure’s success is impressive, says Cullen. From the studies available, around 90% of treated horses had no clinical signs one month from gentamicin injection, and 75% of horses remained free of uveitis one year following the procedure. “However, Appaloosas and horses with leopard spotting were found to have a higher failure rate compared to other individuals,” she notes.
So who’s a good candidate for this procedure? “The ideal candidate will still be visual and their intraocular inflammation well-controlled at the time of injection,” Cullen says. “Blind eyes, especially those with secondary glaucoma, are usually better served by enucleation to permanently eliminate the source of pain.”
Infectious keratitis is a bacterial or fungal infection causing inflammation and infection of the cornea, or the outer layer of the eye. These cases are extremely difficult to manage and often lead to enucleation due to treatment failure.
Some of the most up-and-coming research in ophthalmology focuses on corneal cross-linking—a potential treatment for infectious keratitis. While still very much in the early stages of her research, Cullen describes the treatment and how she hopes to use it to help equids: “The cornea is primarily made up of collagen fibrils. Corneal cross-linking is a procedure that stiffens the cornea by inducing these fibrils (using a B protein called riboflavin and UV light) to form strong covalent bonds between themselves. Corneal cross-linking is used in human medicine to treat a variety of corneal diseases where the cornea becomes thinner or weaker.
“More recently, it has been investigated in other forms of corneal disease, including infectious keratitis,” she continues. “This is the primary area of interest for its use in veterinary medicine. Infection leads to the production of collagenase enzymes, which break down the cornea in a process known as corneal melting.”
Melting of the cornea is a true emergency, and it can lead to rupture of the eye itself. “Corneal cross-linking is thought to counteract this melting process, stabilizing an infected cornea,” Cullen explains. “Additionally, the reactive oxygen species that are produced may damage the infectious organism directly, and the riboflavin may slow replication of the organism.”
Most of my infectious keratitis cases in the northeastern United States and Canada have been bacterial. In other parts of the world, especially the southern U.S., fungal keratitis is much more common. However, corneal cross-linking doesn’t appear to have the same efficacy when facing a fungal foe. “Fungal infections are common in horses, but unfortunately corneal cross-linking appears to be less successful in humans for cases of fungal keratitis relative to bacterial infections,” Cullen says. “It is possible that fungal organisms are more resistant to the effects of this treatment.”
She says her hope is to be able to add corneal cross-linking to veterinarians’ arsenal of treatment options for infectious keratitis in horses. “Access to this treatment is still very limited, even among referral hospitals,” she adds. “In human and especially in veterinary medicine, we are still in the very early stages of learning about this treatment for infectious corneal disease. We still have a lot to learn about case selection and treatment protocol. While it shows promise, the jury is still out on whether it will be beneficial for infectious keratitis in horses. It is very unlikely this will replace traditional treatment with topical medications.”
One of the more progressive and detrimental eye conditions that can affect horses is glaucoma. It occurs when the eye contains too much aqueous fluid, increasing the intraocular pressure (IOP) and causing pain and damage to the retina and optic nerve. Even when treated, this condition can inevitably lead to blindness. Topical treatments are the hallmark, but they rarely halt the disease process, just slow its progression.
Cyclophotocoagulation is a relatively novel surgical procedure for treating glaucoma in animals. “Laser cyclophotocoagulation targets and destroys the ciliary body epithelium, resulting in a decrease in aqueous humor production and ideally a reduction in IOP,” says Abby Sturbaum, DVM, of the Animal Eye Clinic of Spokane, in Washington.
Equine glaucoma is a difficult condition to manage. Like many eye issues, it can require several treatments per day, potentially long-term. “The goals of surgical glaucoma therapy are to maintain vision, decrease IOP, relieve pain secondary to glaucoma, and potentially reduce the need for topical medications,” says Sturbaum. “Chronically affected eyes tend to show a poorer response to treatment.”
While this procedure is still under investigation, it remains an intriguing possibility for horse owners battling equine glaucoma. But, Sturbaum warns it is not a be-all and end-all procedure. “Cyclophotocoagulation, if successful, is a treatment that may help reduce frequency of treatments,” she says. “Additionally, it may aid in reducing IOP in horses no longer responding to medical therapy.”
The most common ocular cancer in horses, squamous cell carcinoma (SCC) is a fickle beast. Located on the eyelid or the cornea, SCC can be a devastating condition. Common in nonpigmented areas (such as on Paints), SCC can be difficult to treat. Topical chemotherapies, surgical debulking, and cryotherapy are treatment options that have varying levels of success.
Surgical removal of SCC masses remains the hallmark of treatment, but, with masses around the eye, vets only have so much tissue to remove. “When surgically removing cancerous tissues, the goal is to remove a certain amount of ‘disease-free’ tissue around the mass, which limits the potential for microscopic disease to persist and spread,” says Alison Clode, DVM, Dipl. ACVO, a board-certified ophthalmologist who works at a number of veterinary clinics in the Northeast, including New England Equine, in New Hampshire, and Rhinebeck Equine, in New York. “Surgery is therefore combined with adjunctive therapies, such as cryotherapy, local chemotherapy application, and/or PDT.”
Photodynamic therapy (PDT) is a laser-assisted surgical procedure performed at specialty clinics and referral hospitals.
“With PDT, the laser is used in conjunction with a photosensitizing dye,” Clode explains. “The dye is introduced, usually by injection, to the affected tissue, and the tissue/dye is then exposed to the laser energy, leading to selective cellular damage. This technique is useful for surface cancerous and inflammatory conditions, such as SCC of the eyelids or globe, or inflammatory conditions of the cornea.
“The general approach is to surgically remove the cancerous tissue and to inject the edges and bed of the resulting wound with the dye,” she continues. “The laser is then directed toward the tissue and the desired tissue/cellular damage results. Surgical mass removal and PDT are performed at the same episode, and generally PDT does not need to be repeated.”
Photodynamic therapy remains an exciting new advancement for equine ophthalmology, and its popularity is growing. “The benefit of PDT is therefore providing experienced practitioners with an additional resource to provide effective treatment for a potentially devastating disease in horses,” Clode says.
Equine medicine is developing by the day. Research is underway, change is happening, and ophthalmology is in that growing populous. Emerging therapies such as these offer hope for owners caring for horses with eye conditions that affect the animals’ sight and quality of life.
Stay on top of the most recent Horse Health news with