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