Disease Du Jour: Equine Recurrent Uveitis  

In this episode, Dr. Nicole Scherrer joined us to discuss clinical signs, risk factors, management options, and ongoing research around ERU.
Appaloosa eye, most at risk for developing ERU
Appaloosas, especially those that are mostly white, are at a much higher risk of developing ERU, and their prognosis is usually worse. | Adobe Stock

In this episode of the Disease Du Jour podcast, Nicole Scherrer, DVM, Dipl. ACVO, joined us to talk about equine recurrent uveitis (ERU). She discussed clinical signs, management options, and prognoses, and she provided a brief overview of a potential new treatment option she was involved in researching. 

What Is ERU? 

Scherrer described ERU as “the horse’s own immune system deciding to attack the uveal structures of the eye.” This leads to pain, which is when owners usually notice the condition, and eventual blindness.  

Clinical Signs of ERU 

Horses with ERU often display clinical signs such as squinting, tearing, and redness. While these signs are unspecific to the condition, Scherrer said if an owner calls in for ocular pain, uveitis is generally always on her list of things to rule out.  

Upon examination, veterinarians might notice some corneal edema or vessels going 360 degrees around the eye. Scherrer said vessels focal to one area are more indicative of corneal ulcers or abscesses than ERU.   

When looking inside the eye, Scherrer said the “slam dunk” for uveitis is aqueous flare, or haziness inside the eye. Horses will sometimes also have a small pupil. The iris color might change from light brown to black, and the corpora nigra can become much smaller in size or disappear completely.  

Scherrer said any time an owner calls about ocular pain in a breed like an Appaloosa, she is more suspicious of uveitis than any other eye problem because of the high prevalence of the condition in the breed. “In particular, Appaloosas that have blankets over them or are mostly white are going to be at the highest risk,” Scherrer said. Leopard Appaloosas have the second-highest risk.  

How to Diagnose ERU 

Scherrer said the ocular examination plays a big part in diagnosing uveitis. “If we see aqueous flare, that means you have uveitis,” she said. “And then the biggest question is, is it equine recurrent uveitis, because there can be other causes of uveitis.” 

If the horse has an infected corneal ulcer or some sort of trauma to the eye, that is likely causing the uveitis; treating the trauma will make the uveitis go away. Equine recurrent uveitis is a different story.  

“The things that kind of key me in to this particular form of uveitis [ERU] is sometimes they’ll have a green, yellowish, murky hue to the vitreous,” Scherrer said. “And again, that darkening of the iris, going from brown to black, tells us that there’s been multiple episodes of uveitis. And then also attachments of the iris to the lens posterior synechia shows us that there’s been multiple episodes of uveitis generally.” 

If signs indicate the horse has had multiple episodes of uveitis in the past, the game plan shifts from treating the individual episode to treating the horse for the rest of its life and monitoring it frequently.  

Treatment Options for ERU 

When considering standard treatment options for ERU, Scherrer divides horses into two groups. For acute episodes with a lot of inflammation, she often uses more potent anti-inflammatories that she wouldn’t consider using long-term. For chronic management, she often uses topical diclofenac or systemic firocoxib.  

Surgical options for managing ERU include intravitreal gentamicin injections and suprachoroidal cyclosporine implants. Scherrer likes the first option best because of its lower cost and great success rates in the early stages. “It doesn’t damage the eye but helps control the inflammation,” she said. However, when she sees a horse with ERU, she always prefers to try medical treatment first because it has the lowest risk to the horse.  

Novel Drug SOCS1-KIR 

Scherrer was recently involved in research for a novel drug (SOCS1-KIR) to potentially treat ERU. “The main reason we’re looking into other drugs is because we have a lot of different treatment options for equine recurrent uveitis, but most of them come with side effects, and not all horses respond to them,” she said.  

SOCS1-KIR is intended to treat autoimmune diseases in humans, including rheumatoid arthritis and some skin diseases. “The nice thing is equine recurrent uveitis is a really nice model of autoimmune disease,” Scherrer said.  

In the study, they were able to show that SOCS1-KIR has anti-inflammatory effects similar to diclofenac, which Scherrer said is promising for both humans and horses. While the drug is not yet commercially available, researchers have started human trials, and Scherrer hopes it will become available in the future.  

Read more about the research on SOCS1-KIR in this article.  

Prognosis for Horses With ERU 

Scherrer said the prognosis for horses with ERU depends on how they’re treated and how frequently they’re monitored. She said veterinarians need to check these horses a few times a year at a minimum. If the horse is having a problem, the veterinarian should check on them immediately and adjust medications. “I think the ones that are really diligent about follow-up are the ones that we see the best success in,” Scherrer said.  

She said 80-90% of horses that have had gentamicin injections keep their vision, but this is short-term data, and more long-term data is needed. Long-term data on cyclosporine implants indicate approximately 80% of horses keep their vision.  

Scherrer said Appaloosas have a much worse prognosis overall, and veterinarians still haven’t determined the best way to treat and manage these horses.  

Prevention Strategies 

Scherrer said genetic testing for the specific gene associated with uveitis and Appaloosas is available through UC Davis. “As a really lofty goal we would breed it out of them,” she said. “Unfortunately, it’s connected to their color, and people want the spots. They want the white. So the chance that we ever are able to breed it out of them is probably really low just because of that.” 

UV light protection is another valuable strategy. Scherrer recommends greater than 90% UV light protection masks. She also recommends having Appaloosas’ eyes examined twice a year, since early intervention is key.  

About Dr. Nicole Scherrer 

Nicole Scherrer, DVM, Dipl. ACVO, currently serves as Assistant Professor of Ophthalmology at University of Pennsylvania’s New Bolton Center, in Kennett Square, Pennsylvania. Scherrer received a bachelor’s degree in biochemistry and graduated summa cum laude at DePauw University, in Greencastle, Indiana. She attended Purdue University, in West Lafayette, Indiana, where she earned a doctorate degree from the Purdue University School of Veterinary Medicine. After graduation, she started an internship at Rood and Riddle Equine Hospital. She then started a second rotating internship at University of Pennsylvania’s New Bolton Center. This experience opened the door into an ophthalmology residency at New Bolton Center. She became a diplomat of the American College of Veterinary Ophthalmologists in 2019. 

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