Equine recurrent uveitis (ERU) is an incurable and painful eye condition that can result in vision loss and reduce affected horses’ function and value. The last time practitioners discussed this disease at the annual British Equine Veterinary Association Congress was in 2011, so Fernando Malalana, DVM, Dipl. ECEIM, FHEA, MRCVS, brought 2018 attendees up to speed on the latest ERU developments. The Congress took place Sept. 12-15 in Birmingham, U.K.
What Is It?
“ERU is a spontaneous disease characterized by recurring episodes of intraocular (within the eye) inflammation,” said Malalana, senior lecturer in equine internal medicine at the University of Liverpool Philip Leverhulme Equine Hospital, in the U.K.
Because of its recurrent nature and positive response to immunomodulatory drugs, researchers believe ERU is an autoimmune disease, he said. The repeated bouts of inflammation are caused by leukocytes (white blood cells) in the eye that become activated, initiating an immune response.
Recently, he said, researchers have identified autoantigens (proteins that are normal but become targeted by the immune system of individuals with autoimmune disorders) in the eye throughout the course of disease. Malalana explained that as the immune system redirects its response from one autoantigen to another, a new uveitis episode occurs, likely explaining the disease’s recurrence.
Research has shown that certain breeds, such as German Warmbloods and Appaloosas, are more at risk of developing ERU than other horses, such as Thoroughbreds. Appaloosas, in particular, are 8.3 to 12 times more at risk, as well as more likely to become blind as a result.
Malalana said researchers have recently identified a group of genes associated with the MHC class I haplotype ELA-A9 that has been linked to ERU in German Warmbloods. In one study, 41% of cases carried these genes. Another genetic marker we now know to be associated with ERU is leopard complex spotting in Appaloosas. Laboratories such as that at the University of California, Davis, now offer testing for it.
Many environmental influences and microbial triggers might be to blame for ERU’s development, but the most common is Leptospira, said Malalana. This pathogen (disease-causing organism) exists in soil everywhere, particularly in tropical, flood-prone, and mild climates. While leptospirosis exposure in horses is common, he said, disease development is rare, and ocular disease typically occurs 12 to 24 months later.
Malalana presented two theories as to how Leptospira might trigger disease: Either the bacteria penetrate the eye, causing persistent low-grade inflammation, or they mimic an existing antigen (molecule that induces an immune response), causing the horse’s immune system to become confused and attack the eye itself, not the organism.
“The two main objectives of therapy are to reduce pain and control the inflammation,” said Malalana.
Veterinarians often administer topical atropine to dilate the pupil and relieve painful muscle spasms in the eye. They reach for non-steroidal anti-inflammatories and, more importantly, corticosteroids to reduce inflammation.
Another therapeutic option is to surgically implant cyclosporine A (CsA, a potent immunosuppressive drug) between the layers of the eye using a suprachoroidal CsA delivery device. So far, these devices have reduced the number and severity of uveitis episodes, said Malalana.
“Around 25% of these horses still have recurrent episodes after surgery, but at least they’re hopefully fewer and easier to treat,” he said, adding that you might need to replace the implant at 48 months.
A less common surgical option is pars plana vitrectomy (PPV, removal of the inflamed vitreous—the jellylike substance behind the lens). It’s most effective in Leptospira-associated cases, said Malalana, but has the potential for complications.
Promising new therapies, he added, include intravitreal low-dose gentamicin injections and intrachoroidal triamcinolone injections (both involving administering medications directly into the affected part of the eye). The latter, in particular, he said, suppressed inflammation for three to four months before needing to be repeated.
As researchers continue to learn more about ERU, Malalana advised owners to have their veterinarians check their horses’ eyes as soon as possible if they have any concerns.
“In some cases ERU causes obvious discomfort that is easy to identify and treat,” he said. “But in others the disease can go unnoticed for months to years and only be detected when it is very advanced and horses are becoming blind.”