Stopping some forms of equine recurrent uveitis might soon be as easy as … sticking a needle in the eye.
Intravitreal (directly into the vitreous, the jellylike part of the eye behind the lens) injections might sound unpleasant. When performed correctly, however, veterinarians can diffuse low doses of the potent antibiotic agent gentamicin into the eye to halt the destruction caused by repetitive bouts of uveitis, said German researchers.
Effective only in cases of leptospirosis-related uveitis, the local injection of low-dose gentamicin is a promising, low-cost alternative to surgical removal of the vitreous—a procedure known as vitrectomy, said André Kleinpeter, Dr.Med.Vet, of the Tierklinik Alt Sammit regional referral equine hospital for northeastern Germany.
ERU: Overview and Treatment
When a horse’s eye repeatedly battles internal inflammation, it’s not only painful but can also have lasting consequences on his vision, Kleinpeter said. Equine recurrent uveitis (ERU) causes multiple bouts of inflammation of the vitreous, each one contributing to destruction of the eye tissues that affect the horse’s vision. Cataract development and blindness are common after years of ERU, he said.
ERU has different causes, including a possible genetic origin that seems to be linked to breeds or colors, appearing more frequently in Appaloosas and Warmbloods, said Kleinpeter. However, some cases can have a bacterial origin, such as from the common soil-based pathogen Leptospirosa. After initial antibiotic treatments following a case of leptospirosis, horses can continue to have relapses of leptospirosis-related uveitis, and treatment becomes challenging.
The Antibiotic Injection Option
Removing the vitreous usually eliminates the cycle of inflammation, but it’s costly and invasive, Kleinpeter said. If the horse is still in the early stages of ERU, without significant damage to the eye itself, he could benefit from intraocular injections.
“Gentamicin is a broad-spectrum, bactericidal aminoglycoside-antibiotic and is especially effective against Gram-negative bacteria and should, therefore, work against Leptospira,” Kleinpeter told The Horse. “It has a high protein-binding capacity and binds on the proteins in the vitreous. There are a lot of proteins and amino acids in degenerated tissue and, after binding to these proteins, gentamicin shows an elongated efficacy with effective levels that lie clearly above (average levels of antibiotic efficacy).”
Laboratory study results suggest gentamicin also affects immune response, which could be helpful in uveitis cases because the disease might have correlations with autoimmune disorders, he added. However, scientists still don’t know how or why gentamicin works in the immune response.
In their study, Kleinpeter and his fellow researchers injected 61 eyes of 50 ERU-affected horses, he said. Nearly all the eyes—56 of the 61—showed no signs of ERU over the long term, with follow-up averaging a little more than two years. The other five eyes only had a single uveitis episode within the two-year follow-up period. The horses tolerated the procedures well with very few complications, and 70% of the horses went on to have long-term, stable vision.
Skill and Immobilization Are Key
Although it’s a less costly and less invasive alternative to vitrectomy, intraocular injection is not a simple, on-the-farm procedure. On the contrary, low-dose gentamicin injections directly into the vitreous require full immobilization of the horse and very skilled needle placement and treatment administration.
“The procedure should not be performed in the field and not in the standing horse,” Kleinpeter said. “It should be done under general anesthesia in lateral recumbency (horse lying on his side). It is very important to immobilize the eye to prevent all eye movements, which you can’t guarantee in the standing horse. Once the needle is injected into the vitreous, even the most minimal movements can result in touching the lens with the needle, which leads to cataract development.”
As for the injection itself, veterinarians should follow the guidelines for intravitreal injection already established in human medicine, said Kleinpeter. “Gentamicin should be injected very slowly to avoid high concentrations in front of the retina,” he added.
Very importantly, the procedure should only be performed when the horse is not having a current episode of uveitis inflammation, Kleinpeter said.
Not for Late-Stage Cases
Intravitreal injections of gentamicin work well for early cases of ERU. But once eye damage from ERU begins to accumulate, it’s not the best treatment method, said Kleinpeter.
“The fewer painful episodes a horse had had at the time of the injection treatment, the better,” he said. “Gentamicin injections can only prevent the recurrence of uveitis; it’s can’t improve vision that’s already limited.”
Cataracts that have already started forming will continue to progress even after gentamicin treatment, he explained.
“Horses with severe alterations within the vitreous are better candidates for vitrectomy, as far as preserving vision is concerned,” Kleinpeter said.