A long-term international study shows that cyclosporine implants can preserve the vision of horses with equine recurrent uveitis.
The leading cause of blindness in horses, equine recurrent uveitis is an autoimmune disease that causes painful swelling of the colored part of the eye. Conventional treatments involve administering topical medication to the affected eye three and even four times a day, a difficult protocol for both owners and horses. About five years ago, however, North Carolina State University researcher Brian Gilgers, DVM, developed an implant that could release the immunosuppressive drug cyclosporine directly and continually into a horse's eye.
Gilger and NCSU researchers recently reviewed the records of 151 horses in the United States and Europe that had received cyclosporine implants in one or both eyes. The duration of the cases, which each spanned from diagnosis to treatment to final follow-up, ranged from 13 months to seven years.
Overall, 79 percent of the horses still had vision at their last exam. This is significantly higher than the 50 percent of horses receiving the standard ERU care that would be expected to retain their sight.
The rate of blindness increased, however, 48 months after the implant placement due to recurrences of ERU. This, Gilger says, indicates that in cases where the implant controlled ERU, it tended to do so for approximately 36 to 48 months, at which point the cyclosporine was depleted. As a result, he recommends the implants be replaced at 36 months.
However, he notes, a significant number of horses did well even after the implant would have finished delivering medication. Only 15 of the 61 horses (25 percent) that had a 48-month follow-up had an ERU recurrence. Although the drug in the implant had been long depleted, says Gilger, its effect on the disease may have persisted. "It may be possible that the immune cells undergo anergy or some other permanent immune suppression, thus not allowing the uveitis to recur."
Future treatments for uveitis will build on this work, says Gilger. One possibility is delivering immunosuppressive medications that last six to eight month through injections into the eye rather than implants. "We are hoping to do a clinical trial of a microparticle formulation of a drug called rapamycin that has been shown to be safe and in pilot studies to be effective in the prevention of recurrence of ERU," hey says.