The reason isn’t fully understood but seems to be linked to the amount of melanin involved in giving Appaloosas their distinctive coat colors.
“Dogs, cats and humans with little melanin tend to have immune problems with their eyes,” Brooks said. “Melanin is more than just a skin pigment; it’s involved in the eye and in the body’s immune system.”
Solving Treatment Issues
“There are all kinds of therapies [for ERU] as well as non-traditional approaches like acupuncture, because this is such a frustrating disease to treat and people are trying everything they can think of,” Brooks said.
“Various drugs have been tried, including one called cyclosporine A, a drug which interferes with T-lymphocytes,” he added. “Cyclosporine interferes with the body’s immune system. If you have a kidney transplant, you would receive cyclosporine as an anti-rejection drug.”
The challenge, Brooks said, is in getting cyclosporine to the location of the lymphoid follicles, to countermand the over-active immune response.
“You can put [cyclosporine] in an eye-drop or an ointment, but it doesn’t penetrate the cornea very well,” said Brooks. “And giving it orally would be both too expensive, at the dosages needed for horses, and too toxic.”
So veterinarian Brian Gilger, of the North Carolina State University’s Comparative Ophthalmology Research Laboratory in Raleigh, developed a cyclosporine implant.
“He tried various kinds of implants, some of the first ones you had to actually put inside the eye,” said Brooks. “But the one we’re now using, you can insert in the sclera, in the white of the eye.”
Even the most well-behaved horses get tired of eye drops, Brooks said. So as ERU progresses, treatment can become more and more difficult, as the patient starts to fight his handlers. That’s where a cyclosporine implant can be a near miracle. It’s designed to slowly release an effective dosage of the drug, exactly where it can do the most good and keep doing it for up to five years.
“By putting the implant in this position in the sclera, it is close to where these T-lymphocyte follicles are,” Brooks explained. “There, it has the best chance of reducing the intensity of uveitis attacks and of prolonging the intervals between the attacks.
“By itself, the implant is not a cure-all, but it can be very helpful in some horses,” Brooks added. “It’s not going to cure ERU, and it won’t restore the sight which has been lost. But if we can control the condition, then we can preserve the sight the horse has, possibly for the rest of his life.”
Brooks said the current style of implant has been in use for about five years, and the results so far are encouraging. Horses who have received the implants have demonstrated no signs of building up a tolerance to the cyclosporine, and if the implant does run out of drug, its silicone composition means it causes next to no irritation and does not need to be removed from the eye. In fact, Brooks believes it would be quite feasible to insert a second implant if necessary.
To an owner who has been fighting a losing battle with ERU, surgery may not seem like an extreme solution for an eye condition.
“Most horses [undergoing the implant surgery] are under general anaesthetic for 45 minutes to an hour,” explained Brooks. “It does qualify as microsurgery, and not every veterinary surgeon will want to do it, but it’s not one of the harder techniques.”
Bruce Watt, DVM, a Caledon, Ont.-based veterinarian, consulted with Brooks in order to become one of the first Canadian veterinary surgeons to offer the implants.
“It’s got to go to a very, very exact spot [in the sclera],” said Watt. “You can’t see the implant once it’s in the eye. It’s a little disc about the size of a baby aspirin, but it’s buried under a few layers [of tissue].”
“The results are better if you do the surgery when the disease is controlled,” Brooks added, “but you can do the procedure on a horse who’s experiencing a flare-up. It’s not always possible to get the inflammation controlled, though it does make the surgery easier. The eyes are inflamed anyway, so much so that the sclera can sometimes be twice its normal thickness. These eyes are sick. If you can make the sickness a little bit better, then the surgery has a better chance of success.”
The success rate is impressive. “Eighty percent of the time, a cyclosporine implant reduces the intensity of the attacks in ERU patients, as well as the frequency. Eighty percent is pretty good,” he said.
But, Brooks emphasized, “it won’t eliminate them completely, because anything that’s an immune challenge can trigger a flare-up. Parasites, viruses, anything that triggers an immune system that is too active anyway can start the flare-up. The disease behaves differently in different parts of the world, and veterinarians have to struggle with that because what works in one location may not work in another. The environment plays a big role.”
If necessary, both eyes can be treated in a single surgery, Watt said, and the aftercare is not difficult, usually consisting of antibiotic treatment to fend off infection. The results, however, are not immediately noticeable.
“It takes about a month before the cyclosporine builds up enough in the tissues for it to start working,” said Brooks.
“So you have to continue on the same topical therapy the horse was on before the surgery [until that happens]. The drug is released at a certain specific rate, so it takes a while before the drug is built up to a level which affects those lymphocytes.”