According to the vet the cataract was caused from recurrent inflammation of her eye (only the right eye is affected) known as equine recurrent uveitis (ERU). There is no known definitive cause though they suspect Lepto infection, genetics, allergies, parasites or an old injury to the eye that wasn't properly treated. Farletta was tested for Lepto and found free of the infection; she did have an unusually high parasite count for a horse that was just dewormed a couple weeks ago though it was not enough to be considered an infestation. He said it likely did not cause the episode but may have aggravated it. If it was genetic it would affect both eyes equally which leaves us with allergies or an old injury as a possible culprits but it is still not definitive.
Anyway, the treatment is medication directly into the eye and bute (anti inflammatory) for two weeks at which point she needs a recheck. She will have to stay on the bute for life though. She will spend her days inside as she is very sensitive to sunlight and the intense light can trigger the episodes of inflammation.
There is no cure and no reversal for ERU. I can do what I can to reduce the intensity of the episodes but they will continue and become more common as she ages. Eventually Farletta will become totally blind in her right eye and might lose it someday but the vet assured me this is likely 7 -10 years from now. Farletta is lucky it was caught early. Her prognosis is good for a horse with ERU.