Poor little cat, we lost our Harry 2 years ago on 3rd dec, he went downhill very quickly, he was fine in the July, but November time he was going blind and the vets were doing tests on him.
He had a mutated form that had badly affected his brain.
There isn't a cure, just things to help keep them healthier for longer. Some take a long time to get sick.
There are 2 types effusive and non effusive.
Dr Addi has a good informative website.