As previously stated the Abyssinian breed is the one most prone to this condition but other breeds can suffer from them.
There is no known cause for these and consequently no knowncure but as they are more common in older guinea pigs it cold be part of the aging process. They do not render the guinea pig blind, leastwise not until they are very elderly. People I have spoken to who suffer from them say that it like looking through a veil.
The cornea of one eye becomes opaque and usually within a month or so the other eye follows suit. The good news is that guinea pigs who suffer from this condition seem to take it in their stride and simply carry on as normal. I used to give a supplement of vitamin A but no longer do so. After observing other people's guinea pig with this condition who were not on this supplement I came to the conclusion that they coped just as well as my own. Though I am a strong believer in the value of supplements for as therapy for specific ailments, if they can do without, all the better.