Someone above mentioned that fly catching syndrome is also associated with other disorders/diseases. One of those is distemper.
I have not a doubt in my mind that every single person on this site has had their dogs vaccinated, so distemper isn't going the be the cause.
Almost 30 years ago I got a puppy from a friend. I just loved that little boy, his name was Ben. I really didn't know anything about vaccinations, etc for dogs. Only about a week after I had Ben, he became ill. I took him to the vet and the vet diagnosed distemper. He said that there was little chance of survival, but that they were experiminting with IV doses of Vitamin C and other medications.
I took Ben to the vet every day for treatment for about a week. The vet advised me that if the treatment wasn't working, Ben would develop brain damage and might hallucinate. That would be a condition called, and I can only give this to you phoneticly, Corea. pronounced like the country, Korea. On about the 7th day, Ben was sitting on the couch, and he suddenly began going crazy, biting at the air in a frenzy. He was totally biting at flies that were not there.
I rushed him back to the vet, and the vet said that it was Corea, and that there was no chance for survival at that point. So Ben was put to sleep.
I can still feel the pain. Ben was only about 3 months old.
So, tho I know every Cav has been vaccinated, this is just a reminder of how serious those vaccinations are.