There is a Stephen King short story that has everyone get something akin to Alzheimer’s in the end. I don’t think it has the two plot points you mention. The story is in one of his anthologies.
Is that the same Arthur Herzog who wrote The Swarm? 'Cause this book sounded like it might be worth a shot, but with that track record, I’m not too sure.
If you’re looking for a book with the exact opposite idea, you could try Brainwave by Poul Anderson. In his book, everyone on Earth suddenly has their IQ go up by several hundred points. Humans become so intelligent that nobody wants to do menial work anymore and animals become intelligent enough to start working against humans.