Oh no. That is awful. I've been thinking about the bate you are using. Rats are very smart and they communicate such things. If only one rat died out of a group, the others might know now and just avoid to eat it too (or again, in case they ate some but not enough). Maybe rather try a different bate? I had roaches chew on my cacti and stick their oothecas onto the cacti. The roach bait I was using did not change that. Someone recommended the brand Talon. I tried roach and ant poison from Talon and both scored great success. Never tried the rat one but might be worth a try. Also spraying the cacti with some contact insecticide might deter them. That stuff should smell and taste bad and at least give them a knock. I keep my fingers crossed!