Oh gosh I'm so sorry. How terrible! It must have been so heartwrenching not being able to stop the bugs while they killed your rat. Hopefully we can fix this problem and you could get a new rat.
If you could describe the bugs (size, shape, color, characteristics) it would help. I can only think of lice, fleas, ticks, and (gulp) ants. Probably not ants because everyone knows what ants look like, but ants are the ones that really swarm.
I think it might be ticks. Ticks actually latch themselves on and suck the blood out. 200 ticks can drain the blood of a horse in two days. Did the bugs get bigger as time went on? Were the bugs moving on your rat or were they just sitting there drinking?
Lice and fleas will bite, but they don't really suck blood I don't think.
Did the bugs have wings? Where did you keep your rats? Did you clean their cage often? Maybe a bug laid a nest in your rats home, and when the baby bugs hatched they swarmed your rat as food.
This is so disgusting. I'm gonna look into this and ask around. Maybe I can find a Literaary Bug Reading Room. We'll get to the bottom of this.
Lila