These guys are really tough to kill still. That made perfect sense early on when the sewers just had rats, but these days-if you hunt rats, even a small group can really mess up a less than fifth level PC.
They're also annoyingly tough for the higher level PCs who can't even gain a use out of their meat.
Perhaps its time to decrease their HP to a level that is appropriate for their racial types.
Say:
Weak Rats about two HP. Sewer Rats about six HP. Fat Rats about twelves HP.
Consider, normal rats have one HP and Dire Rats have five HP.