[size=13pt]Posting of Spoilers:[/size]
The cataclysm was primarily caused (5 days previously) by an extra-dimensional ooze that infects living things. In mammals such as humans and dogs (and later on wolves, bears, deer, etc) it then goes dormant, after the creature dies the ooze activates and proceeds to reanimate and control the corpse.
In constrast, the effect on other things is where we get giant bees/ants/mosquitoes, fungaloids, triffids, etc.
Then of course the teleportation technology that gave the scientists the ooze originally opens portals to other places, so we also get horrifying monstrosities such as amigara horrors or jabberwocks.
Practically speaking this means that early on you can kill a pack of wolves and a few bears or moose and just leave the bodies there, no problem. Later on (after the infection has spread far and wide) they will get back up and hunt you down if you don’t butcher/burn/smash the corpses.
Considering the medium of infection that caused the cataclysmic event in question (and a certain amount of suspension of disbelief), this all sounds perfectly reasonable to me.