My unique belief is that the whole concept of turning people into other animals was a sham for illegal organ harvesting.
Remember in one scene the blood goes to a nearby hospital and the organs are removed from the body and the body tossed. Please note Farrell refused to say what animal he turned the evil woman into (I mean really, how could he do it without having some incredible knowledge, all he likely saw was a morgue or operating room for harvesting). He knew what it was but didn't want to scare others for other reasons.
In the director's other films he takes bizarre situations that could absolutely occur in the real world. Why would this be any different? IMHO most people I think just skimmed the story and missed every single plot point.