I think "replaced" is the wrong word. (and it´s of course not only cats ;) ) Like Sleezy said, it could be just a mutation as sideeffect of the infection. I mean at the moment the Zeds are seeing you from a very far distance. If this is supposed to be, their eyesight could be enhanced because of the infection. I think it´s a good idea and the night atmosphere would be really creepy aswell o.O