I've never posted before, but this really bugs me. In the beginning when the Boov are rounding up the humans and they scan Pig (the cat) the computer tells them that he's not human and to move along. So all the humans are sent to Happy Humans Town and all pets are left behind. Without anyone to feed or care for them, and being locked in their homes they would all die. So essentially when the humans are freed they return to a nation of dead pets. I find that a little disturbing in a children's film.
Which means that pets still have a high chance of being dead if they do not know how to or unable hunt, made easy prey for larger animals, fly long distances or fly at all in the case of birds, or went into shock after change location so quickly for fish. Also what about the pets that were on trains, plains, cars, at the vet or a place for boarding. There are also the pets and animals that needed medicine on a schedule that did not allow a gap of two weeks.
If you think about it, the whole thing is very terrifying. Neither the book or the movie goes into much detail while covering this very serious topic of forced relocation, which would be very hard on pets and people alike. It's crazy that this was chosen as a topic for a kid's book/movie, but I think it was handled rather well.