Board Thread:WARNING ALL USERS False Info\Spoilers/@comment-24534214-20140211203920/@comment-24447558-20140219193547

It's also important to point out the fact that they were dead for a very long time. As LadyX said, it would be long enough to start that process... the brain can only go a very short amount of time without oxygen.

I think it is more likely that the Nogitsune is mimicking the symptoms because of the pain and strife it would cause Melissa, Scott, and most importantly, the Sheriff... all of whom love Stiles and know what Claudia would have endured before she eventually passed.