The Patronus Charm article states that J.K. Rowling was quoted saying that Snape was the only Death Eater that can conjure a Patronus. It is uncited and I, personally, haven't been able to find anywhere where Rowling said this. If it can't be properly cited, it shouldn't appear there, of course. It doesn't seem to make any sense. Umbridge, one of the most despicable people in the Potter universe conjures one and Bellatrix doesn't seem like she'd have too many problems "thinking happy thoughts." Furthermore, the Death Eaters would need to use the Patronus Charms to control the Dementors.
- July 30 2007 chat transcript. The Umbridge question is also answered in that chat. As for the final question, not sure, though I suppose sometimes having a common goal is enough for control. Also, Snape did hint once that there may be other ways of repelling a dementor, though we were never given more info on that, that I know of. ProfessorTofty 04:29, November 4, 2011 (UTC)