I'm sorry, I wish I was more helpful on this. I've not delt with gapeworm yet myself. It would seem odd that she is the only one with worms, but I can't figure out what else it might be. It could be MG like the other poster mentioned and the other two birds are carriers, I just don't have enough of a knowledge base to really make any more suggestions. I keep hoping someone with more information will chime in.